Research results of the past 6 decades on the incorporation of OH point
defects in quartz are summarised and evaluated in terms of their application
to natural samples and processes, and a link between experimental petrology,
natural archives, and model calculations is made. A strong focus is put on
recent studies on quartz as a rock-forming mineral, as a geochemical and/or
petrological tracer, and as a tool for provenance analysis in sediments and
sedimentary rocks.
The most relevant defects for natural specimens are generated by coupled
substitution involving mono- and trivalent cations, the most prominent being
Li+, Al3+, and B3+. OH incorporation is rather a function of the
availability of trace metals and water than of pressure and temperature,
though temperature indirectly influences the incorporation by the solubility
of trace metals in the fluid. Pressure has a negative influence on the
formation of OH defects, so the most pure quartzes are probably formed in
the deep crust close to the quartz/coesite transition.
Natural quartz grains from the Earth's crust have on average 10 wt ppm (weight parts per million) water
(5 wt ppm median), but grains with OH defect contents corresponding to up
250 wt ppm water have been discovered in sedimentary archives, matching the
concentration of quartz from high-pressure experiments <4 kbar
under water-saturated conditions in granitic systems. A rough division into
three classes is suggested: (1) grains with pristine igneous and/or
hydrothermal origin, (2) mildly thermally annealed grains, and (3) strongly
dehydrated grains. While samples derived from the currently exposed
Scandinavian Shield are dominated by the third class, considerable
contributions of the first two classes are found in the younger rock systems
in Central Europe. OH defect contents may be used to estimate mixing ratios
for sediments with different sources, provided that a sufficiently large
data set exists and that the different sources can be clearly distinguished
by their OH inventory. Furthermore, metamorphic overprint leads to a higher
degree of equilibration of OH defects between individual grains and may thus
be used as a geothermometer. Finally, OH defect retention in quartz allows for
estimating timescales of volcanic processes.
Introduction
Quartz is formed under nearly all conditions realised within the Earth's
crust, ranging from the Earth's surface down to the base of the continents.
Depending on crystallisation-relevant physical and chemical parameters
(pressure, stress, temperature, temperature gradients, time, presence and
composition of melts and fluids) and geological circumstances (cracks and
voids in the surrounding rock), the resulting habit and size of quartz
crystals is very diverse, ranging from microcrystalline fibrous chalcedony
to decimetre- and metre-sized euhedral crystals. Although both crystal size
extremes are not representative of the main portion of quartz as
a rock-forming mineral (12 vol % of the Earth's crust; Ronov and
Yaroshevski, 1969), they are very prominent in our mind and important to
human history because they were used since prehistoric times as precious
stones (such as agate, amethyst, and rock crystal) or for tools and weapons
(such as flint and rock crystal). Owing to the high appeal to the human eye
and the technical applicability in modern times (Jung, 1992a, b), it is
understandable that the first systematic studies on “water” in quartz were
performed on large hydrothermal (natural or synthetic) crystals. The major
portion of quartz, however, forms as millimetre-sized, often anhedral crystals in
igneous systems (50 % of the Earth's continental crust consists of felsic
igneous rocks (Wedepohl, 1995) that in turn consist of >20 %
quartz). Furthermore, as a hard and chemically resistant mineral, quartz
survives weathering and transport and comprises a significant fraction in
siliciclastic sediments, sedimentary rocks, and their metamorphic
counterparts.
This review summarises early findings on “water” in quartz and its
influence on physical properties, followed by a review of theoretical and
experimental considerations on OH point defects, and finally reports recent data on
OH point defects in common quartz as a rock-forming mineral from igneous
rocks, metamorphic rocks, and sedimentary archives. As will be outlined in
this study, molecular water follows different incorporation laws than OH
point defects, and, consequently, the wealth of studies on fluid inclusions
in quartz, as well as on OH species (molecular water and OH point defects) in
microcrystalline and/or amorphous silica such as opal and chalcedony (Langer
and Flörke, 1974; Frondel, 1982; Adams et al., 1991; Chauviré et
al., 2017) and moganite (Flörke et al., 1984; Heaney and Post, 1992;
Hardgrove and Rogers, 2013), are not considered in this review.
Brief history
The existence of a significantly polarised absorption band at 3400 cm-1
with a more pronounced absorption for E||o than E||e has been known since the end of the 19th century (Merritt, 1895)
but was not linked to a specific impurity until several decades later. In
the first half of the 20th century quartz became more and more
interesting for the upcoming electronic industries (e.g. chronometry and
radio engineering), and research on physical properties and chemical
impurities of (preferentially large) natural and synthetic quartz crystals
was promoted. The analysis of “water” in quartz reached its first prime in
the early 1960s (Bambauer, 1961; Brunner et al., 1961; Kats, 1962; Bambauer
et al., 1962, 1963), focussed on detailed investigations on the spatial
distribution of chemical impurities, hydrogen mobility, charge balance, and
assignment of infrared (IR) absorption bands to specific OH defects in
natural hydrothermal quartzes. Although a distinction between different
species such as molecular water and OH point defects (protons that are
charge balanced by metal impurities or vacancies) was made, both species
were often subsumed as water. The research on water in quartz was
further fuelled by the discovery of hydrolytic weakening (Carter et al.,
1964; Griggs and Blacic, 1965; Griggs et al., 1966) and its importance for
structural geology. In the following decades research on OH in quartz
experienced several flares accompanying technical improvements of
microanalysis, such as secondary ion mass spectrometry (Rovetta et al.,
1989), transmission electron microscopy (Gerretsen et al., 1989; Cordier and
Doukhan, 1991), and calculation capacities (Purton et al., 1992; McConnell
et al., 1995; Rosa et al., 2005). Predominant study objects during these
periods still were large hydrothermal (natural and synthetic) single
crystals. It was not until the early 21st century that common quartz –
representing the main portion of quartz as a rock forming mineral – came into
focus, and systematic studies on metamorphic (Müller and
Koch-Müller, 2009), sedimentary (Stalder and Neuser, 2013; Stalder,
2014; Stalder et al., 2017, 2019; Jaeger et al., 2019), volcanic (Biró
et al., 2016, 2017), and plutonic (Müller et al., 2009; Stalder et al.,
2017; Potrafke et al., 2020) quartz grains were performed.
Dry and hydrous point defects in quartz
Impurities in quartz can be incorporated by a plethora of cation exchanges,
either by the simple exchange of Si4+ by other tetravalent cations such as
Ti4+ or Ge4+ or by coupled substitutions involving monovalent
cations such as alkalis or protons. Within the scope of this work, defects
involving protons are called “hydrous defects”, and those without protons
are called “dry defects”.
Even before quartz was systematically investigated for impurities and
chemical zoning, it was known that β-eucryptite (LiAlSiO4) is
isostructural to high quartz (Winkler, 1948; Cohen, 1960; London, 1984) and
that a metastable solid solution series exists in the system SiO2 –
LiAlO2 (Roy and Osborn, 1949), indicating the possibility of a “dry”
AlLi defect through the exchange Si4+=Al3++Li+.
Consequently, the first charge balance equations involving incorporation of
defect protons into the quartz structure considered Al3+ and alkali
cations as the most relevant impurities (Bambauer, 1961; Kats, 1962, Chakraborty
and Lehmann, 1976a). Other tri- and pentavalent cations were subsequently
added to the equation, suggesting a charge balance equation,
[Al,B,Fe3+]=[H,Na,K,Li,P],
for natural samples (Müller and Koch-Müller, 2009) that later was
confirmed for experimentally grown quartz crystals from natural starting
material (Baron et al., 2015; Potrafke et al., 2019, 2020), while a modified
equation for small Li contents and crystallisation at moderate pressure
(<10 kbar) was proposed [Al,B,Li]=[H,Na,K,P], taking into
account a neutral LiOH species (Frigo et al., 2016).
Large hydrothermal quartz crystals are very heterogeneous with respect to OH
(Kats, 1962), and the highest concentrations are often found in the centre of
the crystal (Chakraborty and Lehmann, 1976a). A similar systematic zoning
from core to rim was observed for metal impurities such as Al3+
(Müller et al., 2003; Miyoshi et al., 2005), suggesting that metal
impurities are linked to OH defects and that the impurity incorporation is
controlled by the fluid chemistry and growth rate. Strong chemical zoning
was recently also discovered within individual phenocrysts from the Mesa
Falls Tuff pyroclastic succession (Tollan et al., 2019) and from Bishop Tuff
(Jollands et al., 2020b), where a decrease in OH from core to rim mirrored a
strong zoning for Al and the formation of a “dry” LiAl defect at the
outermost rim by the exchange of H by Li.
Hydrous species in quartz
As a chemical component, water in quartz is hosted in at least two
totally different ways: (1) molecular water such as fluid inclusions and
(2) OH point defects, in which protons are charge balanced by vacancies or
metal impurities. Further hosts of water are micro inclusions of melts and
hydrous minerals such as mica (Stalder and Neuser, 2013; Kronenberg et al.,
2017), as well as amorphous gel-like material (Brunner et al., 1961). The
different OH species were identified and characterised by a number of
different strategies, such as freezing behaviour, D/H exchange, and
absorption features in the IR region. Amorphous gel-like material, for
example, causes a rather broad, isotropic absorption band at 3400
or 3500 cm-1 and leads to the formation of molecular water upon heat
treatment (2SiOH=Si-O-Si+H2O) that gives the crystal a milky
appearance (Bambauer et al., 1969). It further does not show evidence for
H/D exchange (Brunner et al., 1961) and probably is formed during low
temperature processes. Such absorption bands are typical for precious
varieties of quartz such as amethyst, citrin, and rock crystal (Chakraborty
and Lehmann, 1976b).
It has already been repeatedly mentioned since the first studies that
molecular water (mostly as fluid inclusions) is the dominant OH species in
quartz (Bambauer, 1961; Aines et al., 1984; Gerretsen et al., 1989; Cordier
and Doukhan, 1991; Müller and Koch-Müller, 2009; Kronenberg et al.,
2017) and that these fluid inclusions in turn are strongly enriched in
alkalis (Bambauer, 1961; Müller et al., 2003). Some of the molecular
water is hosted in nano-inclusions (Gerretsen et al., 1989; Cordier and
Doukhan, 1991) that are often too small to build ice upon freezing (Aines
and Rossman, 1984; Müller and Koch-Müller, 2009). Furthermore, no
correlation could be found between the amount of molecular water
incorporation and OH point defects (Aines et al., 1984; Biró et al.,
2016), which implies that the amount of molecular water in quartz is not
a diagnostic for the formation conditions and thermal history of an individual
grain. While point defect formation is controlled by thermochemical
parameters (Paterson, 1986), the amount of fluid inclusions is often
heterogeneously distributed even within one individual and otherwise chemically
homogeneous grain. Fluid inclusions are often concentrated along healed
cracks and were taken up under deformation, which in early studies led to estimates of water solubility that are too high (Gerretsen et al., 1989). Therefore,
the present article will be focused on OH point defects, and molecular water
will not be discussed in detail.
Proton incorporation as OH defects leads to the formation of characteristic
OH dipoles that can be detected and distinguished depending on the charge
compensation by their absorption frequency in the IR range of
electromagnetic radiation. Since oxygen is the only charge-balancing anion
in the crystal, the relevant geochemical component in mineralogical and
petrological literature is often expressed as a neutral water component.
Therefore, throughout this article OH contents are expressed as weight parts per million (wt ppm) water
(identical to µg/g H2O), in which 0.01 wt % water corresponds to
100 ppm water, 11.1 (=100/9) ppm H, or 667 (=100/0.15) H/106Si.
IR spectra of quartz normalised to 1 mm thickness. Spectra are offset
for graphical reasons. (a) Polarised (E||no and
E||ne) spectra of quartz grains (#655 and #657)
from granites from Vånga, Sweden (Stalder et al., 2017), showing strong
absorptions for molecular water around 3400 cm-1 (#655) and mica
inclusions around 3620 cm-1 (#657). By subtraction of both spectra
(E||no-E||ne) the isotropic signal is
erased, and the absorption feature for the OH defects (655o–e and 657o–e) is
extracted. This procedure was followed in all other spectra in (b)–(d).
(b) Quartz grain from a granite from Vånga, Sweden (Stalder et al.,
2017). The grain was repeatedly thinned out and re-measured. (c) Quartz
crystals grown in high-pressure experiments from Stalder and Konzett (2012),
Baron et al. (2015), Frigo et al. (2016), and unpublished results. Added
phases to the starting material are Ab (albite), Sp (spodumene), and
Tu (tourmaline), and numbers in parentheses indicate the pressure in kilobars (kbar).
Impurity-specific absorption bands (at 3378, 3480, and 3595 cm-1) are
indicated by broken lines. Note that the sample from the B2O3
doped run was further minimised by a factor of 10 for graphical reasons. (d) OH-rich natural quartz crystal from a pegmatitic quartz (Podlesí, Czech
Republic; Breiter et al., 2005) compared to crystals from sedimentary
archives reported in Jaeger et al. (2019) for Japan, Stalder (2014) for
Australia and Sweden, and Stalder et al. (2017) for the Baltic Sea. Impurity-specific
absorption bands (at 3378, 3480, and 3595 cm-1) are indicated by broken
lines. The LiOH band (3480 cm-1) is typically dominant in the
pegmatitic sample but poorly preserved in the sedimentary grains.
Methods to characterise and quantify OH in quartz
Although several different instrumental methods to measure OH in nominally
anhydrous minerals has been established, the by far most used technique to
analyse OH in quartz is IR spectroscopy that detects absorption of IR
radiation due to vibrations in the respective material. Different structural
environments differ in their bond strength, length, and orientation and thus
have different vibration properties. Based on the absorption band
characteristics such as vibration frequency, anisotropy, and band sharpness
(Fig. 1a), IR spectroscopy is able to distinguish between different OH
species, such as (1) molecular water, (2) nominally OH-bearing minerals, and
(3) several specific OH point defects. Since the absorbance is linearly
correlated to the concentration (Eq. 2), the sensitivity depends on the
sample thickness, and for 200 µm thick sections of crystals a
detection limits down to the parts per million (ppm) level can be reached. For thicker samples
even lower concentrations can be detected. Analysis can now be performed on
surfaces below 0.001 mm2 (compared to 10 mm2 in early studies such
as Brunner et al., 1961), enabling the possibility to reveal micrometre-sized
colourless inclusions (Stalder and Neuser, 2013) and complex internal
zonings (Potrafke et al., 2020). In order to turn IR absorptions into
concentrations, OH contents have to be determined by an independent method
on reference material, and in the past 6 decades different methods were
used, such as (1) H-alkali exchange via electrolysis (Brunner et al., 1961),
(2) secondary ion mass spectrometry (Rovetta et al., 1989; Thomas et al.,
2009), (3) proton–proton-scattering (Thomas et al., 2009), and (4) the theoretical calculation of extinction coefficients (Balan et al., 2008;
Jollands et al., 2020a). The formulations deviate from each other with
respect to the measurement protocol (unpolarised versus polarised) and in
terms of the resulting units (concentrations expressed as wt ppm water or
H/106Si), and thus different integral extinction coefficients
ε (Eq. 2) were obtained, which finally all lead to the similar
results: the mineral-specific calibration of Thomas et al. (2009) determined
a value for the extinction coefficient as ε=89000±15000 L mol-1 cm-2, which is in excellent agreement with
ε=246.6(3753-ν) of Libowitzky and Rossman (1997) that
would receive the same value for ε for a wavenumber ν=3390 cm-1. As the main absorption band of OH in quartz is exactly
in this region for most quartz specimens, both calibrations obtain nearly
identical results. Only in a few cases in which high-wavenumber bands are
over-represented (Stalder and Konzett, 2012) does the wavenumber-dependent
calibration lead to systematically higher OH concentrations. Considering
that OH dipoles are nearly totally polarised ||no
(perpendicular to the c axis) and using the Lambert–Beer equation,
c=At⋅ε,
with the concentration c, the extinction coefficient ε, and the
thickness normalised absorbance A/t, Thomas et al. (2009) would receive for one
polarised measurement ||no the relation
c(ppmH2O)=At⋅2⋅MH2O⋅1000DQz⋅89000=At⋅0.153,
with the molar mass of water M=18 g mol-1, the density of
quartz D=2.65 g cm-3, and the thickness t in centimetres. The factor
2 represents the twofold contributions of no (out of the three
directions in space), and the factor 1000 is obtained from the conversion of
litres to cubic centimetres (cm3). The obtained value is equivalent to
cH106Si=At⋅1.02,
while Aines et al. (1984) receive only very slightly higher values:
cH106Si=At⋅1.05.
Taking into account the different measurement protocols and different units,
the extinction coefficients of ε=14000 cm mol-1 used by Brunner et al. (1961), Bambauer (1961), and Chakraborty and
Lehmann (1976a) would be recalculated to the sixfold value ε=84000 L mol-1 cm-2, which is in excellent agreement
with the other calibrations. The mineral-specific calibration of Rovetta et
al. (1989) leads to 30 % higher OH contents. There is an ongoing debate
whether a mineral- or wavenumber-specific calibration is more reliable. On
the one hand, a pronounced dependence of the extinction coefficient on the
wavenumber for OH dipoles in quartz was theoretically predicted by density
functional theory (DFT) calculations (Balan et al., 2008; Jollands et al.,
2020a), and a nearly linear correlation has been observed for OH dipoles in
hydrous minerals (Libowitzky and Rossman, 1997). On the other hand, a less
pronounced dependence was observed for different OH absorption bands in
quartz in the calibration of Thomas et al. (2009), and finally, one
mineral-specific extinction coefficient of 89 000 L mol-1 cm-2 was proposed. While the theoretically calculated
extinction coefficients (Balan et al., 2008; Jollands et al., 2020a) are
significantly higher than suggested by the analytical calibrations and would
lead to lower OH concentrations by up to a factor a 2, the general
wavenumber-specific (and not quartz-specific) calibration of Libowitzky and
Rossman (1997) leads nearly to identical OH contents for most samples
compared to the mineral-specific calibrations of Brunner et al. (1961) and
Thomas et al. (2009). Since all calibrations finally obtain very similar OH
contents, no a posteriori corrections concerning the originally published
values were made in the data compilation (Table 1) of published OH contents
in quartz. A more complicating factor for the comparison of published data
from different references is the circumstance that some studies did not make
a distinction between molecular water and OH from point defects, leading to
higher OH for some samples, especially those in which molecular water was an
important species. Since molecular water gives rise to broad isotropic
absorption bands and hydrous defects are nearly perfectly polarised ||no (Brunner et al., 1961), a distinction between both
fundamentally different groups of species was made early by only taking into
account the sharp polarised absorption bands (Bambauer et al., 1962) or by the
separate quantification of point defects and molecular water (Müller and
Koch-Müller, 2009; Biró et al., 2016). Another strategy to overcome
the contribution of non-defect OH (actually originally proposed by Brunner
et al., 1961) is a revised protocol for the treatment of IR absorption spectra
(Stalder and Konzett, 2012), in which all measurements are performed on
oriented sections ||c for both crystallographic directions
(E||no and E||ne) on the same
spot. Subtraction of both spectra (E||no-E||ne) eliminates the isotropic contributions of molecular water,
hydrous melts, and randomly oriented mica inclusions (Fig. 1a). For the BOH
defect that exhibits one third of its absorbance ||ne, the
proposed protocol was later refined (Stalder and Neuser, 2013; Baron et al.,
2015). This strategy is in general in accordance with many previous studies,
in which all measurements were performed ||no (Bambauer,
1961) and/or only the contribution ||no was used for
OH quantification (Brunner et al., 1961; Aines et al., 1984). Besides the OH
absorption bands, IR spectroscopy also reveals information on the orientation
and thickness of a sample by using lattice overtones (Biró et al., 2016;
Stalder et al., 2017; Jollands et al., 2020b) and offers an independent
alternative to mechanical measurements of the sample thickness that is
needed to calculate concentrations from the measured absorbance (Eq. 2).
OH defects in natural quartz samples.
Source materialMethodNumberNumberOH defects (as wt ppm water) ReferencesamplesacrystalsRange minRange maxAverageGraniteFTIR2131313Müller and Koch-Müller (2009)GraniteFTIR641810Stalder and Neuser (2013)GraniteFTIR132<1285Stalder et al. (2017)GraniteFTIR10097226Potrafke et al. (2020)PegmatiteSIMS1222222Yurimoto et al. (1989)PegmatiteFTIR364520Müller and Koch-Müller (2009)PegmatiteFTIR2354Stalder and Neuser (2013)Comb quartzFTIR1132132132Breiter et al. (2005)bSmoky quartzcFTIR9<131Bambauer (1961)Smoky quartzcFTIR70<161Bambauer et al. (1962)Rock crystalcFTIR13<182Bambauer (1961)Rock crystalcFTIR1151515Stalder and Neuser (2013)“Common” quartzcFTIR295<2103Bambauer et al. (1962)Mimetic quartzcFTIR111018070Bambauer (1961)Mimetic quartzcFTIR60722550Bambauer et al. (1962)HydrothermalFTIR222815Kats (1962)HydrothermalSIMS554020Rovetta et al. (1989)HydrothermalSIMS6183828Yurimoto et al. (1989)HydrothermalFTIR174020Miyoshi et al. (2005)HydrothermalFTIR2687Müller and Koch-Müller (2009)RhyoliteFTIR61127Stalder and Neuser (2013)RhyoliteFTIR4132Biró et al. (2016)RhyoliteFTIR831310Biró et al. (2017)QuartziteFTIR74128Müller and Koch-Müller (2009)QuartziteFTIR6122Stalder and Neuser (2013)QuartziteFTIR145<1102Stalder et al. (2017)GneissFTIR1222Stalder and Neuser (2013)EclogiteFTIR2333Stalder and Neuser (2013)Siliciclastic sedimentsFTIR95<15015Stalder and Neuser (2013)Siliciclastic sedimentsFTIR338<11559Stalder (2014)Siliciclastic sedimentsFTIR246<111416Stalder et al. (2017)Siliciclastic sedimentsFTIR543<16511Stalder et al. (2019)Siliciclastic sedimentsFTIR188<1257d9Jaeger et al. (2019)
FTIR signifies Fourier transform infrared spectroscopy, and SIMS signifies secondary ion mass spectrometry. a Averaged value of several unpolarised measurements on unoriented crystals; b OH content determined in this study; c formed under hydrothermal conditions;d value higher than initially reported (211 ppm) after re-examination.
IR band assignment and their chemical correlation to metal impurities
Several dozens of OH absorption bands in quartz have been reported in the
literature (for a summary see Aines and Rossman, 1984) and linked to
specific defect species based on chemical correlations, thermal stability,
H/D exchange experiments, and spectral characteristics such as sharpness and
polarisation. It is understandable that reported band positions deviate from
each other by some wavenumbers due to different spectrometer calibrations
and different temperatures during analysis (e.g. room and liquid nitrogen
temperature). A particular example of a band shift is reported for the most
prominent AlOH band at 3378 cm-1, which is shifted to 3395 cm-1 at
550 ∘C and drops to 3386 cm-1 at the α/β
transition of quartz at 573 ∘C (Suzuki and Nakashima, 1999).
During cooling from room temperature to liquid nitrogen temperature, the AlOH
band is shifted by 15–20 cm-1 towards lower wavenumbers, while
the LiOH band at 3470 cm-1 (at room temperature) is only shifted by 10 cm-1 (Brunner et al., 1961; Suzuki and Nakashima, 1999). Therefore,
wavenumbers given below may deviate slightly from the values reported in the
original publications.
Apart from the aforementioned broad and isotropic absorption feature for
molecular water, the most important OH absorption bands are strongly
polarised and have been assigned to specific metal impurities. The by far
most important metal impurity is Al3+, followed by Li+ and
B3+.
AlOH band
The most prominent absorption band occurs at 3378 cm-1 and is accompanied by two side bands at 3310 and 3440 cm-1 (Fig. 1). This band is rather sharp and strongly polarised ||no (Kats, 1962), is the most stable band against thermal
treatment (Brunner et al., 1961; Bambauer et al., 1963; Aines and Rossman,
1984), and exhibits the slowest diffusion rates. Based on H/D exchange
experiments this band has unequivocally been identified as the OH absorption
band (Kats, 1962), and due to chemical correlations to the Al content of the
sample, this triplet was assigned as the AlOH defect (Bambauer, 1961; Brown and
Kahan, 1975; Aines and Rossman, 1984) that results from the coupled
substitution Si4+=Al3++H+. The AlOH defect was also
characterised by electron paramagnetic resonance (Halliburton et al., 1981;
Nuttall and Weil, 1981), and the band assignment has in general been
confirmed by DFT calculations (Jollands et al., 2020a).
LiOH band
The second most pronounced sharp absorption band in
most hydrothermal quartz specimens is detected at 3470–3480 cm-1 (Fig. 1). Like AlOH it was identified as the OH band by H/D exchange
experiments, and it is strongly polarised ||no (Kats, 1962)
and accompanied by several side bands between 3400 and 3520 cm-1
(Brunner et al., 1961; Kats, 1962; Aines and Rossman, 1984). Based on
chemical correlations it was assigned as the Li-specific defect and is commonly
referred to as LiOH (Kats, 1962; Bambauer et al., 1963; Aines and Rossman,
1984). Its incorporation into the crystal lattice was proposed as an interstitial molecule (Kats, 1962; Bambauer et al., 1963), as the Li-perturbed
AlOH band (Kats, 1962), or (equivalently) as the proximity of an AlOH to a
dry AlLi defect (Miyoshi et al., 2005) that also contributes to the high-energy band of the AlOH triplet at 3440 cm-1. The formation of dry
defects, in which Li+ concurs with H+, is supported by the general
charge balance equation (Eq. 1) (Bambauer, 1961; Müller and
Koch-Müller, 2009) and has recently been documented in quartz phenocrysts
interacting with its degassing host magma (Tollan et al., 2019). Upon
thermal (or hydrothermal) treatment the LiOH bands decrease irreversibly
while AlOH increases (Brunner et al., 1961; Rovetta et al., 1986; Kronenberg
et al., 1986; Suzuki and Nakashima, 1999; Stalder et al., 2017), suggesting
that the LiOH environment is destroyed, while the OH is retained in the
crystal. As a consequence of the low thermal stability of LiOH, these bands
are much weaker (or even absent) in quartz from metamorphic origin
(Müller and Koch-Müller, 2009). DFT calculations (Jollands et al.,
2020a) corroborate the suggested band assignment.
BOH band
Another frequently observed absorption band occurs at
3595 cm-1 (Fig. 1c) and has been correlated to boron impurities in the
crystal lattice forming by the coupled substitution Si4+=B3++H+ (Staats and Kopp, 1974; Müller and Koch-Müller, 2009).
Its pleochroic behaviour is different from all other impurity-related OH
vibrations, with E||no contributing only two thirds of the
total absorbance and E||ne contributing one third (Thomas et
al., 2009; Stalder, 2014; Baron et al., 2015). Furthermore, it is
thermally stable up to 600 ∘C (Niimi et al., 1999), and
consequently, BOH has a better chance than LiOH to survive metamorphic
overprint. Finally, this band is sharper in natural than in synthetic
amethyst and has been used as discrimination tool (Karampelas et al.,
2005). The band assignment has been confirmed by DFT calculations (Jollands
et al., 2020a).
Intrinsic band
A further absorption band occurs at 3585 cm-1; this band is not relevant for average natural quartz (Stalder,
2014) but frequently observed in synthetic quartz (Chakraborty and Lehmann,
1976a) and natural amethyst (Chakraborty and Lehmann, 1976b; Aines and
Rossman, 1984; Karampelas et al., 2005). Due to its occurrence in pure
synthetic quartz and its missing correlation to metal impurities leading to
an excess OH in the charge balance equation (Eq. 1), it is widely accepted
as an intrinsic defect (Paterson, 1986; Rovetta, 1989; Rovetta et al., 1989;
Stalder and Konzett, 2012). There has been some debate concerning its
possible or probable assignment as a hydrogarnet [4H]Si defect (Paterson,
1986; Stalder and Konzett, 2012). On the one hand, the theoretically
predicted existence of a concentration maximum of [4H]Si (around a
pressure of 10–15 kbar; Paterson, 1986) has experimentally been reproduced
(though at 20–25 kbar; Stalder and Konzett, 2012), and on the other hand, recent
DFT calculations suggest that a hydrogarnet defect should lead to four
different OH stretching bands. The preferred explanation for the 3585 cm-1 band from these calculations are isolated OH- groups with
non-local charge compensation (Jollands et al., 2020a).
Further bands
A further weak and only occasionally visible
band occurs at 3614 cm-1 (Stalder and Konzett, 2012, and unpublished
results), potentially related to the 3585 cm-1 band. This band is only
visible at very high intrinsic defect concentrations and may additionally be
blurred by mica inclusions that exhibit strong absorptions around 3620 cm-1 (Stalder and Neuser, 2013).
In some studies a very weak absorption band was detected at 4500 cm-1
(Brunner et al., 1961; Cordier and Doukhan, 1991) and later was assigned as
hydrogarnet (Cordier et al., 1994). Upon thermal treatment it is replaced by
an absorption band at 5200 cm-1 (Brunner et al., 1961), which was
interpreted as a combination band of molecular water (Cordier and Doukhan,
1991).
Several suggestions for the assignment of a further absorption feature at
around 3200 cm-1 have been put forward. Possibly, this band is
generated by molecular surface water or Si-O overtones (Biró et al.,
2016), and hence it was often ignored for OH quantification in recent
studies. Absorption features for the anisotropic contribution (E||no-E||ne) below 3250 cm-1 have,
however, to be regarded with caution because their thickness-normalised
absorbance changes with thickness (Fig. 1b), which can be explained neither
by Si-O overtones (which should stay constant when normalised to thickness),
nor by molecular surface water (which should be isotropic).
Thermodynamic modelling and ab initio calculations
Several attempts have been made to model OH defect species based on
thermodynamics (Doukhan and Trépied, 1985; Paterson, 1986; Rovetta,
1989) and ab initio calculations (Purton et al., 1992; McConnell et al.,
1995; Rosa et al., 2005; Jollands et al., 2020a). Until recently
calculations were strongly focussed on the two concurring intrinsic defects:
hydrogarnet defect [4H]Si and the so called Griggs defect according to
the reaction Si-O-Si + H2O = 2 SiOH (Griggs et al., 1966). There is
consent that [4H]Si concentrations are positively correlated with
pressure up to 10 kbar (Doukhan and Trépied, 1985; Paterson, 1986) at
oxygen fugacities above NNO-2. As to temperature, both negative (Doukhan and
Trépied, 1985) and positive (Paterson, 1986) correlations were
predicted. Assuming a [4H]Si formation by the reaction of the
dissociated species (Paterson, 1986),
2H2 (gas)+O2 (gas)+SiO2 (crystal)=H4O2 (crystal)+SiO2 (surface),
where H4O2 (crystal) represents the [4H]Si defect. It was
predicted that [4H]Si concentrations drop to nearly zero below oxygen
fugacities of NNO-6 (Paterson, 1986). Similarly, thermodynamic models for
the Griggs-type SiOH defect calculated for pressure conditions at 15 kbar
suggest increasing concentrations with decreasing oxygen fugacity and a
positive correlation to temperature (Rovetta, 1989) and maximum OH
concentrations corresponding to up to 300 wt ppm water. Ab initio
calculations (using density functional theory, DFT) suggest that (i) [4H]Si is energetically the most favourable intrinsic defect species
(Purton et al., 1992; McConnell et al., 1995; Rosa et al., 2005), which (ii) converts to molecular water during the development of dislocations
(McConnell et al., 1995), and (iii) that the Griggs defect is unstable in
unstrained quartz (Purton et al., 1992).
Influence of OH point defects on physical properties
The incorporation of OH species influences the physical properties in
different ways: (1) the high mobility of protons enhance the electrical
conductivity by acting as charge carrier, and (2) protons modify the
chemical bonds and thereby the mechanical properties of the silica network
itself. The electrical conductivity depends on the concentration and
mobility of mobile species and can be described by the Nernst–Einstein
equation:
σ=∑iDicizi2kBT,
where σ is the bulk electrical conductivity, kB is the Boltzmann
constant, T is the absolute temperature, and Di, ci, and zi are
the diffusion coefficients, the concentration, and the charge, respectively,
of the mobile species i. In quartz, the principle charge carriers are protons
and thus crucial for long-term DC (direct current) conductivity (Kronenberg
and Kirby, 1987). In order to calculate the electrical conductivity
according to Eq. (4), the diffusivities of protons from different OH species
(H not linked to Al, and H linked to Al) have to be known.
Diffusivities of H determined on natural quartz crystals. Data are
derived from Brunner et al. (1961) – open large thick circle, Kats (1962)
– open small thin circles, Rovetta et al. (1986) – open small thick
circle, Kronenberg et al. (1986) – open large thin circles and triangles
and filled circles and triangle, Bachheimer (1998) – grey squares, Jollands
et al. (2020b) – dark grey field, and Stalder (unpublished data) – light grey
field. Method and diffusion direction are given in the legend.
A number of different strategies to determine H diffusivities have been
followed, such as hydrogen uptake (Shaffer et al., 1974; Kronenberg et al.,
1986), hydrogen extraction (Bachheimer, 1998), H/D exchange (Brunner et al.,
1961; Kats, 1962; Rovetta et al., 1986), and H/Li exchange (Jollands et al.,
2020b). All methods represent different processes with different
rate-limited steps. In addition, diffusion experiments were performed with
different starting materials (although mostly with natural hydrothermal rock
crystals with AlOH > LiOH). Consequently, they reflect different
diffusion mechanisms, and the results spread by several log units at a given
temperature (Fig. 2): the H/D inter-diffusion coefficient is a good
approximation for the self-diffusion of H; in contrast, H uptake or
extraction can be rate-limited by other diffusivities depending on the
charge balance it is based on and may in fact be slower by orders of
magnitude. The hitherto reported H/D exchange experiments above
700 ∘C all fall into a narrow range (Fig. 2) and exhibit a rather
high activation energy around 200 kJ/mol. Experiments at lower temperatures
reveal a marked drop in the activation energy to approximately 70 kJ/mol
(Kats, 1962). This remarkable change has been interpreted as the transition
between two regimes, in which the diffusing species is dominated by H not
linked to Al at low temperatures in contrast to the results at high
temperatures, in which H is predominantly linked to Al (Kronenberg and Kirby,
1987). H uptake experiments by Kronenberg et al. (1986) show a similar
diffusion law as H/D exchange until the charge neutrality [Al] = [H] is
reached, which suggests that the diffusion probably relies on a similar
mechanism to H/D exchange. IR spectra revealed a defect ratio
AlOH > LiOH at the beginning of the experiment, and
AlOH >> LiOH (with nearly no LiOH) at the end of the
experiments. This finding insinuates that to some extent H/Li exchange (and
H exchange with other alkalis) occurred, but the majority of the protons
incorporated during the experiment has to combine with the existing
AlSi′ defects (a dry defect, in which Al is on a tetrahedral position
and exhibits a local charge deficit) to an AlOH defect. It is furthermore
noteworthy that initial samples used for H extraction (Bachheimer, 1998)
reveal IR absorption spectra that are similar to the hydrated sample of
Kronenberg et al. (1986), but H extraction still does not seem to follow the
reverse process of H uptake. Some of the differences between these two
studies may partly be explained by missing exchange options by alkalis in
the extraction experiments. Interestingly, the activation energies are
similar, but diffusivities for H extraction are shifted towards lower values
by 3 log units (compare H uptake and dehydration in Fig. 2). For crystals
without other defects than those that follow the charge neutrality [Al] = [H], a lower diffusivity for H extraction than from self-diffusion is
expected since another – negatively charged – species is involved. A
possible candidate for this negatively charged species is oxygen, which is
several orders of magnitude slower. Other explanations for the discrepancies
between H uptake and dehydration are the redox behaviour of trace amounts of
Fe (not reported in these studies) or pre-existing other defects.
For the H/Li exchange process in quartz a lower activation energy of 100 kJ/mol was recently determined (Jollands et al., 2020b), which is rather
similar to the low temperature H/D exchange data of Kats (1962) and in
accordance with the interpretation that the diffusing H species is not linked
to Al (Kronenberg and Kirby, 1987). H/Li exchange is influenced by the Li
mobility. Estimates for Li diffusion in quartz yield similar to slightly
higher values than for H (Charlier et al., 2012). Considering the
inter-diffusivity,
DH-Li=XH+XLiDHDLiXHDH+XLiDLi,
with D= diffusivity and X= mole fraction, the mismatch between the
observed H mobility in H/Li exchange ||c (Jollands et al.,
2020b) and H/D exchange experiments < 600 ∘C ||c (Kats, 1962) by up to a factor of 3 (half a log unit) may be
explained (Fig. 2). Taking into account that the samples from both studies
differed strongly with respect to absolute H and Li content, diffusion can
be considered concentration independent and DH and DLi very
similar.
No significant dependence of H mobility on pressure (Kronenberg et al.,
1986) and oxygen fugacity (Jollands et al., 2020b) was observed. Similarly,
no pronounced anisotropy (difference between diffusivity ||c
and ⊥c) was found for H uptake and H/D exchange (Kronenberg et al.,
1986) and H extraction (Bachheimer, 1998); the effect for H/Li exchange was
not quantified with regard to crystal orientation in these studies, but a
slower diffusivity for diffusion ⊥c than ||c was
reported (Jollands et al., 2020b).
The presence of defect protons has an indirect impact on other physical
properties. In this context it has been shown that the incorporation of
protons enhance the diffusivity of O within the crystal lattice and thus
indirectly influence the mechanical properties of quartz (Elphick and
Graham, 1988). After the first observation of hydrolytic weakening of quartz
(Griggs and Blacic, 1965; Griggs et al., 1966), the nature of the relevant OH
species assumed to facilitate the deformation was extensively debated. The
discussion primarily concentrated on intrinsic defects such as SiOH, while
AlOH as a chemically very stable defect was not considered as a relevant species
for hydrolytic weakening (Cordier and Doukhan, 1991). The initial model
assumed that weakening was caused by the formation of Griggs-type defects by
the following reaction (Griggs et al., 1966; Aines et al., 1984):
Si-O-Si+H2O=Si-OHHO-Si.
Later on it was suggested that
actually hydrogarnet defects [4H]Si are the relevant species that help
activating dislocations and thus facilitate hydrolytic weakening (Doukhan
and Trépied, 1985; Cordier et al., 1994). Based on the observation of the
precipitation of molecular water bubbles during the formation of
dislocations (Cordier et al., 1988; Cordier and Doukhan, 1991), a conversion
of hydrogarnet defects to molecular water during dislocation (according to
Eq. 4) was suggested (McConnell et al., 1995). A transformation between
different OH species was also observed during the thermal annealing of
previously deformed quartz, in which recrystallised samples contained less
molecular water due to the conversion into point defects (Niimi et al.,
1999).
OH incorporation in quartz in experimental petrology
It has been shown early by hydrothermal annealing at 15 kbar and
900 ∘C (Mackwell and Paterson, 1985; Rovetta, 1989) that quartz is
able to incorporate high amounts of OH (up to 150 wt ppm water) and,
furthermore, that OH incorporation is enhanced at low oxygen fugacities
(Rovetta, 1989). The interpretation of incorporation as point defects was
soon challenged, and it has been argued that OH is mainly hosted in
fluid inclusion (Gerretsen et al., 1989). Experimental research activities
in the following years were focussed on the role of OH during deformation of
quartz and studying OH in other nominally anhydrous minerals, and
relevant data from experimental petrological studies on OH in quartz, which
were applicable to granitic systems, were not published until the beginning
of the past decade. By using a new analytical protocol (Stalder and Konzett,
2012), the isotropic contribution of molecular water (and species in melts
and other amorphous material) could be erased from the IR absorption
spectrum, and the contribution from OH point defects could be determined even
in sub-millimetres, inclusion-rich crystals from high-pressure experiments.
Synthesis experiments in simple systems allow for the assignment of OH bands to
metal impurities by investigating pure systems and systems doped with the
respective metal ions. This allowed us to verify the nature of intrinsic
defects (in the following referred to as hydrogarnet) and impurity-specific
defects in particular, and the substantial influence of B on OH
incorporation was shown (Fig. 1).
Pressure dependence of OH incorporation in quartz. Data are from
experimental studies in granitic and haplogranitic systems with final run
temperatures between 800 and 900 ∘C. Each data point
represents one measured oriented crystal. Experiments from Baron et al. (2015) were doped with tourmaline and Frigo et al. (2016) with spodumene (blue
symbols) or tourmaline (black symbols).
Quartz from high-pressure experiments in granitic systems exhibit OH defect
contents between 30 and 600 wt ppm water, depending on the pressure and
composition of the system (Fig. 3). The influence of crystallisation
temperature is not very well constrained because melt viscosities and the
P-T circumstances in the phase diagrams for realistic scenarios set rather
narrow limits for successful experiments which are expected to provide
sufficiently large quartz crystals for further analysis. However, no clear
temperature trend on OH incorporation was established in a haplogranitic
system (Stalder and Konzett, 2012). Between 1 and 5 kbar, pressure seems to
have a rather small systematic influence (Potrafke et al., 2019), except for
a narrow region at 4 to 4.5 kbar close to the high/low quartz transition
in which a sudden increase in OH and metal impurities was observed. Between 5
and 25 kbar, a negative pressure trend (Fig. 3) was observed in each studied
system (Stalder and Konzett, 2012; Baron et al., 2015; Frigo et al., 2016),
leading to the suggestion that the most pure quartz (inclusive of the charge-balancing metal ions) is formed at the quartz/coesite transition (Frigo et
al., 2019). IR spectra recorded on quartz from high-pressure syntheses in
granitic systems are usually dominated by the AlOH triplet (Fig. 1c). If the
system is saturated with tourmaline (Baron et al., 2015) and/or Li (Frigo
et al., 2016; Potrafke et al., 2019), specific BOH and LiOH are observed in
addition. More detailed studies (Stalder and Konzett, 2012; Baron et al.,
2015) observed a negative pressure trend for AlOH that may be explained by
the unfavourable size of Al3+ in the tetrahedral site at high pressure
(Potrafke et al., 2019). Furthermore, LiOH is strongly decreased with
pressure especially around the high/low quartz transition (Frigo et al.,
2016), while BOH does not show a clear pressure trend (Baron et al., 2015).
These observed trends lead to the situation that at low pressure the linear
absorbance of the LiOH band may nearly reach the absorbance of the AlOH main
band (Potrafke et al., 2019), and at high pressure the BOH band may even
exceed the linear absorbance of the main AlOH band in some cases (Baron et
al., 2015). The hydrogarnet defect shows a positive pressure dependence,
possibly culminating in a maximum at 20–25 kbar (Stalder and Konzett, 2012),
but it never reaches the absorbance of the AlOH band in granitic systems.
OH in natural quartz from different origin
In contrast to experimentally grown quartz from granitic systems (Baron et
al., 2015; Frigo et al., 2016; Potrafke et al., 2019) in which high OH defect
contents ranging between 30 wt ppm and 600 wt ppm water were observed (Fig. 3),
natural quartz from granitic and hydrothermal systems show large variation
and median values around 20 ppm water (Table 1). Average crustal quartz
represented by siliciclastic sediments hosts only 10 wt ppm water as OH
defects (Stalder, 2014), while metamorphic and volcanic samples cluster
around 5 wt ppm (Table 1). The large discrepancy between natural and
experimentally grown quartz from water-saturated systems may have several
reasons, such as (1) reduced water activity during crystallisation in
natural systems leading to reduced OH incorporation (Stalder and Konzett,
2012) and (2) metamorphic overprint leading to partial or total OH loss
(Stalder et al., 2017). In accordance with the high-pressure experiments, the
highest reported values in natural quartz closely reach values above 200 wt ppm water (Bambauer et al., 1962), and even in the sedimentary archives a
handful of grains with OH defect contents >100 ppm water are
preserved (Stalder, 2014; Jaeger et al., 2019, see Table 1). Interestingly,
most of the OH-rich grains from sediments that retained a high OH defect
content are dominated by AlOH defects, which is in contrast to the late-stage igneous samples with high OH contents (Fig. 1d). Possibly, these grains
underwent very low-grade thermal treatment after formation (and before
sedimentation) that led to a transformation of the LiOH to AlOH absorption
bands, comparable to thermal annealing experiments on quartz (Brunner et
al., 1961; Rovetta et al., 1986; Stalder et al., 2017).
Granites and granitic pegmatites
OH contents in quartz from hitherto analysed granites vary between 0 wt ppm and 72 wt ppm water (Table 1) with a median value around 3 wt ppm for Proterozoic
samples from Scandinavia with ages around 1.8 Ga and median values around
20–35 wt ppm for Variscan samples with ages around 0.3 Ga (Stalder et al.,
2017; Potrafke et al., 2020). Neoproterozoic samples (ages around 0.9 Ga),
and some samples from the Paleoproterozoic Transscandinavian Igneous Belt,
may reach 10–20 wt ppm (Müller and Koch-Müller, 2009; Stalder et
al., 2017). The observed differences may either reflect (i) primary igneous
incorporation, (ii) secondary processes such as dehydration, or (iii) a
combination of both. Specifically, the observed differences could be
explained by the different crustal levels that were sampled (having in mind
the experimentally determined pressure trend) or by a long-term low-grade
metamorphic overprint that caused a partial dehydration. In this context it
is interesting that older sedimentary archives from Scandinavia (such as the
1.4 Gyr old Dalarna subarkose sandstone) reveal information about the upper
portion of the same Paleoproterozoic source that is exposed today. During the
sedimentation of the Dalarna sandstone (1.4 Gyr ago), the igneous rocks had a
similar age as Variscan samples have today, and a significant fraction of
very OH-rich grains was preserved (Stalder, 2014). The interpretation of the
long-term low-grade overprint is supported by the observations that IR
spectra of quartz from old granitic bodies are typically dominated by AlOH
absorption bands, the thermally less stable LiOH band is not observed, and
there is a tendency of higher OH contents towards the centre of the
body (Stalder et al., 2017). Variscan granites are dominated by AlOH, too,
but in contrast often exhibit strong LiOH bands (Stalder et al., 2017;
Potrafke et al., 2020) even if they are subordinate or missing in some
cases (Müller et al., 2009).
Spectral characteristic (ratio of the linear absorbances
I3480/I3378, representing the ratio LiOH/AlOH) plotted against
absolute OH content for quartz from river samples from the Elbe and Rhine (grey
solid dots) compared to quartz from Variscan plutons (coloured fields; see
legend). Sediment data are derived from Stalder et al. (2017, 2019), granite
data for the Black Forest samples from Stalder et al. (2017), and
for Podlesí and Zinnwald from Potrafke et al. (2020). Numbers refer to
the components defined in the text.
In order to better understand the systematics of OH incorporation in quartz
during igneous rock formation, several granitic bodies were investigated in
more detail. For the Variscan samples from the Black Forest, spectral
characteristics show an array with a negative correlation of LiOH versus
total OH (Stalder et al., 2017). A similar trend is observed for the
Zinnwald samples (a modally zinnwaldite-bearing granite) in their dependence on depth, with moderate LiOH and high total OH at low depth, followed by a
decrease in total OH and an increase in LiOH towards greater depth (Fig. 4).
A different trend with increasing depth is observed for the Podlesí
stock granite, in which low LiOH and moderate total OH contents are followed by
an increase in LiOH and subsequently a decrease in LiOH and an increase in
total OH (Fig. 4). Due to the rather small depth interval, the pressure
effect can probably be neglected. The process behind the OH defect
variations is possibly controlled by the activity of Li and water in the
system similar to experimentally determined trends, in which more Li in the
system causes a shift towards higher LiOH and lower total OH due to the
formation of dry AlLi defects (Frigo et al., 2016), and a reduced water
activity generally leads to lower total OH contents (Stalder and Konzett,
2012).
Reported OH contents for quartz from granitic pegmatites (Table 1) cluster
around 20 wt ppm water with a range between 6 wt ppm and 45 wt ppm (Müller and
Koch-Müller, 2009). One pegmatitic comb quartz from the granite stock
in Podlesí, Czech Republic (Breiter et al., 2005), revealed an OH content of
132 wt ppm (Fig. 1d, Table 1).
Rhyolitic tuffs and ignimbrites
OH contents in quartz from felsic volcanic rocks up to 13 wt ppm water and
average values of 6 wt ppm were reported from different studies (Table 1).
IR absorption spectra of all studies reveal nearly exclusively AlOH defects
(Müller et al., 2009; Stalder and Neuser, 2013; Biró et al., 2016,
2017; Tollan et al., 2019), and a clear intra-crystalline zoning pattern was
observed with decreasing Al and OH contents from core to rim (Tollan et
al., 2019). In accordance with the charge balance equation established above
(Eq. 1), protons are charge balanced by Al3+ and are in concurrence
with alkali cations. Diffusive H/Li exchange at the rim can lead to a
further decrease in OH as observed in quartz phenocrysts from Mesa Falls
Tuff, Idaho (Tollan et al., 2019), and from Bishop Tuff, California (Jollands
et al., 2020b). A pronounced decrease from the base towards the middle part
of thick pyroclastic density current deposits in the Bükk Foreland
volcanic area, Hungary, was observed, which was explained by diffusional loss
after deposition and slow cooling (Biró et al., 2017). After all, it
seems to be clear that the observed differences between crystals from the
same eruption cannot be explained by processes in the magma chamber but
rather reflect a snapshot of ongoing late-stage modifications of the OH
defect inventory that can be quantified in terms of diffusional loss and
allows for an estimate on the timescales of long-lasting cooling.
Hydrothermal quartz
Hydrothermal quartz specimens have been investigated for decades, and the
range of OH contents is rather large (0–225 wt ppm water; Table 1);
individual crystals reveal strong internal zoning (Chakraborty and Lehmann,
1976a) with respect to OH and vary even between zero and high defect
contents (>20 wt ppm water) within one specimen (Stalder et
al., 2017). In contrast to quartz from most other formation processes
(notably except pegmatites), hydrothermal quartz often exhibits strong LiOH
absorption bands (Brunner et al., 1961; Chakraborty and Lehmann, 1976b;
Müller and Koch-Müller, 2009) that may match or even exceed the
intensity of AlOH bands (Yurimoto et al., 1989; Stalder and Neuser, 2013).
An increasing LiOH/AlOH defect ratio shows a positive correlation to the
total OH content (Bambauer et al., 1963) and reaches values around unity for
the most OH-rich specimens.
Different populations of hydrothermal quartz have been distinguished
(Bambauer, 1961): (1) “usual” hydrothermal quartz with moderate OH defect
contents with up to 8 wt ppm water and (2) mimetic quartz with OH defect
contents corresponding to up to 180 wt ppm water. Common colourless rock
crystals often have a few weight parts per million water as OH defects (Table 1), while smoky
quartz on average even contains OH contents which are lower by a factor of 2 to 10 (Brunner et al., 1961; Bambauer, 1961; Bambauer et al., 1962).
Higher OH contents (comparable to values of rock crystals) are observed in
weakly coloured smoky quartz and their colourless cores (Bambauer et al.,
1962).
Correlation of OH-poor and OH-rich fissure quartz (data from Bambauer
et al., 1962) to metamorphic isograds (plagioclase composition – Wenk,
1962; isograds in metapelites – Frey and Ferreiro-Mählmann, 1999).
Prl: pyrophyllite; St: staurolite; Sil: sillimanite; An: anorthite.
Solid squares represent OH-poor (<2 wt ppm water) and open circles
OH-rich (>4 wt ppm water) common fissure quartz. Large symbols
summarise several samples from nearby localities.
The areal distribution of OH in fissure quartz of the central Alps (Bambauer
et al., 1962) revealed a regional pattern depending on formation conditions,
locally superimposed by the country rock and fluid chemistry. In detail, a
correlation between the OH contents of fissure quartz and the metamorphic
grade in the regional context could be established (Fig. 5), in which feldspar
compositions towards more Na-rich (up to Ab17) adularia and
anorthite-rich plagioclase in metacarbonates (representing high temperature)
accompany quartz with high OH content, and Na-poor adularia and albite
(representing low temperature) were found in fissures containing quartz with
low OH contents. Even if it was pointed out (Bambauer et al., 1962) that
fissure quartz is formed under different conditions (not only at peak
metamorphism), higher metamorphic temperature implies an earlier start of
crystallisation in which the fluid phase was more enriched in the
charge-balancing trace metals that finally lead to the increased incorporation
of OH defects.
Metamorphic rocks
OH defect contents in quartz from metamorphic rocks are generally low
(<5 wt ppm water) and seldom exceed 10 wt ppm water (Table 1). IR
spectra are generally strongly dominated by the AlOH triplet (Müller and
Koch-Müller, 2009; Stalder et al., 2017) in accordance with the observed
limited thermal stability of LiOH. Thus, low LiOH/AlOH combined with low
total OH is typical – although not unequivocally diagnostic – for
metamorphic quartz. In contrast to LiOH, but in accordance with a high thermal
stability of BOH, the absorption band at 3595 cm-1 was detected in
quartz grains from a tourmaline-bearing Qz eclogite (Stalder and Neuser,
2013). A systematic study on quartzites from Norway (Müller and
Koch-Müller, 2009) suggests a rough tendency towards higher OH contents
for quartz from kyanite-bearing quartzites (9 wt ppm) compared to
kyanite-free quartzites (6 wt ppm). It is not clear whether this putative
difference is caused by pressure and/or availability of Al (both properties
ascribed to the presence of kyanite). Pressure is probably not an important
factor because there is no systematic difference in the equilibration
pressure of the studied kyanite-bearing and kyanite-free quartzites, and,
furthermore, a pressure trend is not expected with respect to the
experimental results. The availability of Al is not a very probable
explanation either because the AlOH defect is dominant in all quartzites
and Al diffusion is much too slow (around 10-22 m2/s at
600 ∘C; Tailby et al., 2018) to account for incorporation of new
AlOH defects at metamorphic conditions. Based on the present data set it is
not clear whether the difference between kyanite-bearing and kyanite-free
samples is significant at all. A clear temperature trend has not been
established, at least not for the kyanite-free quartzites (Fig. 6).
Information concerning the equilibration temperature in quartz-rich
metasediments may be gained from the distribution of OH between different
grains. While a homogeneous defect distribution was observed in a quartzite
from Hohe Tauern, Austria, that was metamorphosed at 600 ∘C (Stalder
et al., 2017), quartzites from Vredefort, South Africa, with a metamorphic
overprint at 350 ∘C revealed strongly different OH contents from
grain to grain, suggesting that at low temperatures the originally variable
sedimentary OH content of the individual grains may (partly) be preserved.
The influence of deformation has not been assessed in detail yet. In
particular, the presence of OH may not only be crucial for hydrolytic
weakening, but in turn, deformation probably also promotes the equilibration
of OH defects.
OH defect contents in quartzites from Norway. Data are from
Müller and Koch-Müller (2009).
Siliciclastic sediments
Most quartz crystals in the Earth originally crystallised in felsic igneous
bodies that comprise about half of the volume of the continental crust
(Wedepohl, 1995) and – due to their low density – sooner or later are
uplifted, exposed, eroded, and transported into sedimentary basins where
they are mixed with other sources from metamorphic rocks and/or recycled
sediments. Finally, a sedimentary quartz grain either retained its original
igneous OH signature or experienced a decrease in LiOH/AlOH by mild thermal
treatment or additionally a strong decrease in total OH by more intense
thermal treatment. Consequently, variations in spectral characteristics of
sedimentary quartz show a fair overlap to their igneous source rocks (Fig. 4)
supplemented by a variable amount of low OH grains from metamorphic sources.
Remarkably, most data points fall within an array that can be defined as
a triangle (similar to the one defined by the quartz crystals from granites)
with the edges at (1) low OH content and low LiOH/AlOH, (2) moderate
OH contents and high LiOH/AlOH, and (3) high OH contents and moderate LiOH/AlOH.
Siliciclastic sediments therefore provide a wealth of information of eroded
parts of felsic plutons. Furthermore, they serve as good proxy for average
crustal quartz (Stalder, 2014).
OH defect distribution of quartz grains from sedimentary samples
(squares: sandstones; circles: river sediments; triangles: beach/dune
sands). The right hand y axis gives the percentage of grains below the
respective defect water content. Data points below 1 wt ppm water are not
plotted due to the large uncertainty. Data for the sedimentary samples are
derived from Stalder and Neuser (2013) for the North Sea and Buntsandstein; Stalder
(2014) for Dalarna, North Africa, Sahara, and average crust; Stalder et al. (2017) for the Rhine and Baltic Sea; and Stalder et al. (2019) for the Elbe, Buntsandstein, and North Africa. Data for European granites (Proterozoic from Sweden and
Variscan from Central Europe are shown for comparison and are derived from
Stalder et al. (2017) and Potrafke et al. (2020). For some systems (Rhine,
Elbe, Buntsandstein, North Sea, North Africa) several samples are plotted
with the same symbol (see text for further details).
Geographical distribution of analysed sediments and Variscan granites
from Central (Germany: D; Czech Republic: CZ) and Northern Europe (Sweden: S). Data for OH-rich (average >20 wt water) granites
are from Stalder et al. (2017) and Potrafke et al. (2020), and those for OH-poor
(average <5 wt ppm water) granites are from Stalder et al. (2017).
Average OH contents for sedimentary quartz are colour-coded. Transport
directions during the Mesozoic (Ziegler, 1990) and Quaternary (Eissmann,
1986) are indicated by blue and red arrows, respectively. Large recent
rivers (Rhine and Elbe) are shown as blue lines and have transport
directions to the north.
Average spectra from large sedimentary reservoirs are dominated by grains
with absorption bands derived from AlOH defects (Stalder and Neuser, 2013;
Stalder, 2014), but average spectra of more local sources and of individual
grains may exhibit strong BOH and/or LiOH (Stalder et al., 2017). OH
contents in quartz from sedimentary systems hitherto investigated exhibit a
wide variability (Fig. 7) and average OH concentrations in quartz sands vary
even within Central Europe by a factor of 5, ranging from 4 wt ppm water for
a beach sand from the Baltic Sea to more than 20 wt ppm water in a river
sand from the Rhine (Stalder et al., 2017). Estimations based on large
siliciclastic reservoirs suggest that 50 % of all quartz grains contain
>5 wt ppm water on average globally (Fig. 7). River sediments
exhibit a large spread, and in Central Europe there is a general tendency of high OH
contents (plotting to the right in Fig. 7) towards the south and west (e.g. the
Rhine) and low OH contents towards the north (e.g. Baltic Sea, North Sea,
and northern Elbe). Taking into account the main transport directions from
north to south during the quaternary glaciations and from south to north for
the recent main river systems (Rhine and Elbe) and during the erosion of the
Variscan Mountains in the Mesozoic, a clear correlation to the OH content of
the granites in the source regions emerges (Fig. 8), linking samples with
predominantly OH-poor quartz to old sources (in this case the Proterozoic
Scandinavian Shield) and OH-rich quartz samples to much younger (in this
case late Palaeozoic) sources. Due to this clear contrast, OH defects in
quartz can even be used to estimate mixing relations of fluvial sources
(from the Variscan basement) and glacial sources (from Scandinavia, also
called “Nordic signal”) in Central Europe. In sediments from the northern
Elbe a significant fraction shows OH defect contents in quartz >10 wt ppm water, suggesting a smaller Nordic fraction of only 30 %–50 %
(Stalder et al., 2019) compared to estimates based on heavy mineral
assemblages and the U/Pb age spectra of detrital zircons (Führing,
2017). Another example that the OH inventory records complementary
information is revealed in Mesozoic sandstones such as the Buntsandstein
(Lower Triassic), in which significant differences in OH were detected in a
sequence, in which neither heavy mineral spectra nor U/Pb ages revealed
significant changes despite a clear change in sedimentary facies. The
observed higher OH content in the lower sedimentary unit (Stalder et al.,
2019) is in agreement with the interpretation that some plutons host the most OH
rich quartz crystals in their roof region (Potrafke et al., 2020) and that
OH incorporation decreases with increasing pressure (Baron et al., 2015;
Frigo et al., 2016). In addition, erosion of deeper parts of a pluton down
to its roots takes place later, which increases the possibility that OH
defects are lost during metamorphism. OH contents in quartz crystals in the
sedimentary succession will thus show a depth profile which is inverse to
the respective eroded plutonic body (Fig. 9).
Conclusions and implications
A combination of the different approaches to study OH point defects in quartz
enables a link between understanding the incorporation behaviour depending
on physical and chemical conditions, the evolution of igneous systems, and
the recycling in sedimentary systems. Ideally, clusters of sedimentary
quartz grains can be identified and then traced back to their possible
sources and can give insights to their geological past. In detail the link
may not be unequivocal, especially if the original sources are removed by
erosion. However, a rough general estimate concerning the thermal history of
the source can still be made. If the origin of quartz grains is unknown, one
may divide them into three large classes: (1) pristine igneous and/or
hydrothermal origin with moderate to high total OH and variable LiOH/AlOH,
(2) mildly thermally annealed (late-stage) igneous grains with moderate to
high total OH and low LiOH/AlOH, and (3) strongly dehydrated grains with
very low OH contents and low LiOH/AlOH such as metamorphic grains or grains
from the middle part of pyroclastic density current deposits characterised
by slow cooling. Different classes are not strictly divided and exhibit a
continuous transition between each other: while group (1) covers nearly the
whole range in Fig. 4, group (2) is confined to the low LiOH/AlOH edge, and
(3) is confined to the low LiOH/AlOH and to the low OH corner. The very
close match of OH contents of 250 wt ppm water in quartz from high-pressure
experiments ≤4 kbar under water-saturated conditions in a granitic
system (Potrafke et al., 2019) and the highest contents observed in natural
grains (Jaeger et al., 2019) suggests that the natural processes can be
realistically simulated by high-pressure experiments. Linking the
sedimentary grains to quartz specimens found in situ (such as directly
sampled from igneous bodies or hydrothermal veins and fissures) is not
straightforward, and candidates that might be the precursors for the most OH-rich sedimentary grains were not identified yet. Basically all hitherto
identified grains from sediments with very high (>100 wt ppm
water) OH contents (Fig. 1d) would belong to the above-defined class (2) but
otherwise cannot be linked to natural specimens found in situ, which either
belong to class (1) and show a much higher LiOH/AlOH (as the mimetic
quartzes; Bambauer, 1961) or have lower total OH contents (Table 1).
Possibly, the OH-rich grains were originally formed with high LiOH/AlOH and
were later annealed at sufficiently low temperature (and/or over
a sufficiently short time) so that LiOH was destroyed, but the high OH content
was preserved. With respect to the low thermal stability of LiOH the
available data led to a consensus that fully explains that LiOH is absent in
old plutonic bodies and metamorphic rocks.
Generally, OH incorporation is probably controlled predominantly by the
availability of Li and water (provided that also Al is present), and an
increase in Li leads to a decrease in LiOH by the formation of dry LiAl
defects (Frigo et al., 2016), and, further, an increase in water activity
leads to an increase in total OH. The observed trend on a larger scale suggests
a general decrease in OH with pressure and depth – that is mainly driven by
an unfavourable Al incorporation on the tetrahedral site (Stalder and
Konzett, 2012; Frigo et al., 2016) – implying that quartz from large
batholiths should exhibit less OH defects in their roots. Consequently, deep
(late) erosion furnishes less OH-rich quartz grains compared to shallow
(early) erosion (Fig. 9). This effect is further reinforced by the longer (and
stronger) subjection to metamorphic overprint of the more basal portions of
the plutonic bodies.
Sequence of schematic continental crust sections with granitic
plutons (marked by crosses) that erode over time. In the sedimentary record
the upper portion of a pluton is deposited as lower units (b)
and the lower portion as upper units (c). Darker colours
represent higher OH contents, taking into account that the OH incorporation
exhibits a negative pressure trend and that lower parts of plutons are
subjected to longer and more intense metamorphic overprint.
In conclusion, OH defects in quartz may be used to quantify geological
processes, provided that a sufficiently large data set is present.
A statistical distribution of OH defects in metamorphic rocks could be used
as a geothermometer, in which a higher degree of equilibration between the
individual grains is reached at higher temperature.
In pyroclastic density current deposits (ignimbrites) the distribution of OH
defects can be modelled in terms of diffusional loss allowing for an estimate on
depositional temperature and the timescales of long-lasting cooling.
In specific cases the classification low OH = old and high OH = young may be a valuable tool. However, the application as a geochronometer is
generally not possible since OH-rich grains primarily reflect their
formation conditions independent of the age of the host rock, and OH-rich
grains may be preserved in old sedimentary rocks, such in the 1.4 Gyr old
Dala sandstone in Sweden.
Provided that a sufficiently large data set exists and that the different
sources can be distinguished by their OH inventory, the quantification of
mixing ratios of different reservoirs is possible.
Open problems and further research directions
Despite the progress during the past decade, several knowledge gaps persist.
Further research directions could be addressed to (1) the formation and
stability of hydrous defects during deformation, (2) experiments in a more
narrow grid at 1–5 kbar, (3) the influence of high/low quartz transition,
(4) experiments <1 kbar to verify the rapid increase in the low
pressure regime and for application to volcanic rocks, (5) Li-H exchange in
quartz from volcanic samples for quantification of eruption timescales, and
(6) the nature of the band at 3200 cm-1.
Other hitherto unsolved questions refer to the link between IR spectroscopy
and other spectroscopic methods such as cathodoluminescence (CL) and optical
stimulated luminescence spectroscopy (OSL). Despite several systematic
studies a clear correlation between the spectral characteristics of IR and
CL could so far not be established, and both methods seem to give
complementary information (Müller et al., 2009; Stalder and Neuser,
2013; Potrafke et al., 2020). Very few studies combining IR and OSL were so
far performed, and the existing data set does not reveal a clear correlation
between IR and OSL characteristics. Hydrous point defects show a vague but
barely significant positive correlation to the intensity of the OSL signal
(Thamóné et al., 2020), and for molecular water (fluid inclusions)
contrasting results – positive trends (Sharma et al., 2017) and no
correlation (Thamóné et al., 2020) – were reported. The fact that
for these methods the ideal grain size is different – typically 90–250 µm for OSL (Rhodes, 2011) and 250–1000 µm for IR – is
unfavourable for combining both methods for the same sample material.
Data availability
No data sets were used in this article.
Competing interests
The author declares that there is no conflict of interest.
Special issue statement
This article is part of the special issue “Probing the Earth: reviews of OH groups in anhydrous and hydrous minerals”. It is not associated with a conference.
Acknowledgements
This project was supported by the Austrian Science Fund (FWF): P29145-N34
and P33038-N. Over the past decade, many colleagues and students contributed
significantly by critical discussions, by providing sample material and
analytical facilities, and by performing experimental and analytical work,
and they have helped to successfully perform current and previous projects on OH in
quartz. In this context, I would like to thank Marzena Baron, Kjell Billström, Karel Breiter, Hilmar von Eynatten, Corinne Frigo, Christoph Hauzenberger, Dominik Jaeger, Jürgen Konzett, Thomas Ludwig, Guido Meinhold, Peter Mirwald, Rolf Dieter Neuser, Alexander Potrafke, Burkhard Schmidt, Henrik Skogby, Michael Strasser and many others for constructive
cooperation. Hans Ulrich Bambauer is thanked for talking about his
work. Axel Müller, Mike Jollands, and an anonymous reviewer are thanked
for their thorough and constructive reviews.
Review statement
This paper was edited by István Kovács and reviewed by Axel Müller, Michael Jollands, and one anonymous referee.
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