In this study we report the synthesis of single crystals of
burbankite, Na3Ca2La(CO3)5, at 5 GPa and 1073 K.
The structural evolution, bulk modulus and thermal expansion of burbankite were
studied and determined by two separate high-pressure (0–7.07(5) GPa) and
high-temperature (298–746 K) in situ single-crystal X-ray diffraction
experiments. The refined parameters of a second-order Birch–Murnaghan
equation of state (EoS) are V0= 593.22(3) Å3 and KT0= 69.8(4) GPa. The thermal expansion coefficients of a Berman-type EoS are
α0= 6.0(2) ×10-5 K-1, α1= 5.7(7) ×10-8 K-2 and V0= 591.95(8) Å3. The thermoelastic
parameters determined in this study allow us to estimate the larger density
of burbankite in the pressure-temperature range of 5.5–6 GPa and
1173–1273 K, with respect to the density of carbonatitic magmas at the same
conditions. For this reason, we suggest that burbankite might fractionate
from the magma and play a key role as an upper-mantle reservoir of light
trivalent rare earth elements (REE3+).
Introduction
Carbonatites constitute important ore concentrations of strategic metals,
including Nb and rare earth elements (REEs) (Simandl and Paradis, 2018).
Carbonates are an important fraction of minerals which concentrate REE
elements, and burbankite, (Na,Ca)3(Sr,Ba,Ce,REE)3(CO3)5,
is one of the REE-bearing carbonates in these ore deposits (Wall et al., 2001; Edahbi et al., 2018). Carbonates form as primary crystallizing minerals
in magmatic environments, and they may often undergo hydrothermal alteration
in successive geologic processes (Zaitsev et al., 2002; Smith et al., 2018).
The burbankite group consists of mineral species which are hexagonal or
monoclinic carbonates, characterized by the general formula
A3B3(CO3)5 (Belovitskaya and Pekov, 2004). In the crystal
structure of hexagonal members of this group there are two independent
cationic sites, A and B, coordinated by 8 and 10 oxygens, respectively, with
the formation of AO8 and BO10 polyhedra. Depending on the
considered mineral species, the A site is primarily occupied by Na, Ca and
REE3+ cations, together with some vacancies, while Sr, Ca, Ba and
REE3+ cations are in the B site (Belovitskaya and Pekov, 2004). Three
carbonate groups are positioned in three different orientations
(Belovitskaya and Pekov, 2004). Minerals of this group can occur as hexagonal
prismatic crystals, but they can be found more frequently as irregular
grains or in their aggregates (Belovitskaya and Pekov, 2004). They are
usually transparent; the lustre varies from vitreous to greasy, and in
general these minerals do not have any cleavage. They can occur in many
different colours (e.g. yellow, green, pale brown, pink), but they are
more frequently colourless or white (Belovitskaya and Pekov, 2004).
Burbankite can be found as a primary crystallizing mineral in magmatic
systems (e.g. Smith et al., 2018) and as a mineral precipitated from alkaline
solutions in lacustrine sediments (Green River Formation in Wyoming;
Fitzpatrick and Pabst, 1977) or in caves (Cioclovina cave in Romania; Onac et al., 2009). It is also the most abundant mineral of the group; in fact in one
type of “rare earth carbonatites” (e.g. Khibiny, Gornoe Ozero),
together with its alteration products, it forms ample accumulations, being
potentially the most industrially interesting material for the extraction of
REEs, Sr and Ba (Belovitskaya and Pekov, 2004).
Recently, a sodium and calcium carbonate with burbankite structure has been
synthesized at high pressure, 6 GPa, and temperature, 1323 K (Rashchenko et al., 2017). The possibility of tuning the chemistry of this material, as a
function of pressure and temperature, also suggests the possible existence
of a new class of synthetic carbonates with promising non-linear optical
properties (Gavryushkin et al., 2014; Rashchenko et al., 2017). Together with
alkaline–alkaline earth fluoride carbonates (e.g. Zou et al., 2011), they
can lead to an expansion of the interest on carbonates in material science.
Alkaline-rich carbonates are relevant as ore minerals, and they might also
play an important role in deep carbonatitic environments as their
high-pressure phases may fractionate elements from a carbonatitic melt
crystallizing in upper-mantle conditions (Shatzky et al., 2016a, b;
Podbordnikov et al., 2019). The experimental determination of
Na2CO3–K2CO3–CaCO3–MgCO3 systems at high
pressures and temperatures has in fact revealed that above 3 GPa, new
classes of Ca-rich alkaline–alkaline earth carbonates are stabilized and are
primary liquidus phases, with a possible important role in fractionation of
deep carbonatites (Shatzky et al., 2016a, b; Podbordnikov et al., 2019).
Burbankite is among these carbonates (Shatzky et al., 2016a, b).
As mentioned above a burbankite-like structure phase was already synthesized
at 6 GPa and 1323 K with a composition Na2Ca4(CO3)5
(Rashchenko et al., 2017). This result reveals the flexibility of this
structure in a wide compositional range. Because of the important role of Ca-rich alkaline–alkaline earth carbonates as candidate minerals in upper-mantle carbonatitic systems and the successful synthesis of
Na3Ca2La(CO3)5 at 5 GPa and 1073 K (Merlini et al., 2020), which proves the incorporation of light REE3+ (LREE3+) in upper-mantle
conditions in a burbankite-like structure, we decided to further investigate
the crystal chemical evolution and thermoelastic properties of this phase.
The paper reports the characterization and the determination of
thermoelastic properties of synthetic Na3Ca2La(CO3)5 at
high pressure and temperature.
Burbankite synthetic batch. Dark grey minerals are burbankite
crystals, while white spots are a Ca-rich La oxide.
Experimental
The synthesis of burbankite single crystals (up to 500×200×100µm3) was carried out at the experimental petrology laboratory at the
Department of Earth Sciences, University of Milan (Italy) (DES-UM), using a
multi-anvil module. Carbonate powders were mixed in a stoichiometric
proportion of the burbankite chemical composition,
Na3Ca2La(CO3)5. A platinum capsule of 3.5 mm length and
2 mm diameter was used to pack the starting composition and later welded.
The multi-anvil experiment was performed with a Cr-doped MgO octahedron of
25 mm edge length combined with tungsten carbide cubes of 32 mm
truncation-edge lengths. For the experiment a graphite heater was employed,
and temperatures were measured by a Pt-PtRh thermocouple (S type).
Temperature is accurate to ± 20 K, with no pressure correction for the
e.m.f. (electromotive force) of the thermocouple. Pressure uncertainties were
assumed to be ± 3 % according to the accuracy of calibrant reactions
(Fumagalli and Poli, 2005). Samples were synthesized at 5 GPa and 1073 K
(ramp rate at about 40 K min-1) with a run duration of 24 h. The
assemblage of the recovered sample is burbankite and a Ca-rich La oxide
(Fig. 1). The two phases were characterized via electron microprobe analyses
(EMPAs) at the DES-UM using a Jeol 8200 electron microprobe operating at 15 nA and 15 kV.
The structure of burbankite was obtained by a single-crystal X-ray
diffraction measurement at the DES-UM at room temperature using a four-circle κ-geometry Rigaku XtaLAB Synergy diffractometer. The
instrument is equipped with a PhotonJet (Mo) X-ray source, operating at 50 kV and 1 mA, with a monochromatized MoKα radiation, and with a hybrid
pixel array detector. A single crystal of ca. 100×50×50µm3 was
picked from the experimental charge and glued on a glass fibre, which was
attached to a metallic pin and mounted on a goniometer head. During the
measurements the detector-to-sample distance was 62 mm and the
measurement strategy was programmed with a combination of scans in ω
with a 0.5∘ step and with an exposure time of 1.25 s at each scan
step for different 2θ, κ and ϕ positions. Data
reductions, including Lorentz–polarization and absorption correction based on
the implemented semi-empirical ABSPACK routine, were performed using the
software CrysAlis Pro (Rigaku Oxford Diffraction, 2019).
In situ high-pressure (PH) single-crystal X-ray diffraction measurements have
been carried out at the beamline Xpress of the Italian synchrotron Elettra
(Basovizza, Italy) up to ca. 7.07 GPa. A synthetic single crystal of
burbankite was loaded in a BX90-type diamond-anvil cell (DAC; Kantor et al.,
2012). The pressure-transmitting medium used in this experiment was a
mixture of methanol : ethanol in 4:1 proportions, which transmits pressure
hydrostatically up to the maximum pressure reached in this experiment (Klotz
et al., 2009). Ruby fluorescence was used as a pressure standard, and the
pressure uncertainty is about 0.05 GPa. The standard PH single-crystal
diffraction setup was used (Lotti et al., 2020), using the MAR3450 imaging
plate detector with an X-ray wavelength of 0.49450 Å. The beam size on
the sample was approximately 80×80µm2, and the crystal size was
ca. 40×40×20µm3. To extract intensity suitable for structure
determination, improvement in data collection protocols has been
specifically designed. Automatic alignment of the sample based on its
absorption assured the full illumination of the sample during the whole
rotation range. The reduced rotation speed during step scans (0.25∘ s-1)
and software synchronization between sample positioning and fast shutter
operation resulted in a significant reduction in the Rint of integrated
intensities down to 2 %, thus allowing a reliable structural data
analysis. The measurement strategy at each pressure step was programmed as
ω scans in the range ± 38∘ with a 1∘ step
scan.
Thermal expansion of burbankite was also characterized by in situ
high-temperature (TH) single-crystal X-ray diffraction measurement at the XRD1
beamline at the Italian synchrotron Elettra (Basovizza, Italy). The
wavelength during the experiment was 0.7000 Å, and the detector used was
a Pilatus 2M. A single crystal of synthetic burbankite, together with a
single crystal of quartz used as standard, was loaded in a quartz-glass
capillary, and during the measurement crystals were kept steady with
quartz-glass fibres. During the experiment a hot air gas blower was used to
increase the T every 30 K in a temperature range from ca. 298 to ca. 746 K,
with T uncertainties of ± 1 K. The beam size on the sample is
approximately 80×80µm2, and the crystal size, for both the
samples, was ca. 60×60×40µm3.
Results
Twenty points were measured on the synthetic batch of the burbankite
experiment by EMPA, and the resulting composition is shown in Table 1. The
average empirical formula calculated from the 20 analyses and based on 5 oxygens p.f.u. is Na2.41(14)Ca0.33(5)□0.26Ca2.06(5)La0.94(3)(CO3)5. Based on the
empirical formula calculated on the EMPA data, some vacancies have been
evidenced.
Major element composition (in wt %) of the synthetic single crystals of burbankite in this study. The chemical formula has been calculated on five oxygens.
The refined unit-cell constants are a= 10.4238(2) Å, c= 6.2910(2) Å and V= 591.97(2) Å3 in ambient conditions. The structural
solution and refinement were performed in the hexagonal P63mc space group
using the crystallographic software Jana2006 (Petricek et al., 2014) in
agreement with previous studies (Belovitskaya and Pekov, 2004). The
burbankite structure is characterized by the presence of two independent
cationic sites, A and B, coordinated by 8 and 10 oxygens, respectively, with
the formation of AO8 and BO10 polyhedra and three carbonate sites,
which all have different orientations (Fig. 2a). The 8-fold coordination
polyhedra form infinite columns disposed at zigzags along the c axis, where
neighbouring polyhedra share their faces. The 10-fold coordination polyhedra
also form infinite columns made of rings (composed of three 10-fold coordination
polyhedra sharing their corners) parallel to the a–c plane, and in the middle of
these rings a carbonate lies parallel to the a–b plane, and they share both corners and edges with
the 10-fold polyhedra. On the top of each 10-fold
polyhedron there is a carbonate group in an oblique direction. These so-formed modules repeat along the c axis but rotated by 180∘ (Fig. 2b). The site occupancy refinement confirms the presence of Na and Ca in
the A site, and of Ca and La in the B site. The principal statistical
parameters of the structure refinement are listed in Table 2. Atomic
coordinates and site occupancies of structure refinements are given in Table 3. Anisotropic displacement parameters and relevant bond distances are
reported in Tables S1 and S2 in the Supplement. The crystallographic information file is also available
in the Supplement.
Crystal structure of burbankite (a) projected parallel to [001] the zigzagged AO8 (yellow) and (b) parallel to (001) where we can observe the 3-fold ring made of BO10 (blue). The
representations of the structure are realized using the program VESTA (Momma
and Izumi, 2011).
Details pertaining to the data collections and structure refinements in ambient conditions of the burbankite studied in this work.
Atomic coordinates, site occupancies and equivalent displacement parameters (Å2) in ambient conditions of the burbankite studied in this work.
SitexyzSite occupanciesUeqA(Na, Ca)0.52317(10)0.04634(19)0.379(6)0.909(14) Na, 0.091(14) Ca0.0171(7)B(Ca, La)0.84168(3)0.68335(5)0.691(6)0.768(4) Ca, 0.232(4) La0.01263(16)C(1)0.6666670.333330.712(6)10.0142(19)C(2)000.358(6)10.0150(17)C(3)0.3946(5)0.1973(2)0.155(6)10.0128(13)O(1)0.8092(4)0.40462(18)0.711(6)10.0238(11)O(2)0.77651(18)0.5530(4)0.342(6)10.0176(11)O(3)0.6322(3)0.7093(3)0.558(6)10.0188(9)O(4)0.1408(4)0.07042(18)0.354(6)10.0302(13)
The evolution of the unit-cell parameters of burbankite at different
pressures (P) is reported in Table 4. The volume decreases smoothly with
increasing pressure, as shown in Fig. 3, up to the maximum hydrostatic
pressure reached in this study of ca. 7 GPa. No phase transition or change
in the deformation mechanisms occurs within the P range investigated. The
P–V data were fitted using a second-order Birch–Murnaghan equation of state
(BM2-EoS; Birch, 1947), since the Eulerian finite strain (fe) vs. normalized
stress (Fe) plot (Fe–fe plot, Fig. S1 in the Supplement) of the data can be fitted by a horizontal line (Angel, 2000). The BM2-EoS coefficients were refined
simultaneously; data were weighted by their uncertainties in P and V, using
the program EoSFit7c (Angel et al., 2014), giving V0= 593.22(3) Å3, KT0= 69.8(4) GPa and K′=4 fixed (χw2= 2.04, and ΔPmax=-0.14 GPa). The axial
compressibility is reported in Fig. S2, and analysis of elastic behaviour
indicates large anisotropy, with the a axis less compressible with respect to the c axis.
Lattice parameters of burbankite at different pressures, collected using methanol : ethanol (4:1) as the P-transmitting medium (P uncertainty ± 0.05 GPa).
P (GPa)a (Å)c (Å)V (Å3)0.00110.4301(2)6.2966(1)593.22(2)0.1610.4273(3)6.2863(7)591.93(7)0.5410.4083(2)6.2824(5)589.41(5)1.2410.3727(2)6.2561(6)582.93(6)2.0910.3417(2)6.2337(7)577.38(6)3.0210.3022(2)6.1990(8)569.78(7)3.7010.2881(4)6.165(1)565.1(2)5.0310.2350(2)6.139(1)556.9(1)6.0610.2020(2)6.109(1)550.6(1)6.6510.1737(2)6.095(1)546.3(1)7.0710.1772(2)6.079(1)545.3(1)
Evolution of the unit-cell volume with pressure of burbankite. The
solid line represents the BM2-EoS fit.
The temperature (T)–volume (V) data collected during the experiment at
ambient pressure are reported in Fig. 4 and Table 5. As can be observed
from Fig. 4, V increases continuously without evidence of phase transition up
to the maximum T reached in this study. The V–T data were fitted using EosFit7c
(Angel et al., 2014) using a Berman-type equation of state (EoS) (Berman, 1988). The thermal
expansion coefficients obtained are α0= 6.4(1) ×10-5 K-1, α1= 3.3(5) ×10-8 K-2 and
V0= 591.86(4) (χw2= 1). The axial thermal expansion is
reported in Fig. S3, and analysis of thermal behaviour of the axes with T indicates large anisotropy, with the a axis less expandable with respect to the c axis.
Evolution of the unit-cell volume with temperature of burbankite.
Data were fitted with a Berman-type EoS (solid line).
Crystal structure refinements at variable pressures indicate that the 8-fold
coordination polyhedra are more compressible than the 10-fold coordinated
ones showing an inverse behaviour of the two polyhedra with respect to their
volume (Hazen and Finger, 1982). In fact, the smaller polyhedron is slightly
more compressible with respect to the bigger one. If we try to fit the
volume variation of the two polyhedra with a BM2-EoS, we obtain KT0= 54(2) GPa for the 8-fold coordinated polyhedra (Table S3 and Fig. 5a),
while for the larger one, KT0= 72(3) GPa (Table S3 and Fig. 5b).
Single-crystal structure refinements at variable temperatures reveal a
similar thermal expansion for both polyhedra (Fig. 6a and b and Table S4).
Lattice parameters of burbankite at different temperatures, collected in a quartz vial (T uncertainty ± 1 K).
T (K)a (Å)c (Å)V (Å3)29810.4233(2)6.2905(1)591.86(2)33210.4296(2)6.2969(1)593.18(2)36210.4353(2)6.3022(1)594.34(2)39110.4404(2)6.3077(2)595.44(2)42110.4461(2)6.3136(2)596.65(2)45010.4521(2)6.3194(1)597.87(2)48010.4581(2)6.3252(1)599.11(2)50910.4637(2)6.3313(1)600.34(2)53910.4695(2)6.3371(1)601.55(2)56810.4753(2)6.3427(1)602.75(2)59810.4839(3)6.3478(2)604.23(3)62810.4891(2)6.3539(2)605.41(2)
Pressure-induced evolution of AO8(a) and BO10(b) polyhedra in the crystal structure of burbankite. The solid black line
represents a BM2-EoS fit to the data.
Temperature-induced evolution of AO8(a) and BO10(b) polyhedra in the crystal structure of burbankite. The fitted thermal
expansion using a one-term Holland-and-Powell curve results in 14.5(7) ×10-5 K and 14.5(6) ×10-5 K coefficients, respectively, in the two
sites.
Discussion and conclusions
The recent discoveries of a new class of Ca-rich alkaline–alkaline earth
carbonates, also including Ca3(Na,K)2(CO3)4 stable above
3 GPa as primary liquidus phases in the system
Na2CO3–K2CO3–CaCO3 (Shatzky et al., 2016a, b;
Podbordnikov et al., 2019), reveal a rich and dynamic environment in
carbonatitic systems at high pressure. Closer to the CaCO3 endmember,
burbankite-type Na2Ca4(CO3)5 was also synthesized
(Rashchenko et al., 2017). According to experimental phase diagrams (Shatzky
et al., 2016a, b; Podbordnikov et al., 2019), these phases necessarily
participate in crystallization and fractionation processes. Our experiments
indicate that La burbankite is still stable at 5 GPa, and these candidate
minerals are indeed possibly able to fractionate LREE3+ and may form a
REE reservoir in the upper mantle.
The thermoelastic parameters determined in this study can provide us with a
first tool to determine the density of La burbankite in upper-mantle conditions. Indeed, the
density of burbankite is ca. 3.2 g cm-3 in P and T ranges of 5.5–6 GPa
and 1731–1273 K, respectively. Density has been calculated using the equation of state for
solids from Holland and Powell (2011). If we compare the density of
burbankite with the density of carbonatitic magmas in the same P and T
conditions, which are 2.058–3.1 g cm-3 (Jones et al., 2013), we can
conclude that the burbankite might fractionate from the magma and play a key
role as an upper-mantle reservoir of light REE3+. Furthermore, if we
take into account our synthesis P–T conditions and the stability field of a
burbankite-like structure phase (e.g. Podbordnikov et al., 2019), its
fractionation might be possible from carbonate-rich fluids metasomatizing
the cratonic lithosphere or cold slabs characterized by low geothermal
gradients. The possible fractionation of this phase demonstrates that La
does not always have an incompatible behaviour as is the case for silicate
systems.
It is worth noting that in this example the two independent A and B
polyhedra might not obey the inverse relationship, as has already been
observed for diopside (Hazen and Finger, 1982). In this study we report a
bulk modulus (KT0= 72 GPa) for the 10-fold coordination polyhedra
greater than the bulk modulus for the 8-fold coordination polyhedra
(KT0= 54 GPa) when we would expect the opposite. Burbankite could
therefore be another example where the inverse relationship is not
rigorously true, and this can be related to the presence of two different
types of cation polyhedra other than the rigid carbonate unit (Hazen et
al., 2000).
Data availability
All data derived from this research are presented in the enclosed tables,
figures and Supplement files.
The supplement related to this article is available online at: https://doi.org/10.5194/ejm-34-351-2022-supplement.
Author contributions
SM and MM conceived the study. SM, DS, PF, BJ, GB and MM performed the
experiments and preliminary data analysis and discussed the results. RB and VC
wrote the software for the beamline handling. SM and MM performed structure
determinations and wrote the paper. All the authors revised the manuscript.
Competing interests
The contact author has declared that neither they nor their co-authors have any competing interests.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Special issue statement
This article is part of the special issue “Probing the Earth: experiments and mineral physics at mantle depths”. It is not associated with a conference.
Acknowledgements
We acknowledge Andrea Risplendente for microprobe analysis. We acknowledge
Elettra Sincrotrone Trieste for providing access to its synchrotron
radiation facilities (beamlines Xpress and XRD1) and for financial support.
Sula Milani, Deborah Spartà, Juliette Maurice, Patrizia Fumagalli and Marco Merlini acknowledge the support of the Italian Ministry of
Education, University and Research (MIUR) through the project “Dipartimenti di Eccellenza
2018–2022”. Constructive reviews by Anna Pakhomova and the anonymous
reviewer, as well as manuscript handling by Stephan Klemme (associate
editor) and Elisabetta Rampone (chief editor), have been much appreciated.
Financial support
This research has been supported by the Italian Ministry of Education (MIUR)
through the project “Dipartimenti di Eccellenza 2018–2022” and by Elettra Sincrotrone Trieste.
Review statement
This paper was edited by Stephan Klemme and reviewed by Anna S. Pakhomova and one anonymous referee.
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