The theoretical vibrational properties of a series of Fe- and Al-bearing
lizardite models have been determined at the density functional theory
level. Each periodic model displays a single cationic impurity substituted
at an octahedral or tetrahedral site of a supercell of lizardite
(Mg3Si2O5(OH)4) containing 162 atoms. The isovalent
Fe2+ for Mg2+ substitution has been considered, as well as the
heterovalent substitution of Fe3+ or Al3+ for Mg2+ or
Si4+. Comparison of the theoretical absorption spectra with previously
reported experimental spectra of natural and laboratory-grown lizardite
samples allows us to propose an interpretation for most of the observed bands.
Although the identification of specific bands related to octahedral
Fe2+ in FTIR spectra is challenging, broad bands at 3584 and 3566 cm-1 reflect the occurrence of octahedral Al3+ and Fe3+,
respectively, in the natural samples. These broad bands likely overlap with
potential contribution related to tetrahedral Al3+. It is suggested
that the modification of the H-bonding pattern related to the incorporation
of trivalent ions at tetrahedral sites has an overall broadening effect on
the interlayer-OH stretching bands of lizardite.
Introduction
The stretching vibrations of OH groups are widely used to probe the local
order of hydrous phyllosilicates. Applications range from the determination
of the stacking pattern in clay minerals (e.g., Brindley et al., 1986;
Prost et al., 1989; Fialips et al., 2001) to the analysis of cationic ordering in
solid solutions (e.g. Petit et al., 2004; Baron and Petit, 2016) and to the
study of structural phase transitions as a function of pressure (e.g.,
Johnston et al., 2002; Welch et al., 2012). The small mass of hydrogen leads
to frequencies significantly higher than those of other vibrational modes,
whereas their relative variations as a function of macroscopic or
microscopic perturbations usually do not exceed a few percents. These
properties facilitate the identification of the corresponding vibrational
spectroscopic signals, most often in the 3000–3700 cm-1 range (e.g.,
Farmer, 1974; Szalay et al., 2002). In addition, the high frequency and weak
dispersion of these “hard modes” result in a localized character (e.g.,
Salje, 1992), which supports the interpretation of observed spectroscopic
changes in terms of modifications of the local atomic-scale environment of
OH groups.
The stretching frequencies of OH groups are dominantly controlled by the
strength of H bonding and by the nature of the neighboring cations (e.g.,
Farmer, 1974; Hermansson, 1991; Libowitsky, 1999). In trioctahedral
phyllosilicates, specific bands involving the structural OH groups located
between the octahedral and tetrahedral sheets of the layers (inner-OH
groups) have been unambiguously related to isovalent cationic substitutions
in the coordination sphere of the oxygen atom (Petit et al., 2004; Blanchard
et al., 2018). The effect of cationic substitutions on interlayer-OH groups,
located on the outer side of the octahedral sheets of 1:1 phyllosilicates
and pointing in their interlayer spacing, is less clearly established. In
this case, the OH stretching modes can involve the coupled motion of the
different interlayer-OH groups occurring in the mineral unit cell, leading
to a 10 to 20 cm-1 splitting of interlayer-OH modes. The local
perturbation of OH vibrations by neighboring cationic substitutions
(typically 14 cm-1 for a Fe2+ or Ni2+ for Mg2+
substitution at octahedral sites; Petit et al., 2004) is thus of similar
magnitude than the effect of the coupling between the interlayer-OH groups.
Furthermore, the potential occurrence of multivalent elements, such as iron,
the simultaneous substitution at octahedral and tetrahedral sites by
trivalent cations (e.g., Serna et al., 1979), and variations of the
cationic ordering and potential intermediate di-tri-octahedral character of
extended solid solutions (Bailey, 1988) may affect the interpretation of the
vibrational spectra of chemically complex synthetic or natural samples. This
is particularly true for the serpentine-group minerals, which can display
significant variations of their chemical composition, with a frequent
departing from the ideal Mg3Si2O5(OH)4 formula related
to the occurrence of iron and aluminum (e.g., Viti and Mellini, 1997). These
elements can play an important role in serpentine-group minerals because the
interplay of chemical composition, electrostatic interactions and structural
strain has been suggested as a potential control of the layer curvature,
driving the growth of the planar lizardite vs. tubular chrysotile variety
(e.g., Mellini, 1982; Wicks and O'Hanley, 1988; Viti and Mellini, 1997). The
presence of aluminum has also been shown to significantly affect the
chemical reactivity of serpentine minerals (Lacinska et al., 2016).
In the present study, we theoretically investigate the effect of Fe and Al
substitutions at low concentration on the vibrational and infrared
absorption properties of lizardite, the more symmetric planar member of
serpentine-group minerals. The theoretical calculation of the IR spectrum of
Fe2+-, Fe3+- and Al3+-bearing lizardite supports the
assignment of some features of the experimental spectra of serpentine-group
minerals to site-specific substitutions and sheds light on the inhomogeneous
broadening mechanisms of the other vibrational bands related to cationic
substitutions. Although the focus is on the OH stretching modes, the effect
of substitutions on the lower-frequency region of the spectra is also
discussed.
Methods
The theoretical modeling framework is the same as that previously used for
investigating the structural and vibrational properties of other
phyllosilicates and hydrous minerals (e.g., Balan et al., 2001, 2002).
Relevant properties were obtained using density functional theory (DFT) with
the generalized gradient approximation (GGA) for the exchange-correlation
functional (xc-functional) as proposed by Perdew, Burke, and Ernzerhof (PBE)
(Perdew et al., 1996) and a plane-wave/pseudo-potential scheme, as
implemented in the PWscf and PHonon codes from the Quantum ESPRESSO package
(Giannozzi et al., 2009; http://www.quantum-espresso.org, last access: 23 October 2021). The ionic cores
were described by optimized norm-conserving pseudo-potentials from the
SG15-ONCV library (Hamman, 2013; Schlipf and Gigy, 2015). The wave functions
and the charge density were expanded in plane waves with 80 and 480 Ry
cutoffs, respectively, as in previous studies (e.g., Balan et al., 2021b).
Structural properties of Fe- and Al-bearing lizardite were determined using
3×3×1 lizardite supercells (162 atoms) containing one cationic impurity
substituting an octahedrally coordinated Mg2+ cation or a tetrahedrally
coordinated Si4+ cation. The chemical composition of the corresponding
system is (Mg1-xXx)3Si2O5(OH)4 or
Mg3(Si1-xXx)2O5(OH)4, where X is the impurity
and x=0.037 or 0.055 for a substitution at the octahedral (M) or
tetrahedral (T) site, respectively. These systems differ from those
investigated by Scholtzová and Smrčok (2005), who considered the
structural effect of coupled substitutions by trivalent cations at both M
and T sites, which would make it difficult to disentangle each individual
effect on vibrational properties. Given the supercell size, the Brillouin
zone sampling for electronic integration was restricted to a 1×1×2 k-point
grid. The unit-cell parameters of pure lizardite-1T were optimized at zero
pressure and then used without further relaxation to produce the Fe- and
Al-bearing supercells. Reduced atomic coordinates were relaxed until the
residual forces on atoms were less than 10-4 Ry/a.u. For the models
displaying heterovalent substitutions, a compensating homogeneous
electrostatic background was spread over the supercell to ensure the
macroscopic neutrality of the periodic system. This electrostatic correction
avoids the divergence of the system total energy but is not expected to
significantly affect the microscopic vibrational properties of ionic systems
(e.g., Leslie and Gillan, 1985). Spin-polarized calculations were performed
on Fe2+- and Fe3+-bearing systems, imposing the high-spin state
of the isolated paramagnetic ion to the supercell.
Harmonic vibrational (displacements and frequencies of the 486 normal
vibrational modes at the Brillouin zone center) and dielectric (Born
effective charge tensors and electronic dielectric tensor) properties were
calculated from the second-order derivatives of the total energy with
respect to atomic displacements and external electric field using the linear
response theory (Baroni et al., 2001). The theoretical powder infrared
absorption spectra were obtained from the low-frequency dielectric tensor
using the approach developed by Balan et al. (2001, 2008). This approach
computes the orientational average of the electromagnetic power dissipated
in an ellipsoidal particle inserted in an infinite isotropic matrix, which
is characterized by a real dielectric constant, assuming that the isolated
particle has a size significantly smaller than the IR wavelength. The
absorption is equivalent to that of a Maxwell Garnet effective medium in the
high-dilution limit (Kendrick and Burnett, 2016). In the calculation of the
low-frequency dielectric tensor (Balan et al., 2001), a damping coefficient
of 2 cm-1 was arbitrarily used to account for the homogeneous width
of absorption bands.
ResultsTheoretical properties of pure lizardite
The unit-cell parameters of lizardite-1T (S.G. P31m) (a=b= 5.37 Å, c= 7.36 Å) compare well with previous values obtained at the same
theoretical level (e.g., Prencipe et al., 2009; Hossain et al., 2001;
Adebayo et al., 2011; Tunega et al., 2012; Ghaderi et al., 2015). As usually
observed in DFT modeling performed at the GGA level, they are overestimated
with respect to their experimental counterparts (e.g., a=b= 5.3267 Å, c= 7.2539 Å; Gregorkiewitz et al., 1996). Each layer of the
structure contains a pseudo-hexagonal silica sheet of corner-shared
SiO4 units linked to a trioctahedral sheet of edge-sharing
MgO2(OH)4 octahedra. Selected theoretical lengths of cation oxygen
bonds (Table 1) are in good agreement with the experimental findings and
consistent with previous theoretical calculations (Balan et al., 2002;
Prencipe et al., 2009). The Mg–O(Si) distance is the longest Mg–O distance,
and the apical Si–O bonds are shorter than the equatorial ones. The
interlayer-OH bonds is longer than the inner-OH bond by 0.004 Å. The
theoretical magnitude of the ditrigonal distortion (α=-4.1∘) is slightly overestimated with respect to the
experimental value of -2.6∘ (Gregorkiewitz et al., 1996).
Selected theoretical bond lengths (Å) in pure and Fe- and
Al-bearing lizardite. The interlayer and inner OH correspond to OH3 and OH4,
respectively.
Vibrational properties of lizardite (Table 2) are consistent with those
previously determined (Balan et al., 2002; Prencipe et al., 2009). Note
however that the theoretical frequencies slightly differ from those of Balan
et al. (2002) due to the full relaxation of the unit cell in the present
study and use of different pseudo-potentials. The inner-OH stretching mode
leads to a weak absorption band at 3783 cm-1 (Fig. 1). The transverse
optical (TO) frequency of the in-phase stretching mode (A1 symmetry) of
interlayer-OH groups is calculated at 3721 cm-1 (Table 2) and leads to
an absorption band at 3740 cm-1 in the spectrum computed for a (001)
platy particle shape (Fig. 1). For a mode polarization along the [001]
direction, this higher frequency coincides with the longitudinal optical
(LO) frequency of the mode (Balan et al., 2005), indicating a LO–TO
splitting of 19 cm-1. This value compares well with those previously
determined (21 cm-1, Balan et al., 2002; 14 cm-1, Prencipe et al.,
2009). The degenerate out-of-phase stretching mode (E symmetry) of
interlayer-OH groups is calculated at 3704 cm-1 (Table 1).
Theoretical absorbance spectra in the OH stretching region of the
Fe- and Al-bearing lizardite models. The reference spectrum corresponds to
pure lizardite. The spectra have been computed for a platy particle inserted
in a KBr medium (εKBr=2.25). Due to electrostatic
effects, the position of absorption bands can differ from the TO mode
frequencies reported in Table 2 (see text). Inset: enlarged view of the main
band related to the stretching of interlayer-OH groups in lizardite. The
presence of cationic impurities leads to a moderate shift and broadening of
this band.
Theoretical transverse optical (TO) OH stretching frequencies
(cm-1) in pure and Al- and Fe-bearing lizardite. The frequencies
reported for Mg3OH in the substituted models correspond to the main
band associated with an in-phase motion of the OH groups and to IR-active
modes displaying out-of-phase displacement patterns. Depending on the
system, the intensity of the latter can be spread over several modes in the
specified frequency range.
At lower frequency (Fig. 2), the stretching modes involving apical or
equatorial Si–O bonds correspond to absorption bands at 1062 and 880 cm-1, respectively, with the former being calculated at the LO frequency
because of the platy morphology of the particle (Balan et al., 2002). The
bands at 643, 607 and 590 cm-1 correspond to the libration of
interlayer-OH groups, the libration of inner-OH group and the hindered
translation in the [001] direction of interlayer-OH groups, respectively.
The bands at 422 and 401 cm-1 correspond to more complex displacement
patterns involving the interlayer-OH groups and contributions of Mg and Si
cations. Finally, the band at 274 cm-1 involves a collective atomic
motion corresponding to an internal shearing of the layers parallel to the
(001) plane.
Theoretical absorbance spectra in the mid-infrared region of the
Fe- and Al-bearing lizardite models. The reference spectrum corresponds to
pure lizardite. The spectra have been computed as in Fig. 1. Note the very
small modifications related to the occurrence of Fe2+ ions in
the octahedral site.
Theoretical properties of Fe2+-bearing lizardite ((Fe2+)M
model)
Cationic substitution at the octahedral site reduces the 3m symmetry of the
system to a mirror symmetry. The average Fe2+ oxygen bond length
increases from 2.09 to 2.13 Å due to the slightly larger ionic
radius of Fe2+ (0.78 Å) compared to that of Mg2+ (0.72 Å)
(Shannon, 1976). The length of inner-OH (OH4) and interlayer-OH (OH3) groups
increases by 0.001 to 0.002 Å with respect to those in pure lizardite
(Table 1). The three interlayer-OH groups linked to the substituting cation
are not equivalent. The interlayer-OH group located on the mirror plane
(noted OHm) can be distinguished from the two other OH groups (noted
OHe), which are symmetrically equivalent by mirror operation. In the
Fe2+-bearing system, the OHm is 0.001 Å shorter than the two
OHe groups.
These local structural changes affect the vibrational properties. The
presence of a Fe2+ cation in the coordination sphere of the inner-OH
group shifts its vibrational frequency from 3783 to 3761 cm-1 (Table 2). The Born effective charge tensor of hydrogen is also affected, and the IR
absorption coefficient of the corresponding OH stretching band increases by
33 % with respect to that in pure lizardite. The stretching modes related
to the interlayer-OH groups are also shifted at lower frequency with respect
to the interlayer stretching modes of pure lizardite (Table 2). Due to the
symmetry reduction, the mode at 3686 cm-1 is dominantly related to the
shorter OHm bond, whereas the mode at 3676 cm-1 involves the
out-of-phase stretching of the two other OHe groups. The intermediate
mode at 3678 cm-1 combines the in-phase stretching of the OHe
groups with an out-of-phase motion of the OHm group. Compared with the
splitting of interlayer-OH stretching modes of A1 and E symmetry in
pure lizardite (∼ 20 cm-1), the frequency splitting of
the OH modes associated with the substitutional defect is reduced to less than
10 cm-1.
The effect of the substituting Fe2+ cation on the stretching frequency
of the bands related to the other more distant OH groups, which are only
linked to Mg cations, does not exceed 1 cm-1 (Fig. 1). The presence of
Fe2+ has a weak effect on the other vibrational modes of the system
(Fig. 2), which is consistent with the limited structural distortion of the
octahedral sheet induced by the impurity, also previously reported by
Scholtzová and Smrčok (2005). A 3 cm-1 splitting of the band
related to the basal Si–O stretching mode leads to a shoulder at 877 cm-1 (not visible on Fig. 2). A more significant change affects the
collective mode at 274 cm-1, which is split and downshifted to 264 cm-1. This suggests that the opposite shift reported by Baron and Petit (2016) as a function of Ni concentration in the lizardite-népouite series is
predominantly due to the decrease in the system molar volume. This effect is
not accounted for by the present modeling approach, which aims at determining
the properties of defective systems in the high dilution limit. Weak
additional bands related to a modification of the OH libration modes are
observed in the 630–670 cm-1 range.
Theoretical properties of Fe3+- and Al3+-bearing lizarditeTrivalent cations at octahedral sites ((Fe3+)M and
(Al3+)M models)
The occurrence of trivalent Fe3+ and Al3+ cations at octahedral
sites also reduces the system symmetry from 3m to m. The shortening of
cation–oxygen bond lengths and contraction of the substituted site are
stronger for Al3+ than for Fe3+, as expected from their respective
ionic radii (Fe3+: 0.645 Å, Al3+: 0.535 Å), which are both
significantly smaller than that of the octahedrally coordinated Mg2+
ion (0.72 Å) (Shannon, 1976).
The presence of a trivalent cation affects the OH groups belonging to its
coordination sphere. The lengthening of the inner-OH (OH4) bonds by 0.003
and 0.002 Å (Table 1) downshifts their stretching frequency by 47 and 25 cm-1 for Fe3+ and Al3+ cations, respectively. A significant
lengthening of interlayer-OHm and OHe bonds is also observed
(0.007 Å for Fe3+, 0.004 Å for Al3+). For the
Fe3+-bearing system, the three interlayer-OH bonds have a similar
length despite the lowering of the site symmetry (Table 1). Their coupling
leads to three modes respectively computed at 3612 cm-1 for the
in-phase motion of the three groups and at 3606 and 3596 cm-1 for their
out-of-phase motion (Table 2). A different vibrational pattern is observed
for the Al3+-bearing system. In this case, the stronger distortion of
the octahedral site induces a more significant difference in the length of
the interlayer-OH bonds. The longer OHm bond leads to a mode at 3631 cm-1 (Table 2). The two other modes at 3682 and 3679 cm-1
correspond to the in-phase and out-of-phase motion of the OHe groups,
respectively (Table 2).
Compared to the Fe2+-bearing system that mostly show local perturbation
of vibrational properties, the presence of octahedrally coordinated
trivalent cations has a more significant effect on the vibrational spectrum
of lizardite because the perturbation extends toward more distant atoms. The
main interlayer-OH stretching band is downshifted by 4 to 6 cm-1,
whereas several resonances involving out-of-phase displacement patterns
appear in the 3691–3717 cm-1 range (Fig. 1). The apical Si–O
stretching band is downshifted by ∼ 9 cm-1, whereas the
equatorial stretching modes are split and the main band is upshifted by
∼ 5 cm-1. In the Al3+-bearing system, the splitting
of the equatorial Si–O modes is stronger than in the Fe3+-bearing
system, reaching 7 cm-1 (Fig. 2). For both systems, the OH libration
and hindered translation modes are significantly affected. The splitting of
vibrational modes to a significantly higher number of IR active modes tends
to spread the IR absorption over a range of frequencies (Fig. 2), inducing a
complex broadening of the signals in the 560–660 cm-1 range.
Trivalent cations at tetrahedral sites ((Fe3+)T and
(Al3+)T models)
The substitution of larger Al3+ and Fe3+ for Si4+ cations
preserves the 3-fold symmetry of the tetrahedral site. It has a significant
effect on the cation–oxygen bond lengths, which increase from 1.61 to
1.73 and 1.81 Å for the apical T–O bond and from 1.67 to 1.78 and
1.90 Å for the basal T–O bond for Al3+ and Fe3+,
respectively (Table 1). Due to the relative stiffness of the T–O bonds, the
site expansion leads to a rotation of the tetrahedron with a di-trigonal
angle locally increasing to ∼-10∘ and to
∼-16∘ for the Al3+- and Fe3+-bearing
system, respectively (Fig. 3). This increasingly negative value indicates a
displacement of the basal oxygen atoms away from the octahedra of the same
TO layer (Mellini, 1982). This displacement shortens the H bonds between the
basal oxygen atoms of the tetrahedron and the three interlayer-OH groups
belonging to the adjacent layer. Similar influence of tetrahedral
substitutions on the H-bonding pattern of interlayer-OH groups was
previously pointed out by Scholtzová and Smrčok (2005). The length
of the three OH bonds pointing to the basal oxygens of the substituted
tetrahedron increases to 0.973 and 0.976 Å for the Al3+- and
Fe3+-bearing systems, respectively (Table 1). Due to the 3-fold
symmetry, the vibrational pattern involving these three interlayer-OH groups
corresponds to an in-phase stretching mode at 3536 and 3602 cm-1 (Table 2) for the Fe3+- and Al3+-bearing system, respectively, and two
degenerate out-of-phase stretching modes leading to weaker bands at a
frequency ∼ 10 cm-1 lower. The in-phase stretching mode
carries the intensity at 3541 and 3605 cm-1 in the corresponding
theoretical absorption spectra (Fig. 1). In comparison, the neighboring
inner-OH groups are less affected, with a moderate lengthening inducing a
decrease in the OH stretching frequency of 14 and 9 cm-1 for the
Fe3+- and Al3+-bearing systems, respectively.
View of the structure of the (Fe3+)T model along the
[001] direction. Blue triangle: basal face of silicate tetrahedra; light
blue triangle: basal face of the substituted site; red: oxygen atoms; light
pink: H atoms; white and brown: faces of the MgO6 octahedra. The
presence of the substituting cation induces a rotation of the tetrahedra
moving the basal oxygens away from the octahedral sites, which corresponds
to a local increase in the absolute value of the α angle. Structure
drawing made with the VESTA software (Momma and Izumi, 2011).
As could be anticipated from the distortion of the tetrahedral sheet, the
presence of trivalent cations substituted for Si4+ cations
significantly affects the Si–O stretching modes involving the basal oxygen
atoms and the related absorption bands (Fig. 2). The modes below 400 cm-1 are also strongly affected. In comparison, the OH libration and
hindered translation are less modified.
Discussion: interpretation of experimental lizardite spectra
Theoretical spectra can be compared with previously reported experimental
spectra of lizardite (Fig. 4), corresponding to a pure laboratory-grown
sample (Mg_100; x=0) (Baron and Petit, 2016); a reference
lizardite sample from Monte Fico, Elba (Viti and Mellini, 1997; Fuchs et
al., 1998; Balan et al., 2002); and a lizardite sample from New Caledonia
(Lz1) previously investigated by Fritsch et al. (2016, 2021). As attested by
X-ray diffraction patterns, the crystalline quality is better in the two
natural samples than in the synthetic one. The detailed chemical formula
proposed for the Monte Fico lizardite is
(Mg2.74Fe0.102+Fe0.053+Al0.11)Σ=3.00(Si1.94Al0.05Fe0.013+)Σ=2.00O5.05(OH)3.95 (Fuchs et al., 1998). Note that this
lizardite has been shown to correspond to skeletal crystals with a minor
proportion (∼ 10 %) of chrysotile and polygonal serpentine
(Capitani et al., 2021). The chemical composition of the Lz1 lizardite
corresponds to the formula
(Mg2.76Fe0.18Ni0.03Cr0.02)Σ=2.99(Si1.99Al0.01)O5(OH)4. Compared to the Monte Fico
lizardite, the Lz1 from New Caledonia is thus richer in iron and almost
Al-free. It is noteworthy that a significant fraction of iron in this sample
occurs under the trivalent state, as attested by X-ray absorption
spectroscopy investigations (Fe2+/Fe3+= 0.15; Farid Juillot, personal communication, 2021). Assuming that all iron ions are located in octahedral sites, this
ratio leads to the formula
(Mg2.76Fe0.022+Fe0.163+Ni0.03Cr0.02)Σ=2.99(Si1.99Al0.01)O5(OH)4. Accordingly, it is
expected that the Lz1 lizardite spectrum displays specific features
dominantly related to Fe3+, whereas the Monte Fico lizardite spectrum
should reflect the dominant occurrence of Al3+ and octahedral
Fe2+.
Experimental FTIR spectra of lizardite samples: New Caledonian Lz1
sample from Fritsch et al. (2021), Monte Fico sample from Balan et al. (2002) and laboratory-grown Mg_100 sample from Baron and Petit (2016). The arrow points to the shoulder at ∼ 3664 cm-1
in the spectrum of the Monte Fico sample.
Vibrational properties of lizardite in the OH stretching range
The experimental FTIR spectra of lizardite display the signals previously
interpreted in the light of the theoretical modeling of pure lizardite
(Balan et al., 2002; Prencipe et al., 2009). They are ascribed to the inner-OH stretching at 3703 cm-1 and to the in-phase (A1 symmetry) and
out-of-phase (E symmetry) stretching of interlayer-OH groups at 3684 and 3645 cm-1, respectively (Fig. 4). Note that the frequency
of the stronger in-phase stretching band of the interlayer-OH groups can
display some variability depending on the sample microstructure and
experimental geometry. These variations are related to long-range
macroscopic electrostatic interactions affecting the vibrational spectra of
polar crystals and are commonly observed in the powder spectra of hydrous
phyllosilicates (Farmer, 1998, 2000; Balan et al., 2001, 2002, 2005) and
layered hydroxides (Balan et al., 2008). In the case of powder spectra
recorded on samples with arbitrary particle shapes, the intense band can be
observed at a frequency intermediate between the corresponding TO and LO
mode frequencies, with a width similar to the LO–TO splitting. For more
specific shapes, such as blocky or cubic particles, the spectra can display
asymmetric bands with components close to the LO and TO frequencies that can
manifest as shoulders or inflection points (Fuchs, 1975). In the case of
lizardite, the main band observed in the infrared spectrum of a thin film
has been decomposed in two components at 3669 and 3688 cm-1, accounting
for its asymmetry (Trittschack et al., 2012). Similarly, a component is
observed at ∼ 3664 cm-1 as a shoulder on the low-frequency
side of the main band at 3684 cm-1 in the FTIR powder spectrum of the
Monte Fico lizardite (Fig. 4). Manifestations of long-range electrostatic
effects are also observed in the Raman spectra recorded on oriented
single crystals (Farmer, 2000). As discussed by Farmer (2000) for the Raman
spectrum of dickite, when the spectrum is recorded on a (001) section in a
backscattering geometry, the exchanged wave vector is parallel to the [001]
polarization of the in-phase stretching mode of interlayer-OH groups, which
enhances the signal at the higher LO frequency. In the case of lizardite,
Compagnioni et al. (2021) reports a shift of the main band from 3688.5 cm-1 in the micro-Raman spectrum recorded on an isotropic lizardite
section parallel to the (001) plane to ∼ 3678 cm-1 for
measurements made on the perpendicular section. In this latter case, the
main band appears asymmetric with a component at ∼ 3668–3670 cm-1 (average 3669 cm-1). Accordingly, the LO frequency of the in-phase stretching of interlayer-OH groups should be close to
∼ 3688 cm-1, whereas the component systematically
observed at ∼ 3669 cm-1 would correspond to its TO
counterpart. The related splitting is consistent with the theoretical LO–TO
splitting, which is predicted to be in the 14–21 cm-1 range.
These interpretations enable a comparison of the experimental frequencies
with their theoretical counterparts (Fig. 5). The comparison reveals an
overestimation of the theoretical stretching frequencies of interlayer-OH
groups amounting to 50 cm-1, which is consistent with the observations
made on antigorite (∼ 57 cm-1, Balan et al., 2021b). It
is also in the range expected from previous investigations on hydrous
defects in oxides and silicates (Balan et al., 2020; Balan, 2020). The
difference between theoretical and experimental frequencies results from the
partial cancellation of two types of errors, one being due to the use of an
approximate exchange-correlation functional, which tends to underestimate
the vibrational frequencies, and the other to the use of the harmonic
approximation, which artificially increases the OH stretching frequencies
with respect to their anharmonic values (Balan et al., 2007). The
theoretical correlation between bond length and OH frequencies (Tables 1
and 2) for the series of investigated models (Fig. 6) is also consistent
with those previously determined at the same theoretical level on a series
of hydroxylated defects in diopside and corundum (Balan et al., 2020; Balan,
2020). This relation between the vibrational frequency and a geometrical
parameter rules out a contribution of the mass of the surrounding cations to
the stretching dynamic of the OH groups, which is decoupled from the motion
of other atoms.
Relation between the theoretical frequencies (Fig. 1) and
experimentally observed vibrational bands. Open squares: FTIR spectrum of
Monte Fico lizardite (Balan et al., 2002); full squares: Raman frequencies of
Monte Fico lizardite (Compagnoni et al., 2021). Black circles: signals
ascribed to cationic impurities in the vibrational spectra of the Monte Fico
and Lz1 lizardite. The average value of theoretical frequencies related to
(Fe2+)M (3683 cm-1) almost coincides with the
higher-frequency contributions related to Al3+ cations (3682 cm-1). The theoretical model overestimates the experimental stretching frequencies of interlayer-OH groups by ∼ 50 cm-1.
Relation between the theoretical stretching frequency (Table 2)
and length (Table 1) of OH bonds in the lizardite models. For stretching
modes displaying coupled OH contributions, the average values have been
considered. Circles: inner-OH groups; diamonds: interlayer-OH groups.
Vibrational signatures associated with cationic impurities in lizardite
The 21 cm-1 downshift of the inner-OH stretching frequency induced by
the presence of Fe2+ in its coordination sphere is similar to that
determined at the same theoretical level for a Ni2+ for Mg2+
substitution in talc (23 cm-1, Blanchard et al., 2018). As in talc, the
theoretical value however slightly overestimates the experimental
observation. Based on the analysis of overtone bands in the near-infrared
range (Balan et al., 2021a; Fritsch et al., 2021), the shift between
Mg3–OH and Mg2Fe–OH in lizardite (16 cm-1) is close to that
determined in talc (14 cm-1, Petit et al., 2004). Accordingly, the
inner-OH stretching band corresponding to a Mg2Fe2+ environment
should occur at ∼ 3687 cm-1, overlapping with the strong
band related to the in-phase interlayer-OH stretching band of lizardite.
The signal of interlayer-OH groups in a Mg2Fe2+ environment is
theoretically determined at a frequency ∼ 20 cm-1 lower
than that of the out-of-phase stretching of interlayer OH in a Mg3
environment (Fig. 1). However, its contribution is not resolved in the
experimental FTIR spectra (Fig. 4) and should mostly contribute to the
broadening of the out-of-phase (E symmetry) stretching band identified at
3645 cm-1. Interestingly, the micro-Raman spectrum of lizardite
reported by Compagnoni et al. (2021) reveals the occurrence of two
components with different polarization properties at ∼ 3653 and ∼ 3642 cm-1. The relative intensity of the
component at ∼ 3653 cm-1 is enhanced in the spectrum
recorded on an isotropic section parallel to the (001) plane. Based on the
comparison with the theoretical frequencies, this component could correspond
to the out-of-phase stretching band of pure lizardite, whereas that
determined at ∼ 3642 cm-1 could be ascribed to the
stretching of interlayer-OH groups in a Mg2Fe2+ environment (Fig. 5). It is noteworthy that the splitting of the bands associated with the
substituted site is relatively weak (8 cm-1, Fig. 1), potentially
leading to the experimental observation of a single band at the average
frequency. However, a contribution of trivalent cations (mostly octahedrally
coordinated Al3+ in the Monte Fico sample) cannot be excluded as they
lead to theoretical absorption signals at similar frequencies (Fig. 1).
Assuming that a difference between experimental and theoretical frequencies
close to 50 cm-1 is also valid for the interlayer-OH groups in
Al3+- and Fe3+-bearing models, the bands respectively observed at
3584 and 3568 cm-1 in the Monte Fico and Lz1 samples (Fig. 4) are most
likely associated with the occurrence of trivalent cations. Consistently,
this type of signal is lacking in the laboratory-grown pure lizardite
sample. The shift of the band observed between the Monte Fico and Lz1
samples is also consistent with the respective predominance of octahedrally
coordinated Al3+ and Fe3+ in these samples (Fig. 5).
It is however challenging to discriminate the occurrence of octahedral or
tetrahedral Al3+. Despite a difference in the associated stretching
frequencies amounting to 27 cm-1, both could contribute to the broad
signal centered at 3584 cm-1 (half width at half maximum of
∼ 35 cm-1). The contribution related to tetrahedral
Fe3+ is expected at a frequency 75 cm-1 lower than that associated
with octahedral Fe3+, corresponding to an expected experimental
frequency of ∼ 3490 cm-1. The absence of such a signal
suggests that the concentration of tetrahedral Fe3+ is too low to be
detected in the samples. Additional features can also complicate the
identification of bands related to trivalent cations at tetrahedral sites.
The expected signal likely overlaps with the broad absorption by water
molecules adsorbed at the surface of the particles, which is centered at
3405 cm-1 in the pure and finely divided synthetic lizardite sample
(Fig. 4). Moreover, the frequency of the interlayer-OH stretching bands
appears to be controlled by the perturbation of the silicate sheet geometry,
which affects the H-bonding pattern. Recalling that the present theoretical
models correspond to a perfectly ordered cationic distribution because of
the periodic boundary conditions, they are expected to only provide a local
description of the changes induced by the presence of the impurity. The more
random cationic ordering occurring in experimental samples likely induces a
distribution of the tetrahedra rotation angles and of the related H-bond
lengths, causing a broadening of the OH stretching signal.
To this respect, the occurrence of tetrahedral trivalent cations should
efficiently contribute to the broadening of the whole vibrational properties
of lizardite, not only affecting the stretching modes of interlayer-OH
groups but also the other vibrational modes observed in the mid-IR range. In
comparison, low concentration of cations at octahedral sites should have a
more local effect and weaker broadening efficiency. It is however noteworthy
that higher substitution levels of divalent or trivalent cations can also
lead to a significant broadening due to the combined effect of the induced
structural strain and increasing variability of cationic configurations,
which include the electrostatically favored coupled substitution of
trivalent cations at both M and T sites (e.g. Yariv and Heller-Kallai, 1973;
Serna et al., 1979; Velde, 1980; Baron and Petit, 2016).
Data availability
PWscf and PHonon codes (Giannozzi et al., 2009) are available at
http://www.quantum-espresso.org/ (last access: 21 September 2021). The
pseudo-potentials are available at
http://www.quantum-simulation.org/potentials/sg15_oncv/ (last
access: 21 September 2021, Schlipf and Gigy, 2015). Specific requests should be addressed to Etienne Balan
(etienne.balan@sorbonne-universite.fr).
Author contributions
EB, GR and LP performed the calculations. All co-authors
contributed to the discussion of the results and preparation of the
manuscript.
Competing interests
Some authors are members of the editorial board of European Journal of Mineralogy. The peer-review process was guided by an independent editor, and the authors have also no other competing interests to declare.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
We thank Georgios Chryssikos and one anonymous reviewer for their
insightful comments. This research is an outcome of the projects
“Conditions of formation of Mg/Ni silicate ores from New Caledonia” (2010,
CNRS INSU CESSUR) and “Ni/Co mineralization factors of laterites derived
from ultramafic rocks of New-Caledonia” (2010–2014, CNRT Nickel and its
environment). Calculations have been performed using the computing resources
of IMPMC (Sorbonne Université-CNRS-MNHN) and the HPC resources of IDRIS
under the allocation 2020-A0080910820 attributed by GENCI (Grand Équipement
National de Calcul Intensif).
Review statement
This paper was edited by Tiziana Boffa Ballaran and reviewed by two anonymous referees.
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