SiO4 Groups Research Articles

High-pressure synchrotron x-ray diffraction and infrared microspectroscopy:applications to dense hydrous phases

Synchrotron x-ray diffraction (XRD) and infrared (IR) absorption spectra ofhydrous and ‘anhydrous’ forms of phase X were measured to 30 GPa atroom temperature. Three OH stretching modes were found in the hydrousphase, and surprisingly one sharp OH mode was observed in the previouslycharacterized anhydrous phase. All OH stretching modes soften and broaden withincreasing pressure and become very weak above ∼20 GPa. XRD indicatesthat the crystal structure remains stable up to 30 GPa. Combining IRabsorption and XRD results, the behaviour is attributed to pressure-induceddistortion of the Si2O7 groups and disorder of the hydrogen atoms. The bulkmoduli of the hydrous and ‘anhydrous’ phases are in the region of 74 GPa.

The Crystal Chemistry of Beryllium

Beryllium is not a very abundant element in the Earth, but, being an incompatible element in common rock-forming silicate minerals, it is susceptible to concentration via fractionation in geochemical processes. Moreover, its properties are such that Be does not tend to show extensive solid-solution with other elements, and hence usually forms minerals in which it is a discrete and essential constituent. Beryllium (atomic number 4) has the ground-state electronic structure [He]2 s 2 and is the first of the group IIA elements of the periodic table (Be, Mg, Ca, Sr, Ba, Ra). The first (899 kJ/mol) and second (1757 kJ/mol) ionization enthalpies are sufficiently high that the total energy required to produce the Be2+ ion is greater than the compensating energy of the resulting ionic solid, even when the latter involves extremely electronegative elements. Hence bond formation involves covalent (rather than ionic) mechanisms. However, Be has only two electrons to contribute to covalent bonding involving four orbitals ( s , p x, p y, p z), resulting in Be being a strong Lewis acid (i.e., a strong electron-pair acceptor) with a high affinity for oxygen. The cation radius of Be2+ is 0.27 A, (Shannon 1976) and Be is known only in tetrahedral coordination in minerals, although BeO3 groups are known in synthetic compounds (e.g., Ca12Be18O29, Y2BeO4 and SrBe3O4; Harris and Yakel 1966, 1967, 1969). The BeO4 and SiO4 groups have a marked tendency to polymerize in the solid state. Although [4]Be (r = 0.27 A) and [4]Si (r = 0.26 A) are very similar in size, solid solution between Be and Si is inhibited by the difference in formal charge of the two species (Be2+ vs. …

Crystal structure of phase X, a high pressure alkali-rich hydrous silicate and its anhydrous equivalent

Phase X, ascribed by Luth (1995) to a hydrous K-rich silicate formed from the breakdown of K-amphibole at high pressures, was synthesized at 1250–1300 °C and 10–16 GPa in four different compositions: Na1.78(Mg1.89Al0.13)Si2.02O7 (anhydrous sodic phase X), Na1.16K0.01(Mg1.89Al0.14) Si2.02O7H0.65 (sodic phase X), K1.85Mg2.06Si2.01O7 (anhydrous phase X), and K1.54Mg1.93Si1.89O7H1.04 (phase X). A general chemical formula for these phases can be expressed as A2− x M2Si2O7H x , with A = K and/or Na, M = Mg and/or Al, and x = 0–1. Structure determination from single-crystal X-ray diffraction data shows that anhydrous sodic phase X is trigonal with space group P 31 m , whereas the other three have an identical structure with space group P 63 cm . Both P 31 m and P 63 cm structures are characterized by MgO6 octahedral layers that are stacked along the c axis and inter-linked together by Si2O7 tetrahedral dimers and K or Na cations. Within the MgO6 layers, each MgO6 octahedron shares three edges with neighboring MgO6 octahedra to form brucite-like layers with one out of three octahedral sites vacant. Large K or Na cations are situated right below and above each occupied octahedron in the MgO6 layers, whereas the Si2O7 groups are located below and above each vacant octahedron in the layers. The two types of structures, however, differ in the relative orientation of MgO6 octahedral layers, the coordination of K or Na, and the configuration of SiO4 tetrahedral dimers. By comparison, the Na2Mg2Si2O7 phase synthesized by Gasparik and Litvin (1997) appears to have the stoichiometry identical to anhydrous sodic phase X. Hence, these two high-pressure phases are likely to possess the same structure, or at least are closely related to each other structurally.

Structure and elasticity of wadsleyite at high pressures

The athermal equilibrium structure, the equation of state, the elastic constants, and O atom charges were calculated for Mg2SiO4 wadsleyite over a range of pressures using a plane-wave pseudopotential method. The zero-pressure volume is 2% lower and the bulk modulus is 4.5% higher than experimentally observed. After correcting for zero point motion and the 300 K temperature difference between theory and experiment, using a Debye model, the calculated zero pressure volume is within 1% of experiment and the bulk modulus agrees within experimental error. The structure compresses anisotropically with linear moduli for the a, b, and c axes of 610 GPa, 599 GPa, and 454 GPa, respectively. The compression is largely taken up by the Mg octahedra M1, M2, and M3 which are much softer than the Si tetrahedra, with polyhedral bulk moduli of 161 GPa, 159 GPa, 157 GPa, and 331 GPa, respectively. The M1 and M3 octahedra were found to compress anisotropically which explains the greater compressibility of the c axis. The geometry of the Si2O7 group is characterized by a small Si-O-Si angle of 121.2°; compression of this group is largely accommodated by shortening of the Si-O bonds, while the inter-tetrahedral angle is almost pressure independent. We find that our results at ambient pressure are consistent with previously established systematics relating bulk modulus to volume, and Si-O-Si angle to Si-O bond length. However the variation of these quantities upon the application of pressure leads to trends that are distinct from the systematics. The calculated zero pressure elastic constants agree to within 10% with available Brillouin scattering data, with the exception of C12 which is 15% higher than experimentally observed. The calculated isotropically averaged bulk and shear modulus and their pressure derivatives are K0 = 182 GPa, K0' = 4.23, and G0 = 116 GPa, G0' = 1.10, respectively. The aggregate velocities and their pressure derivatives are VP = 9.75 km/s, VP' = 0.056 km/s/GPa, and VS = 5.72 km/s, VS = 0.012 km/s/GPa. We find that the elastic anisotropy of wadsleyite is intermediate between the two other Mg2SiO4 polymorphs, forsterite, and ringwoodite. The anisotropy is only weakly pressure dependent and decreases with increasing pressure. The azimuthal and polarization anisotropy for S waves 14.5% and 12.8% respectively, is almost pressure independent, while the azimuthal anisotropy for P waves decreases from 12.5% at ambient pressure to 10.5% in the upper part of the transition zone (14-17 GPa). Our calculated O atom charges suggest that Ol is the most likely hydroxyl site and remains so throughout the stability field of wadsleyite.

Characterization of 2M pyrophyllite associated with argillic alteration in steam-heated environment, Solo, Mabini, Philippines.

Localized deposit of Fe-rich pyrophyllite was encountered in the kaolinite-dominated alteration halo in Solo, Mabini, Philippines. The environment of formation of this dioctahedral mineral is related to low-temperature advanced argillic alteration found in shallow volcanic region. The pyrophyllite crystallizes as a two-layer monoclinic (2M) structure with the following unit cells, a=5.16A, b=8.95A, c=18.70A, β=99.87° while its dehydroxylate form yields a slightly different unit cell parameters with a=5.17A, b=9.02A, c=18.76A, β=99.80°. The expansion of b on dehydroxylation arises largely from the rearrangement of the Al ions in the octahedral layer and the insertions of oxygen ions in the Al planes. The obtained tetrahedral rotation (twisting of SiO4 groups) of 11°52' in pyrophyllite is closely comparable to the 9°40' angle of twist in the dehydroxylate. This strongly suggests that the tetrahedral networks of the original pyrophyllite are almost similarly twisted in the dehydroxylate phase. X-ray powder data demonstrate that the dehydroxylation process is accompanied by a significant enlargement of basal spacing with an observed expansion of about 1.99%. Thermal decomposition of pyrophyllite shows a single dehydration event, which takes place over a temperature range of 500-700°C with its peak at 604°C. The occurrence of this low-temperature dehydroxylation can be regarded as a consequence of significant replacement of Al by Fe in the octahedral layer and the presence of some impurities. Chemical data of pyrophyllite obtained from EDX analysis give an average structural formula of Ca0.01K0.10(Al1.81Fe3+0.16Ti0.01)(Si3.88Al0.12)O10(OH)2 with an average Si/Al3++Fe3+ ratio of 1.86. Analyses of representative samples indicate that the octahedral site is filled with 1.76 to 1.84 Al cations, which is relatively lower than the 1.90 to 1.95 Al cations exhibited by ordinary pyrophyllite. This significant deficiency of Al is believed to be compensated by considerably high Fe3+ occupancy in the octahedral site. The source of Fe can be correlated to the previous decomposition of minor amount of pyrite associated with argillic alteration. SEM observations show common disoriented stacks of pyrophyllite plates, but the most visible component of the samples are the loosely compacted stacks in which thin elongated blade-like particles are randomly dispersed in the form of relatively porous aggregates. The coexistence of pyrophyllite and kaolinite in this environment is a consequence of sluggish crystallization of quartz caused by local variation of silica activity, which explains the crystallization of pyrophyllite instead of stable kaolinite at low temperature (<300°C).

Analysis of Harmonic Displacement Factors

In my role as crystallographic editor for the American Mineralogist and The Canadian Mineralogist , I examine many papers that include crystal structure refinement data and discussions about this data. It is clear to me that understanding and working with the displacement parameters is a challenge to many researchers. The purpose of this chapter is to clearly define the meaning of these parameters and the mathematics needed to interpret them. Throughout this chapter, I will provide illustrative examples from the structure of quartz refined by Kihara (1990) at a variety of temperatures. Figure 1. An image of the displacement ellipsoids for a Si2O7 group in α-quartz, SiO2, at 298 K and 838 K (Kihara 1990). The ellipsoids, as drawn, enclose 99.0 % of the probability density. It is well established that an atom in a crystal vibrates about its equilibrium position. This vibration can be attributed to thermal and zero-point energy. For example, in Figure 1 the anisotropic displacement parameters are drawn for the Si and O atoms in quartz at 298 K and 838 K. The displacements are significant, with maximum amplitudes for O of 0.138 A, and 0.259 A, at 298 K and 838 K, respectively, in the direction perpendicular to the Si-Si vector. Therefore, in a crystal structure refinement, it is imperative not only to find the mean position of an atom, but also to describe the region in space where there is a high likelihood of finding it. This region of space can be mathematically defined with a probability distribution function (p.d.f.). If we assume harmonic restoring forces between the atoms, or in other words, that the forces between the atoms are quadratic and obey Hooke’s Law, then it can be shown that the p.d.f. can be represented by a Gaussian function (Willis …

An infrared spectroscopic study of lawsonite to 20 GPa

The infrared spectrum of CaAl2Si2O7 · H2O-lawsonite, has been characterized to pressures of 20 GPa at 300 K. Our results constrain the response to compression of the silicate tetrahedra, hydroxyl units, and water molecules in this material. The asymmetric and symmetric stretching and bending vibrations of the Si2O7 groups (at zero pressure frequencies between 600 and 1000 cm−1) increase in frequency with pressure at rates between 3.6 and 5.9 cm−1/GPa. All silicate modes appear to shift continuously with pressure to 20 GPa, although the lowest frequency stretching vibration becomes unresolvable above 18 GPa, and a splitting of the main bending vibration is observed near this pressure. The O-H stretches of the hydroxyl units exhibit a discontinuity in their mode shifts at ∼8–9 GPa, which we interpret to be produced by a pressure-induced change in hydrogen bonding. The stretching and bending vibrations of the water molecule are relatively unaffected by compression to 20 GPa, thus demonstrating that the structural cavities in which water molecules reside are relatively rigid. Significant changes in the amplitude of the O-H stretches of the hydroxyl and water units are observed at this pressure as well; nevertheless, our results demonstrate that the dominant structural units in lawsonite persist metastably at 300 K with only modest structural modifications well beyond the known stability field of this phase.

A study of Li2TiOSiO4 and Li2TiOGeO4 by X-ray powder and electron single-crystal diffraction, 17O MAS NMR andO K-edge and Ti L2,3-edge EELS.

X-ray powder and electron single-crystal diffraction of crystals of Li(2)TiOSiO(4) and Li(2)TiOGeO(4) showed them to be tetragonal, space group P4/nmm unit-cell parameters a = 6.4379(2), c = 4.40032(2) Å for Li(2)TiOSiO(4) and a = 6.6110(8), c = 4.4372(6) Å for Li(2)TiGeO(4). The compounds are isostructural with their sodium analogues but are considerably compressed along the c axis owing to the smaller size of lithium compared with sodium atoms. Square-pyramidal TiO(5) groups are joined in these compounds by tetrahedral SiO(4) and GeO(4) groups, respectively. (17)O nuclear magnetic resonance spectra of the two compounds, isotopically enriched with (17)O, showed peaks due to the apical titanyl, Ti-O, and basal, bridging, Ti-O-Si or Ti-O-Ge, oxygen atoms of the title compounds. By comparison with reference spectra, oxygen K edges and titanium L(2,3) edges of electron energy-loss spectra were tentatively assigned.

Lattice-dynamical evaluation of atomic displacement parameters and thermodynamic functions for phenakite Be 2 SiO 4

Using empirical potentials derived from fitting the vibrational frequencies of a group of silicates and oxides (not including the mineral under study), a Born-von Karman rigid ion lattice-dynamical model has been applied to the whole Brillouin zone in phenakite (Be2SiO4), a silicate which can be described as a three-dimensional network of tetrahedral SiO4 and BeO4 groups strictly linked together by sharing the oxygen atoms at their corners. The atomic anisotropic displacement parameters (ADPs) derived from accurate crystal-structure refinement at room temperature agree very well with the corresponding theoretical estimates, thereby confirming the validity of the model and the procedure followed. The experimental values of the bond distances corrected for thermal motion show that the observed invariance of the Si-O bond length with respect to temperature is only apparent. Furthermore, the disagreement with the experimental values of thermodynamic functions such as the specific heat C p and entropy at room temperature is 1.3% and 0.9%, respectively, and such a disagreement remains below 3% for a wide range of temperature (200 to 1000 K). As for many oxides and silicates so far examined, the zero-point contribution to the ADPs of the Si and O atoms amounts to about one half of the corresponding value at room temperature, and that of the Be atoms is even higher (60%).

Strontium Metasilicate, SrSiO3

Strontium silicon trioxide has been synthesized for the first time and its structure solved. It is isostructural with the germanium analogue SrGeO3 reported by Hilmer [Sov. Phys. Crystallogr. (1963). 7, 573–576], but the true symmetry is monoclinic, rather than hexagonal as reported earlier. The structure has alternate layers of ternary rings of SiO4 groups and close-packed Sr atoms stacked along [001]. Viewed as a polytype, this compound has a six-layer structure and the calcium analogue, α-CaSiO3 [Yamanaka & Mori (1981). Acta Cryst. B37, 1010–1017] has a four-layer structure.

Sodalite, Na4Si3Al3O12Cl: structure and ionic mobility at high temperatures by neutron diffraction

Crystal data: Na4Si3Al3O12Cl, cubic, space group P43n, Z = 2, F(000) = 233.06 fm, μ n = 0.06 cm−1, lattice parameter ao (Å) [T] (K) at eight temperatures: 8.882 (1) [295]; 8.902 (2) [500]; 8.912 (1) [600]; 8.923 (1) [700] 8.951 (2) [800]; 8.971(1) [900]; 8.988 (1) [1000]; 9.037 (1) [1200]. The crystal structure has been determined at six temperatures (295 ≤ T ≤ 1200 K) based on neutron diffraction data with (sin θ/λ) &lt; 0.80 Å−1. Besides conventional parameters, the least-squares refinement model included thermal tensor parameters up to fourth-order for sodium and chlorine (295 ≤ T ≤ 1200 K) and up to third-order for oxygen (T ≥ 700 K), together with the 1:1 coupled site occupancy factors of sodium and chlorine (T = 1200 K). The indices-of-fit, wR(F 2), are in the range 0.015–0.028 with observation-to-parameter ratios from 7.0 to 8.6. Bond lengths and angles in the aluminosilicate framework have average e.s.d.'s less than 0.002 Å and 0.08°. Between 295 and 1200 K, the observed Si—O (Al—O) bond lengths differ by −0.015 Å (−0.012 Å); corrections for librating rigid SiO4 (AlO4) groups change the difference to +0.004 Å (+0.006 Å), compared with the 295 K value of 1.620 Å (1.741 Å). The unique Si—O—Al angle increases from 138.24° (295 K) to 146.87° (1200 K), while the Si and Al valence angles are virtually unchanged. Between 295 and 1200 K the [Na4Cl] clusters expand with increases in the Na—Cl bond lengths of 0.200 Å, with simultaneous increases in Na—O bond lengths of 0.145 Å and decreases in the shortest Na...O contact distances of 0.126 Å. The thermal expansion of sodalite is attributed to the increasing amplitudes of coupled translational motion of the Na+ ions and the librational motion of the [Al/ SiO4] tetrahedra, leading to the untwisting of the aluminosilicate framework. Maps of the probability density functions for Na+ and Cl− indicate ionic diffusion paths along (111) directions. There is a finite probability of finding the Na+ ion within the plane of the next-nearest O atoms, suggesting that Na+ jumps from an occupied to an unoccupied site in the next-nearest cage through the six-membered ring of [Al/SiO4] tetrahedra.

Hochdrucksynthese und Strukturverfeinerung von Strontium-Danburit, SrB2Si2O8

Abstract Single crystals of strontium danburite, SrB2Si2O8, were found after high-pressure treatment of SrSiO3/SiO2/Al2O3 mixtures at 40 kbar and 1400°C, obviously formed by an unwanted side reaction with the boron oxide-containing boron nitride capsules used. The crystals are orthorhombic, space group Pnma, with a = 8.1891 (5), b = 7.9311 (6), c = 8.9284(5) Å and Z = 4. The structure was refined to R = 0.031, Rw = 0.028 for 590 independent, absorption-corrected reflections. SrB2Si2O8 is isostructural with danburite (CaB2Si2O8). The structure comprises a tetrahedral framework of ordered B2O7 and Si2O7 groups which contains Sr in irregular coordination.

Syntheses, Structural Analyses, and Unusual Magnetic Properties of Ba2CoSi2O7 and BaCo2Si2O7

The compounds Ba2CoSi2O7, 1, and BaCo2Si2O7, 2, were obtained by heating mixtures of BaCO3, CoCO3, and SiO2 in the proper stoichiometries to 1300 °C for 48 h. Both materials were characterized by single-crystal X-ray diffraction and magnetization measurements on powder and partially oriented single crystals. Each cobalt ion in 1 is surrounded by four oxygen atoms derived from the [Si2O7]6- groups in the form of a distorted tetrahedron. The cobalt ions are bridged by O−Si−O and O−Si−O−Si−O groups from the [Si2O7]6- ions in an extended two-dimensional arrangement. The barium ions are arranged in layers that run parallel to the Co[Si2O7] layers. In 2 each cobalt ion is also surrounded by an arrangement of four oxygen atoms derived from the Si2O7 groups in a distorted tetrahedral shape. These tetrahedra are linked into chains through the sharing of one of the oxygen atoms from the Si2O7 groups. The CoO4 chains are crosslinked via O−Si−O−Si−O bridges from the Si2O7 groups. The barium ions occupy channels that run parallel to the crystallographic c-axis. Curie−Weiss behaviors and temperature independent paramagnetisms (TIP) coexist in the ranges 40−300 K and 100−300 K for 1 and 2, respectively. For both compounds the values of the Curie constant and TIP are consistent with tetrahedral Co(II) ions having three unpaired electrons. While compound 1 remains paramagnetic down to 5 K, 2 undergoes a transition into a state having a field-induced weak ferromagnetic response at Tc = 21 K. The weak ferromagnetic properties below Tc can be explained by spin canting and spontaneous domain formation. The canting angle is estimated to be about 5° at 5 K. Interestingly, 2 could represent the first example among the tetrahedral Co(II) compounds with the transition temperature (Tc = 21 K) being higher than the zero-field splitting of the 4A2 electronic ground state. Crystal data: for 1, formula Ba2CoSi2O7, a = 8.450(1) Å, b = 10.729(1) Å, c = 8.474(1) Å, β = 111.365(8)°, monoclinic, space group = C2/c, Z = 4; for 2, formula BaCo2Si2O7, a = 7.2131(6) Å, b = 12.781(1) Å, c = 13.762(1) Å, β = 90.299(8)°, monoclinic, space group = C2/c, Z = 8.

Monoclinic members of the epidote group: effects of the AI ? Fe3+ ? Fe2+ substitution and of the entry of REE3+

Minerals of the clinozoisite-epidote series (with Fe3+ ranging from 0.24 to 1.14 a.p.fu.) and allanites (with REE up to 0.93 a.p.f.u.) were studied from a crystal chemical point of view. Structure refinements and microprobe analyses were performed on twentythree single crystals; the determined site populations were correlated with structuralgeometrical parameters. Important modifications affect the M3 octahedron, which is the preferential site for the entry of Fe3+ and Fe2+ substituting for Al. M1 can accomodate iron too (up to 0.18 a.p.f.u.) and, in allanites, small amounts of divalent cations. Minor geometrical variations occur in the A1 and A2 polyhedra, as well as in the M2 octahedron. Tetrahedra are quite constant in shape and dimensions; however, significant variations of their mutual orientation in the Si2O7 group are observed as a function of the total iron content (p value) and the amount of REE. These two chemical variables also affect the donor-acceptor distance. The relationships between unit cell parameters and chemical composition are underlined.

Raman study of fresnoite-type materials: Polarized single crystal, crystalline powders, and glasses

Raman spectra are reported for fresnoite-type [Ba2Ti(Si, Ge)2O8] materials. Single crystal and glasses of the endmember compositions, Ba2TiSi2O8 (BTS) and Ba2TiGe2O8 (BTG), along with crystalline powders along the pseudobinary join, Ba2TiSi−xGe2−xO8, are discussed. The polarized single-crystal Raman spectra of BTS show the vibrational bands of the Si2O7 and TiO5 molecular groups. The v (Si–O–Si) mode is found at 666 cm−1, and is relatively weak. The strongest bands in the BTS single crystal spectra are found at 858 cm−1 and 873 cm−1. These are attributed to the v (SiO3) modes and the stretching of a short Ti–O bond, with mixing between them evident. The BTG single crystal polarized Raman spectra show the vs(Ge–O–Ge) mode at 521 cm−1, and a strong band at 841 cm−1 assigned to the stretch of the short Ti–O bond. The angular dependence of the TO-LO splitting is cataloged for the A1(z) stretching modes. The vibrational characteristics of both BTS and BTG are dominated by the molecular groups, and do not show behavior indicative of a sheet structure composed of TiO5 and (Si, Ge)2O7 groups. Raman investigation of the crystalline powders points to the presence of Si2O7, Ge2O7, and SiGeO7 groups within the nonendmember compositions. The BTG glass spectra show correspondence with the BTG crystalline Raman spectra; the vs(Ge–O–Ge) mode occurs at 518 cm−1 in the glass. Fivefold coordinated titanium may be present in the BTG glass, as revealed by a very strong band at 824 cm−1 in the I spectrum. BTS glass spectra also show some similarity with the BTS crystalline spectrum; the strongest band is found at 836 cm−1 in the I glass spectrum. Through comparison with other titania-silicate glasses, we conclude that the BTS glass has a structure similar to the crystalline phase.

Studies of surface NiO species in NiO/SiO2 catalysts using temperature-programmed reduction and X-ray diffraction

Three different kinds of nickel-containing species have been found in silica-supported nickel oxide samples according to their different reducibilities during temperature-programmed reduction (TPR). Species A is similar to bulk phase nickel oxide, species B is attributed to SiO4 containing nickel oxide particles and species C is a stoichiometric or non-stoichiometric nickel silicate. Examination of the effects of silica-supported NiO particle size on the reducibility shows that the difference in the ease of reduction of NiO crystallites may be caused by the penetration of silica into the lattice of NiO; moreover, the presence of SiO4 groups can retard the sintering of NiO.

Vibrational spectroscopy of end-member silicate garnets

Infrared reflectance (IR) and Raman spectra were collected on small (ca. 500 micron) single crystals of 5 natural garnets with nearly end-member compositions: pyrope (98% Mg3Al2Si3O12), almandine (83% Fe3Al2Si3O12), spessartine (98% Mn3Al2Si3O12), grossular (97% Ca3Al2Si3O12), and andradite (99% Ca3Fe2Si3O12). Frequencies and symmetry assignments were determined for all 17 IR modes and all 25 Raman modes. By using factor group analysis and by correlating the bands by their intensities, bands were assigned to either one of the SiO4 internal motions, as a rotation, or to a type of translation. The assignments are supported by (1) the distinct trends of frequencies with cell size and cation masses for each of the different types of motion, (2) the similarity of garnet energies for each of the different types of motion to those of olivine with the same cation, and (3) the closeness of the T1u IR frequencies to the T2g Raman frequencies. Mode mixing appears to be weak. Correlations between frequencies and structural parameters suggests a direct dependence of force constants on lattice parameter. This relationship arises from bond lengths in the garnet structure being constrained by the size and compressibility of adjacent polyhedra through edge-sharing. Comparison of our endmember data with previous powder IR studies of intermediate garnets indicates that dodecahedral (X) and octahedral (Y) sites alone exhibit two-mode behavior for those solid solutions involving two ions with considerably different masses. However, for solid solutions involving cations of much different ionic radii, two-mode behavior is found for the translations of SiO4 groups. This is the first report of two-mode behavior that is unrelated to mass, and instead is due to significantly different force constants in the pyralspites compared to the ugrandites.

A study of silicon ELNES in nesosilicates

Si L2,3-absorption edges have been collected using EELS from four nesosilicates; Forsterite, Fayalite, Phenacite and Willemite. Nesosilicates, also known as orthosilicates or island silicates, are the simplest materials to be found in the silicate mineral class, consisting of isolated tetrahedral SiO4 groups. While many features in energy-loss spectra are common to all of the materials, remarkable extra intensity was observed for Forsterite. This appears to arise from distortion of the SiO4 tetrahedron; a hypothesis supported by XANES calculations. The concept of a simple

Modelling of structural and elastic changes of forsterite (Mg2SiO4) under stress

The method of crystal static deformation, including inner strain effects, was applied to calculate the structure configuration and the elastic constants of forsterite under anisotropic and isotropic pressure. A Born type interatomic potential is used, with optimized atomic charges and repulsive radii; SiO4 tetrahedra are approximated as rigid units. Computations were carried out in the range 1–8 GPa, with steps of 1 GPa, for the three uniaxial stresses τ1, τ2, τ3 and for pressure p. By interpolation of results, interatomic distances and elastic tensor components are shown to depend quadratically on stress. A non-linear behaviour generally appears above 4 GPa; the importance of inner strain and non-linear effects is analyzed. Mg-O bond lengths and O-O edges of coordination polyhedra respond differently to anisotropic and to isotropic stresses, according to the topological features of the structure. Elastic and structural results for hydrostatic pressure are compared to experimental literature data, discussing the range of validity of the rigid body approximation for SiO4 groups.

Vibrational spectra of olivine composition glasses: The Mg-Mn join

The vibrational frequencies of a series of splatquenched, olivine glasses spanning the compositional range from Mg2SiO4 to Mn2SiO4 have been determined using both infrared and Raman spectroscopies. The spectra of all glasses show evidence of tetrahedral coordination of silicon (possibly with some slight distortions), and largely octahedral coordination of magnesium. Spectra of Mn-rich glasses indicate that there is some manganese in 4 or 5-fold coordination. The frequencies observed for the fundamental vibrations of the silica tetrahedra are similar to those previously observed for SiO4 groups in both crystalline and glassy orthosilicates. Additionally, there is evidence for a small amount of silicate polymerization in all glasses characterized: vibrations attributable to Si2O7 groups are visible in both infrared and Raman spectra.