Volume 82, Issue 7-8

Calculations that minimize the energy and optimize the geometry of all atomic coordinates for two proposed kaolinite crystal structures were performed using a first-principles, quantum chemical code based on local density functional theory. All calculations were performed using published unit-cell parameters. Inner- and interlayer H atom positions agree well with those determined by Bish (1993) from neutron diffraction data and confirm a unit cell with C1 symmetry.

We present a first-principles study (local density approximation) of the structural prop­erties of forsterite under pressure. This highly anisotropic magnesium orthosilicate is the most abundant phase of the Earth’s upper mantle, and its elastic properties determine the rheology of this region. We perform full structural optimizations and investigate its compressive behavior up to 25 GPa. We obtain a pressure dependence of lattice parameters that agrees well with experiments to 17.2 GPa. We predict that the coordination polyhedra compress essentially isotropically, and we explain the anisotropy of forsterite in terms of the nonuniform distribution of coordination polyhedra having different but nearly uniform compressibilities. In agreement with Brodholt et al. (1996), we do not find theoretical evidence for sudden changes in compression mechanisms in this mineral as had been suggested from experiments. Our results support the hypothesis that such compressive anomalies are caused by solidification of the pressure medium.

Optical properties of fayalite, Fe 2 SiO 4 , have been obtained from ab initio calculations on the basis of the self-consistent energy band structure. The semi-relativistic, extended linear-augmented plane wave method (ELAPW) was used. Comparison of the calculated polarized optical spectra with experimental absorbance spectra shows satisfactory agreement. This observation allows a semiquantitative interpretation of the origins of the ob­served d-d transitions. Energy level diagrams for Fe 2+ ions in the M1 and M2 sites have been constructed using the Xα-scattered waves cluster method. These calculations quantitatively justify the use of the relationship ΔE ∞ 1/R 5 , which is often used for the derivation of crystal-field stabilization energies at high pressures.

X-ray diffraction rocking curves of titanite, CaTiSiO 5 , were measured using a novel high-resolution diffractometer. Three Bragg reflections were recorded as a function of temperature and their profiles analyzed in terms of a Gaussian Bragg peak and a diffuse scattering component with an overall Lorentzian shape. The temperature dependence of the Gaussian intensities of the rocking peaks of superstructure reflections, hkl with k + l odd, scale with the long-range order parameter as I ∞ Q 2 ∞ |T - T c | 2β , where β = 0.14(1) is the effective order parameter exponent. The diffuse scattering intensity changes little with temperature at T < T c - 20 K. Strong diffuse scattering is found at T > T c with centers of their Lorentzian diffraction profiles shifted by Δω ≈ 0.5° with respect to the position of the equivalent Bragg peak. A second phase transition at 825 K is confirmed and the possibility of a third transition at ~ 1150 K is discussed.

Unit-cell parameters of magnesite have been measured to high precision between 0 and 7 GPa using single-crystal X-ray diffraction. The isothermal bulk modulus of magnesite determined from fitting a Birch-Mumaghan third-order equation of state to the volume compression data is K T = 117(3) GPa with Kʹ T = 2.3(7), and K T = 111(1) GPa if Kʹ T is constrained to a value of 4. Crystal structure parameters have been determined from X-ray intensity data at room pressure, 2.26, 3.09, 4.16, 4.77, and 6.05 GPa. The principal struc­tural change with increasing pressure is compression of the MgO 6 octahedra while the CO 3 group remains invariant (within the experimental uncertainty) throughout the pressure range studied. The effect of the polyhedral compression is reflected in the anisotropic compression of the unit-cell parameters with the c axis approximately twice as compressible as the a axis. The polyhedral bulk modulus of the MgO 6 octahedron is 113 GPa, which is greater than that observed in other rhombohedral carbonates, but significantly smaller than values observed in many oxides and silicates. The distortion of the octahedra, though already small, decreases slightly with pressure. No phase change or change in compression behavior was observed throughout the pressure range studied.

Unit-cell parameters of synthetic (Mg 5 Fe)SiO 3 ortho- and clinopyroxenes were deter­mined at regular intervals in the temperature range 293-1094 K using powder X-ray diffraction techniques. Volume thermal expansion coefficients calculated from these data show that orthopyroxenes expand faster than clinopyroxenes (i.e., α opx > α cpx ), irrespective of their composition along the MgSiO 3 -FeSiO 3 join. For both ortho- and clinopyroxenes, α MgSio₃ exceeds α FeSio₃ . Axial thermal expansion coefficients calculated for each of the py­roxene phases studied here are a complex function of the changes in structure at high temperature. Thermodynamic calculations of the position of the phase boundary between MgSiO 3 ortho- and clinopyroxene show excellent agreement with the experimentally re­versed boundary.

The new Cr 2+ -containing silicate compounds BaCrSi 4 O 10 and SrCrSi 4 O 10 were synthesized both from alkali-borate fluxes and by high-T subsolidus solid-state reactions. The gillespite-type crystal structures (space group P4/ncc, Z = 4) were determined from single­crystal X-ray diffraction data. The unit-cell parameters are a = 7.4562(4), c = 15.5414(4) Å for SrCrSi 4 O 10 , and a = 7.5314(3), c = 16.0518(4) Å for BaCrSi 4 O 10 . Comparison with previously published data shows that A (= Ba, Sr, Ca) cation substitution in ABSi 4 O 10 gillespite-type compounds mainly affects the c lattice parameter whereas the substitution of the B (= Cu, Cr, Fe) site leads to only small changes, mainly in a. The Cr 2+ cation occupies a square-planar coordinated site unique in oxide crystal chemistry, with a Cr-O bond length of 1.999 ± 0.002 Å in all three Cr compounds. The rigidity of these bonds leaves the CrSi 4 O 10 layers within the structure with only one significant degree of freedom, that of rotation of the four-membered Si 4 O 10 rings in response to substitution on the A cation site. The magnitudes of these rotations are independent of the identity of the B cation. In addition the AO 8 polyhedron becomes more elongated // c with increasing radius of the A cation. The increasing aplanarity of the O(3)X 3 configuration is almost exclusively determined by occupational changes on A, whereas the aplanarity of the square-planar BO(3) 4 group can be related to the positional shifts induced by the individual substitutions on both A and B sites. Polarized optical absorption spectroscopy was conducted on (hk0) sections of SrCrSi 4 O 10 and BaCrSi 4 O 10 . Absorption bands at ~19500, ~14900, and ~22070 cm -1 could be assigned to 5 B 1g → 5 B 2g , 5 B 1g → 5 A 1g (E ⊥ c), and 5 B 1g → 5 E g , (E // c) spin- allowed d-d transitions for Cr 2+ in a square-planar configuration. The crystal-field stabili­zation energies of 13110 ± 150 and 13220 ± 180 cm -1 are indistinguishable for both compounds reflecting the very similar CrO 4 geometries.

Amphiboles were synthesized at 750 ℃, 1 kbar (H 2 O) for compositions at 20% intervals along the join potassium-richterite-tremolite. Structural variations, site occupancies, and modal analyses of the experimental products (amphibole + minor diopside, quartz, and enstatite) were characterized by Rietveld structure refinement, with final R Bragg indices in the range 4-6%, and by infrared spectroscopy in the principal OH-stretching region. Amphibole compositions were determined by (1) site-scattering refinement for the A and M4 sites that are occupied by (K, ⃞) (⃞ = vacancy) and (Na,Ca), respectively; and (2) mass- balance calculations involving the modal analysis and the nominal experimental product composition. These measurements agree within 1% absolute and show close agreement with electron-microprobe compositions for the two samples that we could analyze. Deviations from nominal amphibole composition are up to 19% absolute. The resulting relations between cell dimension and composition are linear. The major change in cell dimensions is a decrease of 0.25 Å in a with increasing tremolite component. The infrared spectra show two principal peaks at 3735 and 3675 cm -1 , corresponding to the local arrangements MgMgMg-OH- A K (the Kr band) and MgMgMg-OH- A ⃞ (the Kr band), respectively. The relative variation in peak intensity as a function of amphibole composition shows that the molar absorptivities of the two bands are significantly different. The ratio of the molar absorptivities for the two bands is 2.2.

Kinetic studies conducted primarily between 465 and 525 ℃ demonstrate that the rate of the polymorphic anatase to rutile transformation increases dramatically when the reacting anatase is very finely crystalline. Coarsening of the reactant anatase and product rutile crystallites occurs simultaneously with the transformation. Kinetic behavior and quantifi­cation of transformation rate as a function of average crystallite size indicate that the increase in favorable nucleation sites is a likely cause of increase in transformation rate at small crystallite sizes. Additionally, experimental evidence supports the reversal of stabilities of anatase and rutile at small crystallite sizes. It is proposed that the reversal of stabilities is the result of rutile having a higher surface energy than the anatase phase. Data for coarsening kinetics of anatase and rutile supports the prediction that the surface energy of rutile is significantly larger than that of anatase. Thermodynamic data and theoretical estimates are used to show that a 15% greater surface energy for rutile causes the total free energy of rutile to be greater (less negative) than anatase at crystallite diameters in the few nanometer range. Given the fact that anatase and rutile structures have no poly­merized octahedral fragments in common, this may be significant in determining the nature of the nucleated phase.

The continuous structural transformation of the natural zeolite stilbite (Na 3.62 K 0.44 Ba 0.03 Ca 6.32 Sr 0.28 Mg 0.04 [Fe 3+ 001 Al 17.33 Si 54.64 O 144 ] · 60H 2 O) upon dehydration has been studied using Rietveld structure analysis of temperature-resolved powder diffraction data collected with synchrotron radiation. In the initial stage of heating, the monoclinic F2/m stilbite structure (the so-called A phase) behaves as a noncollapsible framework, featuring only a slight framework distortion and a slight cell-volume contraction. At about 420 K, a first-order phase transition occurs changing the symmetry to an orthorhombic Amma phase, whose framework is collapsible and shows a large cell-volume contraction with temperature. The cell contraction is related to the process of T-O-T bond breaking and leads to a high-temperature stilbite phase with the same Amma space group and a collapsed structure similar to the previously described B phase in stellerite and barrerite. The struc­tural refinement indicates that the dynamics of bond breaking is related to the shift of the Ca cations in the channels to achieve optimal coordination after the release of the H 2 O molecules. Refined statistical occupancies of the tetrahedral atoms involved in the bond­breaking process (T1 and T1P) are consistent with a random rupture and re-formation of the T-O-T bonds. This is the first experimental study of the dynamic bond breaking of T-O-T bonds in a framework structure.

The structures of synthetic Mn 3 Al 2 Si 3 O 12 spessartine and Ca 3 Al 2 Si 3 O 12 grossular garnet have been refined using single-crystal X-ray diffraction methods at 100 K, 293 K, and 500-550 K. The divalent X-site cations, located in large dodecahedral sites, show measur­able anisotropic dynamic disorder in contrast to the rigid vibrational behavior of the SiO 4 tetrahedra and AlO 6 octahedra. The amplitudes of vibration of Mn 2+ in spessartine are similar to those of Fe 2+ of almandine, in the plane of the longer X-O(4) bonds, and both are about twice that of Ca 2+ in grossular, despite the lighter mass of the latter. Heat capacities measured between 300 and 1000 K on synthetic poly crystalline spessartine and two natural nearly end-member spessartine crystals are similar to those of almandine. In addition, the IR active modes of spessartine at low frequencies are very similar to those of almandine suggesting that their heat capacities are also similar at lower temperatures. The low-energy phonon spectra of pyrope and grossular are probably considerably distinct from the two transition metal-containing garnets as suggested by their different low frequency IR active modes, reflecting the different bonding properties for Mg and Ca in garnet. The large pressure-temperature stability field of spessartine, relative to the other aluminosilicate garnets, does not appear to be due to any sort of intrinsic entropy stabilization.

Unit-cell parameters of synthetic, end-member titanite (CaTiOSiO 4 ) critically depend on the synthesis conditions, as is shown for studies reported in the literature and for new samples reported here. Our study suggests that phase-pure samples are likely to be obtained only if they are synthesized entirely below the solidus. In contrast, samples synthesized either directly from melt or by annealing of glass tend to have higher unit-cell volumes, contain Si-rich and Ca-Si-rich phase impurities, and may be nonstoichiometric. The ob­served variations in cell parameters among the samples strongly correlate with synthesis methods and can be explained by vacancies in the Ca or Si site or both. This result is particularly important because the thermodynamic properties currently in use for titanite are based on samples synthesized from melts of stoichiometric composition and thus are suspect even though they have been determined carefully. To establish a reference point for future studies concerned with the chemical and physical properties of this material we report our findings along with a redetermination of the unit-cell parameters [a = 7.062(1), b = 8.716(2), and c = 6.559(1) Å; β = 113.802(9)°, V=369.4(3) Å 3 ] from powder X-ray data of synthetic, stoichiometric titanite.

The enthalpy of drop solution of a quickly quenched, clear, titanite glass sample and several synthetic, polycrystalline, samples was measured in a Calvet-type calorimeter in 2PbO · B 2 O 3 solvent at 702 ℃. The enthalpy of formation of stoichiometric, end-member (CaTiSiO 5 ) titanite is redetermined to be -119.59 ± 2.24 kJ/mol from the oxides and -2610.13 ± 2.90 kJ/mol from the elements at 25 ℃. The new value is either 8.725 or 13.525 kJ/mol more negative than the currently accepted values derived from calorimetry and phase-equilibrium experiments. The presence of impurities and possibly nonstoichiom­etry in the original sample, used in the previous calorimetric studies, are probably respon­sible for this discrepancy. This interpretation is further supported by the correlation of the variations in the measured enthalpies of drop solution to the documented physical differences among the synthetic polycrystalline samples used in this study. At 25 °C the enthalpy of formation of CaTiSiO 5 glass is -38.775 ± 3.37 kJ/mol from the oxides and -2529.31 ± 3.84 from the elements and the enthalpy of vitrification of titanite is 80.78 ± 3.59 kJ/mol.

For the first time, chemically pure, nearly single-phase antigorite, Mg 48 Si 34 O 85 (OH) 62 , was synthesized directly without using seeds. Starting material was a stoichiometric mix­ture of previously synthesized talc and brucite. Synthesis conditions were 50 kbar, 500 ℃, and 120 h. TEM studies show that the dominant wavelength for the structural modulation of antigorite is about 4.5 nm, which corresponds to m = 17 in the general antigorite formula M 3m-1 T 2m O 5m (OH) 4m-6 and thus to the structure of most natural antigorites. Selected-area electron diffraction patterns of single crystals exhibit the hk0 reciprocal net diagnostic of antigorite.

A complete solid solution has been hydrothermally synthesized between the two end- members LaPO 4 and (Ca 05 Th 0.5 )PO 4 at 780 ℃ and 200 MPa, indicating that there is no limitation in temperature and pressure conditions corresponding to those of granitic mag­mas for Th insertion in natural monazites. The composition limits of the (A 3+ 1-2x B 2+ x C 4+ x )PO 4 compounds crystallized in the monazite structure-type are determined by both r average = (1 - 2x) [9] r A³+ + x [9] r B²+ + x [9] r C⁴+ and r ratio = (1 - x) [9] r A³+ + [9] r B²+ /(1-x) [9] r A³+ + x [9] r C⁴+ parameters (where [9] r A is the ionic radius of the A element in ninefold coordination). The upper and lower values of these parameters are 1.216 Å ≥ r average ≥ 1.107 Å and 1.238 ≥ r ratio ≥ 1. The incorporation of large amounts of trans-uranium ele­ments in the monazite structure is deduced from this model. The limitations and geochronological inferences of this model are discussed.

Solid solutions of (Ce 5 Y)PO 4 were synthesized hydrothermally between 300 ℃ and 1000 ℃ at pressures of 2, 5, 10, and 15 kbar. Experiment products were analyzed by electron microprobe, and their lattice parameters were refined by Rietveld analysis of XRD powder patterns. Over a wide range of bulk compositions, two immiscible phases formed: monazite with a P2 1 /n structure (REEO 9 polyhedron) and xenotime with an I4 1 /amd structure (REEO 8 polyhedron).I Variations of the unit-cell dimensions are directly correlated with variations in composition. The boundaries of the asymmetric miscibility gap are strongly dependent on temperature. At 2 kbar and from 300-1000 ℃, maximum Y concentrations in monazite coexisting with xenotime increase from 3-16 mol%. With increasing pressure, this limb is shifted to higher Y contents so that at 15 kbar it ranges from 6 to about 25 mol% for the same temperature range. In contrast, Ce concentrations in xenotime do not exceed 3 mol% at 1000 ℃ over the entire pressure range. Our experimentally determined boundaries fit surprisingly well with empirical calibrations of the (LREE-HREE,Y)PO 4 -solvus boundaries derived from a suite of low pressure metapelites (3 to 5 kbar) that equilibrated at peak temperatures ranging from 400-700 °C. The behavior of the natural (REE 5 Y)PO 4 system is probably well described by the simple (Ce 5 Y)PO 4 binary, at least for metapelitic compositions. Many natural samples of monazite coexisting with xenotime show prograde growth zonation with respect to the incorporation of HREE + Y The combination of our thermometer along with U-Pb and Sm-Nd age determinations of high spatial resolution may thus provide information about prograde branches of metamorphic PTt paths.

The rate and mechanism of a key diagenetic reaction, laumontite → wairakite + H 2 O, have been determined in experiments with durations as long as three months at P H₂O = 100 MPa and temperatures of 350-450 ℃. In the lower temperature range, 350-400 ℃, nucleation of wairakite occurred on the smallest laumontite fragments in the starting material. Growth then proceeded by the dissolution of large laumontite grains, transport within the fluid, and precipitation of euhedral to subhedral wairakite. At higher temperatures, 425 and 450 ℃, each sample contains two product phases: wairakite and an unidentified pla- gioclase-like phase. The plagioclase-like silicate was stabilized by the uptake of Na and formed early as ~10 μm wide skeletal grains along laumontite grain boundaries. The wairakite grains subsequently nucleated on and grew into the interiors of large laumontite grains. Nucleation rates at 425-450 ℃ were 10-100 wairakite grains per square meter of laumontite surface per second. Growth rates varied from 1.5 × 10 -11 m/s at 350 ℃ to 2.1 × 10 -10 m/s at 450 ℃; the low-temperature data can be fit with an apparent activation energy of 72 ± 13 kJ/mol and a pre-exponential “interface jump distance” of ~1 × 10 -18 m. This activation energy and these growth rates are comparable to those calculated by Walther and Wood (1984) to characterize interface-controlled reactions in silicates under H 2 O - saturated conditions. Our data predict transformation rates for geologic conditions that are too fast to account for the commonly observed incomplete natural reaction of laumontite → wairakite, indicating that transformation in nature must be limited by slower nucleation rates or by slower intergranular diffusion-perhaps as a result of lower H 2 O activity or slower heating rates.

Competition among atomic or molecular species for occupancy of crystallographic sites exaggerates correlations among chemical elements in suites of mineral chemical data, a phenomenon known as closure. Such exaggerated correlations can lead to incorrect conclusions about ionic substitution mechanisms and the petrological forces that drive them. Expressing mineral compositions in terms of a single additive component and molar concentrations of exchange components, eliminates the effects of closure. Statistical analysis of data so transformed can, in some instances, lead to conclusions distinct from analysis of the same data expressed in terms of ionic abundances. The chemical variability of Active and naturally occurring amphiboles serves to illustrate the potential difficulties brought about by closure and the benefits of its elimination.

Dessauite, a new mineral in the crichtonite group, occurs within cavities in calcite veins as tabular {001} rhombohedral, black, millimeter-sized crystals at Buca della Vena Mine, Apuan Alps (Tuscany, Italy). It is associated with derbylite, hematite, rutile, karelianite, siderite, and calcite. Dessauite is trigonal, space group R3̅, with a = 9.197(1) Å, α = 68.75(2)°. Optically, dessauite is opaque and shows low bireflectance and very weak pleochroism. Electron microprobe analyses led to the following simplified chemical formula: (Sr,Pb)(Y,U)(Ti,Fe 3+ ) 20 O 38 . The crystal structure of dessauite was refined from single-crys­tal X-ray diffraction data to R = 0.065. Dessauite is isostructural with the others members of the crichtonite group; a peculiar structural feature is the presence of additional, partially occupied octahedral sites. A comparison of the crystal-chemical formulas of all the minerals within the crichtonite group is presented. In view of the structural information, the analytical data have been re-arranged on the basis of the crystal-chemical formula ABC 18 T 2 O 38 , rather than AM 21 O 38 .

Sediments of playa Lake Salines, SE, Spain, contain a carbonate mineral characterized by X-ray diffraction peaks very similar to, but systematically shifted from those of pure magnesite. Analyses (SEM, IR and Raman spectroscopy, DTA., TGA, and ICP) indicate the mineral is a hydrous Ca-bearing magnesium carbonate with the chemical formula (Mg 0.92 ,Ca 0.08 )CO 3 · 3H 2 O. Thermal characteristics of the mineral are similar to those of other known hydrated magnesium carbonates. X-ray and electron diffraction data suggests a monoclinic system (P2 1 /n space group) with unit-cell parameters of a = 6.063(6), b = 10.668(5), and c = 6.014(4) Å and β = 107.28°.