Volume 85, Issue 7-8

Garnet-biotite experiments now exist from Ferry and Spear (1978, FS), Perchuk and Lavrent'eva (1983, PL), and Gessmann et al. (1997, GE). Recent garnet Margules parameters exist from Berman and Aranovich (1996, BA), Ganguly et al. (1996, GA), and Mukhopadhyay et al. (1997, MU). A comparison of ∆G mix plots for garnet binaries shows similarity among models with some significant differences. In all, 20 garnet-biotite geothermometers were retrieved from the various data using a modified form of the approach of Holdaway et al. (1997, HO) such that experimental vs. calculated values of T, in °C, were regressed stepwise to a line with intercept of zero and slope of one, maximizing r 2 . All tests assumed the GA model for garnet MnMg and the HO Fe 3+ correction and ∆W Ti Bio . The experimental combinations (sets) were: (1) GE + FS with GE values of VI Al Bio in FS; (2) PL + FS with GE VI Al Bio in FS; (3) PL + FS with GE VI Al Bio in FS, increasing W Gr MnMg by 5 kJ; (4) PL + FS with no VI Al Bio in FS, increasing W Gr MnMg by 5 kJ; (5) PL + FS with 0.10 VI Al Bio in FS, increasing W Gr MnMg by 5 kJ; (6) PL + FS + GE with 0.10 VI Al Bio in FS, increasing WGr MnMg by 5 kJ. Models are designated by a number for the experimental set and letters to designate the above garnet model, e.g., 5BA. Experimental sets vary in maximum r 2 as follows: (1) 0.892-0.896; (2) 0.943-0.956; (3) 0.943- 0.955; (4) 0.980-0.982; (5) 0.980-0.983; and (6) 0.939-0.944. Application to the HO Maine biotitegarnet data gives the following sequence of increasing quality from experimental sets: 1 < 2 < 3 < 6 < 4 ≡ 5. The various garnet models show only small differences in r 2 and in quality with the Maine data base. For all 20 models, average T (°C) for the Maine M3 staurolite zone varies between 554(15) and 588(13). The best models appear to be 5BA, 5GA, and 5MU, which allow for 0.10 VI Al in FS biotite. A model based on an average of the three garnet Margules models, 5AV, has some justification and provides a slight improvement. Model 5AV gives (in J) ∆G ex = 40198 - 7.80T; W Bio FeMg = 22998 - 17.40T; ∆W Al Bio = 245559 - 280.31T , and Maine M3 average staurolite zone T = 571(12) °C. The high quality of sets based mainly on PL experiments (2-5 above) results in part from their experimental P of 6 kbar. The results suggest that FS biotite contained only minor VI Al. The wide ranges of derived exchange and biotite Margules parameters, which provide reasonable calibrations, indicate that while accuracy in parameters is important, consistency is even more important. These geothermometers (5BA, 5GA, 5MU, 5AV) are available on PC disk. I recommend geothermometer 5AV.

The volumes of mixing of six garnet binaries in the four-component system almandine-pyropespessartine- grossular have been analyzed and compared. The almandine-pyrope join is thermodynamically ideal and the others show positive deviations from ideality. The magnitude of the excess volume of mixing of a given binary, as described by a symmetric mixing model, is shown to be a function of the volume difference between the two end-member components. The relationship between the magnitude of nonideality and volume difference can be described with a linear or quadratic function, but the latter has a physical basis in strain theory. Positive deviations from ideal thermodynamic mixing are primarily a result of strain resulting from the substitution of dodecahedral X-site cations of different sizes, but second-order chemical effects (e.g., electronic, coulombic) also appear to contribute to thermodynamic properties. XRD and IR spectroscopic measurements show that nonlinear behavior of microscopic properties, such as bond lengths and polyhedral distortions and volumes, across a solid solution is strong for pyrope-grossular garnets and very small to nonexistent for almandine-pyrope solid solutions. The magnitude of the deviations from linearity in the microscopic properties can be related to the excess volume of mixing for a given binary. A good correlation exists between microscopic structural strain and the macroscopic volume of mixing.

The 1.85 Ga paleosol at Flin Flon, Manitoba, Canada, was one of the first paleoweathering profiles taken as evidence for a dramatic rise in the oxygen content of the Paleoproterozoic atmosphere. Diagenesis and greenschist-facies metamorphism have modified the abundances of some major elements (e.g., K and Fe) in the Flin Flon paleosol. Inductively coupled plasma-mass spectrometry (ICP-MS) analyses reveal a positive Ce anomaly on both chondrite- and Amisk basalt-normalized REE patterns and marked Th/U fractionation in the uppermost maroon paleosol. The Ce anomaly is confirmed by the occurrence of cerianite and by the compositions of other REE- and U-rich accessory minerals (i.e., secondary monazite and uraninite) in the uppermost maroon paleosol. Th/U fractionation is supported by the presence of uraninite in both the paleosol and overlying Missi sedimentary rocks. Chemical ages of uraninite (1.85 to 1.0 Ga) suggest that this mineral might have formed during weathering or diagenesis but was susceptible to disturbance. Although igneous monazite and zircon are well preserved in even the uppermost maroon paleosol, primary fluorapatite exhibits variable degrees of weathering in the basal green paleosol and has been obliterated completely in the upper maroon paleosol. Published paleomagnetic data suggest a tropical paleolatitude for the Flin Flon region at the time of formation of the paleosol. The positive Ce anomaly in the Flin Flon paleosol confirms previous studies for an oxic atmosphere (PO 2 ≥ 10 -1.5 to 10 -2 PAL) at 1.85 Ga. However, extreme caution must be exercised in the interpretation of Th/U fractionation in paleosols. Moreover, this study demonstrates the advantage of a detailed characterization of REE- and U-rich accessory minerals in the application of trace-element geochemistry of paleosols for paleoatmospheric reconstruction. Eh-pH diagrams for the stabilities of cerianite and uraninite in their respective systems Ce-P-C-H-O and UP- C-H-O at 25 °C and 1 bar reveal the importance of phosphate complexes in the formation of Ce anomalies and Th/U fractionation in both modern and ancient weathering environments.

New experimental results are reported on the partitioning of Os between Ni 94 Os 6 (at) alloys and silicate melt at 1400 °C at one atmosphere total pressure and oxygen fugacities ranging from 10 -6 to 10 -8 atm. Specially designed experiments allowed us to avoid nugget formation, which was responsible for the anomalously high Os solubilities reported by Borisov and Palme (1998). Experimental glasses were analyzed by instrumental neutron activation analysis and by isotope dilution analysis. Results reveal a strong dependence of Os solubility on f O₂ , with Os contents decreasing from about 100 ppb to about 5 ppb as f O₂ decreases. The slope of log (Os solubility) vs. log f O₂ implies that Os 3+ is the dominant Os valence state in silicate melts. The relative geochemical behavior of Os and Pt is also compared. Results suggest that Pt is more siderophile, and may also be more chalcophile than Os under the conditions considered.

The compositions of aqueous fluids in equilibrium with enstatite + forsterite were investigated at temperatures from 900 to 1200 °C and pressures from 1.0 to 2.0 GPa. Experiments, performed in a piston-cylinder apparatus, involved the location of phase boundaries between the stability fields of enstatite and enstatite + forsterite, and enstatite + forsterite and forsterite. The intersection of these two phase-boundaries near the H 2 O apex was used to define the fluid composition. The results indicated a systematic increase in the concentration of silica in the fluid phase with increasing temperature. The experiments indicated that the concentration of dissolved MgO was below 0.3 mol% and not resolvable using our techniques. This finding was corroborated by microprobe analyses of quench precipitates from the fluid phase, which gave on average 0.2 mol% MgO. Because of the low MgO concentrations, the mean values of the intercepts with SiO 2 -H 2 O binary of the fitted lines representing, respectively, the phase boundaries between the enstatite and enstatite + forsterite and between the enstatite + forsterite and forsterite stability fields were taken to represent the concentrations of dissolved silica at the various temperatures and pressures of the present study. The concentrations increased from 0.6 mol% at 900 °C to 3.9 mol% at 1200 °C at 1.0 GPa. The pressure effect from 1.0 to 2.0 GPa at 1000 °C appeared to be minor and not resolvable using our techniques. At 1.0 GPa, the base 10 logarithm of the molal concentration of dissolved silica in equilibrium with enstatite + forsterite was obtained combining data from the present study with those from Nakamura and Kushiro (1978) and Manning and Boettcher (1994): logm SiO2(aq) En-Fo = 6.869 - 1.335 × 10 4 / T(K) + 5.544 × 10 6 / T(K) 2 . Comparison with studies of the solubility of quartz (Manning 1994) indicated that thermodynamic properties of aqueous silica derived from silica-saturated systems may not be applicable to calculations in silica-deficient systems at high pressure.

Amphiboles were synthesized at 1.5 kbar P H₂O and 850 °C along the join pargasite-fluoropargasite. Structural variations in the amphibole as a function of F have been characterized by a combination of SEM-EDS, X-ray powder diffraction, and infrared spectroscopy. SEM-EDS analyses show that, for increasing F in the system, there is a decrease in Altot in the amphibole and a significant decrease of F incorporation in the structure. In agreement with the EDS data, the variation in cell parameters and IR spectra show that incorporation of F in pargasite is restricted to about 1.0 atoms per formula unit (apfu). The OH-stretching spectra show fine structure caused by F replacing OH at the O3 anion site, and are consistent with two-mode behavior typical of A-site-filled amphiboles. The agreement between calculated and observed relative band intensities suggests complete short-range disorder of OH and F at the O3 anion site.

Apatites provide a mechanism for in-situ remediation of Pb-contaminated sites, by supplying phosphate which combines with dissolved Pb to form highly insoluble pyromorphite. This study expanded upon previous research using hydroxylapatite (HAP), to focus on how the surface properties of natural chlorapatite (CAP) and fluorapatite (FAP) affect pyromorphite nucleation and growth, when aqueous Pb 2+ ([Pb] < 50 mg/L as PbCl 2 ) is reacted with apatite at pH = 4.2, 22 °C. A combination of atomic force microscopy (AFM), scanning electron microscopy (SEM), optical microscopy, energy dispersive X-ray fluorescence spectroscopy (EDS), infrared spectroscopy (FTIR), and X-ray diffraction (XRD) was used for in-situ and ex-situ examination of the interface and the reaction products. A variety of experiments were performed, and pyromorphite [PY = Pb 10 (PO 4 ) 6 Cl 2 ] was the only identified reaction product. PY nucleated both homogeneously (directly from solution) and heterogeneously on CAP and FAP; relationship between PY and HAP could not be determined definitively. The nucleation mechanism was sensitive to initial experimental conditions, which suggests that care should be taken to consider initial conditions in designing sorption and nucleation experiments. Heterogenous nucleation on FAP and CAP showed strong evidence for epitaxy. Epitaxial crystals of PY on CAP and FAP grew away from the substrate surface, which was the only source of phosphate available for crystal growth. Epitaxial crystals showed distinctly different morphologies from homogeneously nucleated crystals, and appeared to grow by a spiral growth mechanism.

Transmission 57 Fe Mössbauer measurements were acquired in the temperature range 11-745 K from a suite of nine synthetic Ca-free P2 1 /c Mg-Fe clinopyroxenes (cpx) along the MgSiO 3 -FeSiO 3 join. The paramagnetic Mössbauer spectra (MS) consist of one doublet produced by Fe 2+ ions at an almost regular octahedral M1 site and a second doublet at a more distorted octahedral M2 site. The temperature dependencies of the Fe 2+ center shifts were fit to equations derived from the Debye model for the lattice vibrations, allowing the determination of the characteristic Mössbauer temperatures for the two Fe sites. The temperature variations of the M1 and M2 quadrupole splitting ∆E Q (T) are consistent with the higher distortions of the M2 octahedra. Applied-field MS revealed that the principal component of the electric field gradient, V zz , is positive, implying a tetragonal compression of both octahedral sites. The crystal-field model was used to analyze ∆E Q (T) and to calculate the energy gaps ∆ 1 and ∆ 2 of the first excited electronic states within the 5 D orbital term, both at M1 and M2. The various physical quantities derived from the MS are discussed in terms of the Fe/(Fe + Mg) ratio.

The single-crystal X-ray technique was used to calibrate a new intracrystalline geothermometer based on equilibrium Mg-Fe* fractionation (Fe* = Fe 2+ + Mn 2+ ) between M1 and M2 sites of natural P2 1 /c pigeonite. Suitable crystals free of exsolution textures and sharp diffraction maxima were selected by careful TEM and XRD investigations from a large number of samples. Two single crystals, PCA82506-3 (Wo 6 En 76 Fs 18 ) from the Pecora Escarpment 82506 Antarctic ureilite, and BTS308- 2 (Wo 10 En 47 Fs 43 ) from the BTS308 Paranà rhyodacite, were annealed at temperatures ranging from 600 to 1000 °C. The TEM investigation, carried out on fragments of selected single crystals both before and after thermal treatment, shows heating-induced texture modifications preliminary to spinodal decomposition in both crystals and a size increase in antiphase domains in BTS308-2. The two geothermometric equations calculated by linear regression of ln K D * vs. 1/T are: ln K D * = -3291(± 269)/T(K) + 0.971(±0.253); (r 2 = 0.974) ln K D * = -2816(± 83)/T(K) + 0.542(±0.083); (r 2 = 0.995) respectively for PCA82506-3 (X Fe * = 0.20) and BTS308-2 (X Fe * = 0.49) [X Fe * = Fe*/(Fe* + Mg)]. These results imply negligible compositional effects on Mg-Fe* site partitioning within the range of compositions encompassed by the samples. Comparison of the intracrystalline fractionation data of pigeonites with those of Pbca orthopyroxenes shows a similar degree of ordering for both at a given temperature. This result suggests only a small effect of Ca on Fe*-Mg ordering in pyroxene with Ca content up to Wo 10- .

A transmission electron microscopy (TEM) investigation was performed on Ca-rich feldspars along the join CaAl 2 Si 2 O 8 -SrAl 2 Si 2 O 8 (An-SrF). Two series of samples were synthesized by dry (T = 1450 °C, t = 96 h) and hydrothermal (PH2O = 2 kbar, T = 600 °C, t = 300 h) annealing with compositions between An100 and An 50 SrF 50 . TEM investigation at room temperature showed the presence of sharp c-type reflections only in hydrothermal An100 and An 80 SrF 20 feldspars, whereas diffuse and elongated c reflections were found in all other feldspars in the range An 80 SrF 20 -An 50 SrF 50 . The intensity of c reflections decreases with increasing Sr content, and fully disappears only at An 50 SrF 50 . In-situ, high-temperature TEM investigation (HT TEM) on hydrothermally treated An100 and An 80 SrF 20 feldspars, showed a significant decrease in the temperature (Tc) of the P1̄-I1̄ phase transition with increasing Sr content. In An 80 SrF 20 feldspar, a precise critical temperature cannot be defined, and the transition is smeared over a T range between 100 and 150 °C. A P1̄-1̄ phase transition is therefore observed both as a function of composition and temperature. The transition dependence on composition is induced by the increase in the non-tetrahedral cation radius that causes a static disorder and affects the intensity of c reflections. Dynamical disorder adds to the static disorder induced by Sr across the transition at HT.

The crystal structures of stuffed derivatives of quartz within the Li 1-x Al 1-x Si 1+x O 4 system have been refined by Rietveld analysis of powder synchrotron X-ray diffraction (XRD) data. Our results reveal an Al-Si order-disorder transition at x = ~0.3 and a β-α displacive transformation at x = ~0.65. Structural variations across the series result from an interplay of three mechanisms: tetrahedral tilting associated with Al-Si order-disorder; Li positional disorder along structural channels parallel to c; and tetrahedral rotation related to the β-α transition. At both microscopic (local bonding) and macroscopic (spontaneous strain) scales, the substitution of Li + and Al 3+ for Si 4+ closely mimics temperature in its effect on the quartz framework.

The high-temperature transformation of andalusite (Al 2 O 3 ·SiO 2 = A) single crystal to 3:2-mullite (3Al 2 O 3 ·2SiO 2 = M) and non-crystalline silica (SiO 2 ) taking place between 1500 and 1600 °C was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Electron diffraction studies confirmed the topotactical orientation relationship ([100] A ‖ [010] M , [010] A ‖ [100] M , [001] A ‖ [001] M ) between the primary product (A) and the product phase (M), which was predicted by Pannhorst and Schneider (1978) on the basis of single-crystal X-ray diffraction studies. The transformation starts at the (001) A surface and proceeds rapidly along to the c A,M -axis. The overall reaction front is parallel to (001) of andalusite and mullite, though on the microscopic scale it exhibits a zigzag shape with facets parallel to {011} A and {201} M . The development of {011} A and {201} M microfacets is favorable, because both lattice planes exhibit a small misfit, which is close to zero at the transformation temperature. The undistorted and direct transition from andalusite to mullite and the high velocity of the reaction along [001]A suggests that the AlO 6 octahedra which occur in both structures are being preserved during the transformation. Other structural units, however, have to be decomposed and reconstructed. The newly formed mullite crystallites display an acicular shape elongated to the c-axis with an average size of about 250 nm along their small dimension. Excess non-crystalline SiO 2 is exsolved between the mullite crystallites in small channels running parallel to [001] M . The major amount of the exsolved silica liquid is transported to the (001) surface of the former andalusite crystal by means of capillary forces.

The transformation of an andalusite (Al 2 O 3 ·SiO 2 = A) single-crystal to 3:2-mullite (3Al 2 O 3 ·2SiO 2 = M) and non-crystalline silica (SiO 2 ) was investigated at the (100) A and (010) A faces using electron microscopy. The transformation starts topotactically at the surfaces producing a dense layer of mullite and vitreous silica on the surface and underneath the mullite layer. Next, the reaction proceeds by dissolution of andalusite in the vitreous silica and by diffusional transport of alumina to mullite crystals occasionally nucleated at the dissolving andalusite. Transformation along [100] A proceeds twice as fast as along [010] A , which is explained by the higher stability of the (010) A face against dissolution. Transformation by dissolution and precipitation is one order of magnitude slower than the topotactic transformation along [001] A . The transformation reaction of andalusite to mullite and silica is highly anisotropic. Combining the results along the directions [100] A , [010] A , and [001] A predicts the type of transformation which will take place at an arbitrary andalusite face. Only if the c A axis is oriented more than 80° off the surface normal, will the reaction proceed by the dissolutionprecipitation process, whereas for all other orientations andalusite transforms by the fast topotactic reaction along the c A axis.

Single crystals of tetragonal Ca 3 Mn 2 Ge 3 O 12 garnet (space group symmetry I4 1 /a, a = 12.3098(7) Å, c = 12.3277(9) Å) were characterized by X-ray diffraction and transmission electron microscopy. Their structure is topologically isosymmetric to tetragonal high-pressure garnets such as majorite, displaying the same two distinct types of macroscopic twin mechanisms. Twinning occurs as pseudomerohedral ferroelastic twin lamellae with preferred orientation of the twin-domain boundaries parallel to {101} tet , whereas merohedral ferrobielastic twin domains occur without any orientational preference. The crystal structure was determined from single-crystal X-ray diffraction data of a crystal fragment completely free of pseudo-merohedral twin domains. It shows two different Jahn- Teller distorted MnO 6 octahedra, with a different orientation of the axis of polyhedral elongation. The ordering scheme of these Jahn-Teller distorted octahedra follows in an alternating pattern the densest rod packing, and the cooperative effect of the electronically induced octahedral distortion was found to be responsible for the cubic-to-tetragonal symmetry breaking in Ca 3 Mn 2 Ge 3 O 12 garnet. The extent of polyhedral distortion indicates a partially dynamic character of the Jahn-Teller effect. The distortion of the tetrahedral T2 sites controls the cooperative effect of the lattice strain induced by the Jahn-Teller distortion of the Mn 3+ octahedra and thus is responsible for the overall lower symmetry.

The volume of lawsonite, CaAl 2 Si 2 O 7 (OH) 2 · H 2 O, has been measured up to 7 GPa and 873 K using in situ energy dispersive powder diffraction and a multi-anvil high pressure-temperature cell at the Synchrotron Radiation Source, Daresbury Laboratory, U.K. Measurements were made on isotherms at 298, 323, 373, 473, 573, 673, 773, and 873 K within the pressure range. Sample pressure was measured from a NaCl standard mixed with the sample; the unit-cell volume of lawsonite was taken from the same diffraction pattern. The data gave an ambient temperature isothermal bulk modulus of K 298 112 ± 6 GPa, similar to previous values. This value overestimates the temperature stability of lawsonite in thermodynamic calculations. A fit of the Birch-Murnaghan equation of state to the whole high pressure and temperature data set gave an isothermal bulk modulus of K 298 = 125 ±5 GPa and a dK/dT of -0.01 ± 0.01/K, with K 1 set to a value of 4 and the expansivity set to 3.16 × 10 -5 /K. Using these values to calculate the pressure-temperature positions of three of lawsonite’s dehydration reactions improved the agreement between observed and calculated positions of the lawsonite dehydration reactions to within experimental and calculation error. This work shows that the ambient temperature bulk modulus and ambient pressure expansivity do not adequately describe the volume behavior of lawsonite at combined high pressure and temperature.

Synchrotron radiation B K-edge XANES spectra are reported for 19 borate and borosilicate minerals. The spectra are characterized by three prominent features; a sharp peak (A) at ~194 eV (the edge feature of trigonal B; III B), a broader peak (B) at 197-199 eV (the edge feature of tetrahedral B, IV B), and a broad peak (C) at 200-201 eV for tetrahedral B and 203-204 eV for trigonal B. The area of peak A is very sensitive to content of III B and its position yields information on B-O bond length and linkage of the BO 3 group. The area of peak B is proportional to content of IV B but quantification is limited by overlap with peak C. The width of peak B increases with increasing divergence of B-O bond lengths, responding to splitting of σ*(t 2 ) antibonding orbitals. The tetrahedral component of peak C appears to be a s-shape resonance. For trigonal B minerals, the relative intensity of peak C and its satellite peaks increases with increase in mean size and/or atomic number of next-nearestneighbor cations, C being most intense in vonsenite (Fe 2 Fe 3+ BO 5 ). Priceite (Ca 4 B 10 O 19 ·7H 2 O), of unknown structure, has 80% IV B, and an isolated BX3 group with III B-O = 1.373(5) Å. Comparison of B K-edge XANES spectra collected using total electron yield (sampling depth <60 Å) and fluorescence yield (sampling depth <1100 Å) shows that borates and borosilicates readily reconstruct in surface and near-surface environments. More profound structural damage involving conversion of IV B to III B occurs in minerals with high contents of IV B, and hydrous Mg borates with interstitial H 2 O are unstable in the high vacuum of the spectrometer.

A lack of direct spectroscopic evidence linking cations to the clay mineral structure is a primary reason for many ambiguities regarding the location and structure of the binding sites. This study attempts to obtain such evidence through the observation of nuclear-spin interactions between adsorbed cations ( 113 Cd 2+ ) and nuclei present in the lattice structure of the clay ( 1 H and 27 Al) using solid-state NMR. 1 H- 113 Cd variable-amplitude cross-polarization (VACP) and 27 Al- 113 Cd spin-echo double-resonance (SEDOR) experiments were successfully performed on dried, Cd 2+ -exchanged beidellite, montmorillonite, and vermiculite samples, demonstrating for the first time a direct interaction between the adsorbed cations and the clay. VACP provides much greater cross-polarization (CP) efficiency which enhances signal intensity necessary for these experiments. Signal intensity varies between octahedral and tetrahedral binding sites because hydroxyls in the ditrigonal cavities of tetrahedral sites produce greater CP efficiency presumably due to shorter 1 H- 113 Cd distances in this environment. CP efficiency decreased in the order vermiculite>beidellite>montmorillonite. The observation of nuclear-spin coupling between cadmium and either 1 H or 27 Al in the minerals is consistent with the presence of the cations in the ditrigonal cavities of the tetrahedral sheet. Inter-sample comparison of the strengths of coupling interactions follow expected trends based on charge location in the crystal structure and Al speciation.

Oxygen isotope exchange between natural analcime and water vapor at low pressure is studied by 17 O NMR, which can distinguish Si-O-Si and Si-O-Al sites and provide information on site specific exchange kinetics and mechanisms. Si-O-Al sites in analcime are found to exchange faster than Si-O-Si sites at lower temperature (400 °C). At higher temperature (500 °C), the exchange rates for the two types of site are similar, suggesting that the exchange process for Si-O-Si has a higher activation energy, as predicted by published theoretical calculations. The overall exchange reaction appears to be controlled largely by reaction at sites, not diffusion. Static 17 O spectra show an anomalously high ratio of Si-O-Si to Si-O-Al sites, suggesting the presence of a few percent of Al-O-Al sites, which are indeed resolved and observed in triple quantum magic-angle spinning (3QMAS) spectra.

The geometrical features and electronic structure of molecular clusters models of two octahedrally coordinated cations in edge-sharing octahedra were studied by means of Hartree-Fock ab initio molecular orbital calculations at LANL2DZ and 6-31+G* levels. These models represent the different cation pairs among Al 3+ , Fe 3+ , and Mg 2+ of the octahedral sheet of clays. These models reproduce the experimental values of the main geometrical features in the corresponding minerals. The vibrational frequencies of the bridging hydroxyl groups (M-OH-M’) were calculated and compared with experimental data. A good agreement between theoretical and experimental results was found. The relative differences of ν(OH) and δ(OH) frequencies calculated among these (M-OHM’) cation pairs are similar to the experimental behavior in clays. Theoretical g(OH) frequencies were also calculated and presented as an estimation of the experimental. Correlations between the atomic weights and the atomic Mulliken charges of the cations with the experimental and theoretical OH vibration frequencies have been also determined and a similar behavior was found.

The dynamical behavior of Cl - and H 2 O molecules in the interlayer and on the (001) surface of the Ca-aluminate hydrate hydrocalumite (Friedel’s salt) over a range of temperatures from -100 to 300 °C was studied using isothermal-isobaric molecular dynamics computer simulations. This phase is currently the best available model compound for other, typically more disordered, mixed-metal layered hydroxides. The computed crystallographic parameters and density are in good agreement with available X-ray diffraction data and the force field developed for these simulations preserves the structure and density to within less than 2% of their measured values. In contrast to the highly ordered arrangement of the interlayer water molecules interpreted from the X-ray data, the simulations reveal significant dynamic disorder in water orientations. At all simulated temperatures, the interlayer water molecules undergo rapid librations (hindered hopping rotations) around an axis essentially perpendicular to the layers. This results in breaking and reformation of hydrogen bonds with the neighboring Cl - anions and in a time-averaged nearly uniaxial symmetry at Cl - , in good agreement with recent 35 Cl NMR measurements. Power spectra of translational, librational, and vibrational motions of interlayer and surface Cl - and H 2 O were calculated as Fourier transforms of the atomic velocity autocorrelation functions and compared with the corresponding spectra and dynamics for a bulk aqueous solution. The ordered interlayer space has significant effects on the motions. Strong electrostatic attraction between interlayer water molecules and Ca atoms in the principal layer makes the Ca···OH 2 bond direction the preferred axis for interlayer water librations. The calculated diffusion coefficient of Cl - as an outer-sphere surface complex is almost three times that of inner-sphere Cl - , but is still about an order of magnitude less than that of Cl - in bulk aqueous solution at the same temperature.

Tetragonal Mg 2.12 Fe 3.17 Ge 2.56 O 12 is a new germanate forming at 1 atm pressure in the MgO-Fe 2 O 3- GeO 2 system. It is an analogue of the high-pressure silicate mineral TAPP occurring as inclusions in diamonds of lower-mantle origin. Its crystal structure was determined by X-ray diffraction using single-crystals grown from a potassium molybdate flux. It crystallizes in the I4̄2d space group with a = 6.8153(4) Å, c = 18.669(2) Å, Z = 4. The refinement of its cation distribution, VIII (Mg 0.52 Fe 0.33 ) VI (Mg 1.60 Fe 2.40 ) IV (Ge 2.56 Fe 0.44 )O 12 , shows the presence of a partially filled (85%) dodecahedral site characterized by an unusual geometry with two very different bond lengths [2.169 Å (×4) and 2.609 Å (×4)]. The structure determinations of the Ca 2+ - and Y 3+ -substituted phases show that the larger cations are completely partitioned in the dodecahedral site, increasing its occupancy slightly (up to 92% in the case of Ca 2+ ). Cubic Mg 2.35 Y 2.00 Fe 0.97 Ge 2.59 O 12 is a new garnet phase forming at 1 atm pressure in the MgO-Y 2 O 3 -Fe 2 O 3 -GeO 2 system. Its crystal structure and cation distribution were also determined by singlecrystal X-ray diffraction: VIII (MgY 2 ) VI (Mg 1.35 Fe 0.56 ) IV (Ge 2.59 Fe 0.41 )O 12 , Ia3̄d space group, a = 12.232(1) Å, Z = 8. In spite of similarities in their chemical compositions, the tetragonal and garnet phases are structurally distinct, with different ratios of dodecahedral to octahedral sites, viz., VIII A VI B 4 IV T 3 O 12 and VIII A 3 VI B 2 IV T 3 O 12 respectively. As a consequence, the tetragonal phase forms in systems containing smaller cations, such as Mg 2+ , Fe 3+ , Ni 2+ , Co 2+ , whereas larger cations, such as Y 3+ , show a strong preference for the garnet phase.

Dicalcium aluminate (Ca 2 Al 2 O 5 ) was prepared in a piston cylinder apparatus at 1250 °C and 2.5 GPa. The compound is orthorhombic with space group symmetry /2mb, a = 5.2281(1) Å, b = 14.4686(2) Å, c = 5.4004(1) Å (Z = 4, D calc = 3.481 g/cm3), and belongs to the brownmillerite structure family. Main building units are (1) layers of perovskite type corner connected AlO 6 -octahedra perpendicular to [010], and (2) zweier single chains of AlO 4 -tetrahedra running parallel [100]. The alternate stacking of the layers and sheets of single chains results in a three dimensional network in which the calcium ions are incorporated for charge compensation. The present structure is the first example for an alkaline earth aluminate with zweier single chains of tetrahedra.

Suredaite, ideally PbSnS 3 , is a new mineral species from the Pirquitas Ag-Sn deposit (Province Jujuy, NW-Argentina). It was observed in symmetrically banded veins in the Oploca district, and is associated with sphalerite, arsenopyrite, pyrite-marcasite, cassiterite, cylindrite, franckeite, hocartite, rhodostannite, and various Ag-Sb and Ag-Bi sulfosalts in minor amounts. Suredaite occurs in layers up to 1 cm in thickness as aggregates of radially arranged tabular-prismatic (single) crystals, has a metallic lustre, and a dark grey streak. VHN 50 ranges between 18.2 and 20.6 (mean 19.6) GPa, the Mohs hardness is 2.5-3. It has perfect cleavages parallel to {001}, {101}, and {100}. The measured density varies between 5.54 and 5.88 g/cm 3 , D x was determined to be 5.615 g/cm 3 . In reflected plane-polarised light, it is white and is not perceptibly bireflectant or pleochroic. It lacks internal reflections and is weakly anisotropic with metallic blue, mauve to brown rotation tints. Specular reflectance percentages in air and in oil are tabulated from 400 to 700 nm and compared graphically with those for the type specimen of teallite, PbSnS 2 . Electron microprobe analyses showed suredaite to be chemically inhomogeneous with respect to the compositional variations (in wt%): Pb 42.3- 48.5, Ag 0.3-1.1, Fe 0.3-1.0, As 0.2-2.1, Sn 27.7-30.2, S 23.1-24.7. The crystal structure determined from single-crystal X-ray diffraction data revealed orthorhombic symmetry [space group Pnma, Z = 4, a = 8.8221(3), b = 3.7728(3), c = 14.0076(3) Å; V = 466.23(4) Å 3 ]. The atomic arrangement is isostructural to the NH 4 CdCl 3 structure type which exists in a series of isotypic sulfides and selenide compounds. The suredaite structure, which is the natural analogue of synthetic PbSnS 3 , consists of columns of double-edge sharing octahedra running parallel to the b axis, which house the Sn atoms. These columns are linked by rods of eightfold-coordinated Pb atoms. On the basis of the structure determination, the empirically determined idealized formula follows a [8] (Pb, As,Ag, Sn) [6] (Sn,Fe)S 3 stoichiometry. Crystalchemical arguments suggest Ag possibly to occupy interstitial sites according to the alternative formula [4] (⃞,Ag) [8] (Pb, As, Sn) [6] (Sn,Fe) S 3 . The name of this new mineral species is in honor of R.J. Sureda Leston, head of the Department of Mineralogy and Economic Geology, University of Salta, Argentina.

Kozoite-(Nd), Nd(CO 3 )(OH), occurs in cavities and fissures of alkali olivine basalt exposed at Niikoba, Hizen-cho, Higashi Matsuura-gun, Saga Prefecture, Japan, in association with lanthanite- (Nd) and kimuraite-(Y). The crystal structure was refined by the Rietveld method in space group Pmcn; a = 4.9829(1), b = 8.5188(2), c = 7.2570(2) Å, Z = 4 using powder diffraction data obtained by a combination of a Gandolfi camera and monochromatic synchrotron radiation. Kozoite-(Nd) is pale pinkish-purple to white with a vitreous to powdery luster. The calculated density is 4.77 g/cm 3 . It has high birefringence, α = 1.698(2), γ = 1.780(5). The four strongest lines in the X-ray powder patterns [d(Å), I/I 0 , hkl] are (4.29, 100, 110); (2.93, 89, 102); (2.33, 78, 131) and (2.06, 78, 221). Kozoite-(Nd) is the first naturally occurring RE(CO 3 )(OH) end-member in the ancylite group.

Florenskyite is a new phosphide species from the Kaidun chondritic meteorite, which fell in South Yemen in 1980. Kaidun is a unique chondritic breccia containing a huge variety of fragments of different chondritic types. Florenskyite was found as four dispersed grains with a maximum dimension of 14 μm within a single mass of Fe-rich serpentine within one Kaidun clast. Florenskyite is associated with submicrometer-sized grains of pentlandite and small (up to 1.5 μm in width) laths of a still uncharacterized Fe-Cr phosphide. Florenskyite is creamy white in reflected light, and its luster is metallic. The average of three electron microprobe analyses gave (wt%) Fe 40.52, Ti 30.08, Ni 5.47, Cr 0.93, V 0.91, Co 0.60, P 21.69, Si 0.59, sum 100.79, corresponding to Fe 1.01 (Ti 0.87 Ni 0.13 Cr 0.03 V 0.02 Co 0.01 ) 1.06 (P 0.97 Si 0.03 ). Single-crystal structure analysis was performed on florenskyite using a Laue pattern collected from a multiple crystal by in-situ synchrotron X-ray diffraction. Florenskyite crystallizes in the space group Pnma, and has the anti-PbCl 2 structure. Previously determined cell constants of synthetic material [a = 6.007(1), b = 3.602(1), c = 6.897(1) Å] were used in the single-crystal data reduction. We used the POWD12 program to calculate a powder XRD pattern; the 5 most intense reflections are d = 2.301 (I = 100), 2.188 (88), 2.307 (47), 1.938 (45), and 1.801 Å (45). Florenskyite is only the fourth phosphide to be described from nature. Its paragenesis may be unique, and may be due to melting of a mineral assemblage including Fe-Ni metal, schreibersite, daubreelite, osbornite, or heideite and subsequent crystallization of phosphides from the melt.

We report the first atomistic computer simulations of the partitioning of divalent cations between forsterite (Mg 2 SiO 4 ) and coexisting MgSiO 3 melt at ~1600 °C and atmospheric pressure. Our results, using new Monte Carlo techniques, are compared with new experimental determinations of forsteritemelt partitioning for the same elements (Ca, Mn, Ni, Co, Cu, Zn, Sr, Cd, Ba) in the same system under identical conditions. Over seven orders-of-magnitude variation in the Nernst partition coefficient (D), experiment and simulation agree typically within a factor of 2 and at worst to within a factor of 4.2 (D Sr ). Our simulation techniques therefore herald a novel means of determining crystalmelt partitioning that may be especially valuable under extreme conditions of pressure and temperature not readily amenable to experimentation.