Volume 90, Issue 2-3

This is a comparative planetary mineralogy study emphasizing the valence-state partitioning of Cr, Fe, Ti, and V over crystallographic sites in olivine, pyroxene, and spinel from planetary basalts. The sites that accommodate these cations are the M2 site (6 to 8-coordinated) and M1 site (6-coordinated) in pyroxene, the M2 site (6- to 8-coordinated) and M1 (6-coordinated site) in olivine, and the tetrahedral and octahedral sites in spinel. The samples we studied are basalts from Earth, Moon, and Mars, and range in oxygen fugacity conditions from IW-2 (Moon) to IW+6 (Earth), with Mars somewhere in between (IW to IW+2). The significant elemental valence-states at these f O₂ conditions are (from low to high f O₂ ): Ti 4+ , V 3+ , Fe 2+ , Cr 2+ , Cr 3+ , V 4+ , and Fe 3+ . V 2+ and Ti 3+ play a minor role in the phases considered for the Moon, and are found in very low concentrations. V5+ plays a minor role in these phases in oxidized terrestrial basalts because it is probably lower in abundance than V 4+ , and has an ionic radius that is so small (0.054 nm, 6-coordinated), that it is almost at the lower limit for octahedral coordination, and can even be tetrahedrally coordinated. The role of Cr 2+ in the Moon is significant, as Cr 2+ predominates in basaltic melts at f O₂ less than IW-1. Lunar olivine has been found to contain mostly Cr 2+ , whereas coexisting pyroxene contains mostly Cr 3+ . Fe 3+ is very important in Earth, less so in Mars, and nonexistent in the Moon. The importance of the Fe 2+ to Fe 3+ transition cannot be overstated and, indeed, their crystal-chemical differences, in terms of behavior (based on size and charge), are similar to the differences between Mg and Al. We note that for pyroxene in six of the seven terrestrial suites we studied, Fe 3+ (in the M1 site) coupled with Al (in the tetrahedral site) is one of the two most important charge-balance substitutions. This substitution is of lesser importance in Mars and does not exist in lunar basalts.

Mineralogical-textural analysis of a multi-layered plagioclase (Pl) + orthopyroxene/biotite (Opx/Bt) vermicular microstructure around garnet from the Bolangir anorthosite massif is presented. X-ray images demonstrate the garnet was zoned prior to decomposition, with Mg decreasing toward its rim, and Mn, Ca, and XFe [= Fe/(Fe + Mg)] increasing. On decomposition, the Ca zonations in garnet were truncated by the consuming garnet margin. By implication, the retreat velocity of the garnet margin was faster than the diffusive velocity for Ca within the garnet. By contrast, the pre-decomposition Fe, Mn, and Mg variations in garnet were considerably modified adjacent to Opx + Pl, but were unaffected adjacent to Bt + Pl intergrowths. In the corona, X Fe Opx and X Fe Bt decrease toward garnet, whereas the An content in associated Pl increases from An 50 to An 80-83 . Thermo-barometry involving nearest neighbor Opx, Grt, Pl, and Qtz indicate that the corona formed at 6.0 ± 1.0 kbar, 750 ± 50 °C. At the isothermal-isobaric condition of decomposition, the compositional variation across the Bt + Pl corona is explained best by the inward retreat of the garnet margin at a velocity faster than the fastest diffusing element (Fe, Mn). In zones where garnet decomposed to Opx + Pl, the velocity of the consuming garnet margin was such that the intra-garnet Fe, Mn redistribution was aborted in transit. The compositions and volume proportions of vermicular orthopyroxene and plagioclase in the multi-layered corona are best approximated by the NCFMAS reaction Grt ss + Pl 1 (± Qtz) → Opx + Pl 2 . The early formed/matrix plagioclase (Pl 1 ) and inward-retreating garnet (Grt ss ) margin provided the necessary components for successive Opx + Pl 2 layers to grow at Grt-Pl 1 interfaces. Tie-line topology in ACFN space indicates Pl2 to be An-rich relative to Pl 1 . The mineralogical relations indicate the CFMAS reaction, Grt ss + Qtz → Opx + An, to be an inadequate analogue of the actual decomposition. Consequently, to assume that the texture indicates near-isothermal decompression across the low-dP/dT CFMAS reaction is possibly an oversimplification.

Dellaventuraite is a new amphibole species from the Kajlidongri manganese mine, Jhabua District, Madhya Pradesh, India. It occurs with leakeite, kornite, albite, braunite, and bixbyite associated with cross-cutting epigenetic veins that have reacted with regionally metamorphosed rocks containing Mn-rich minerals (braunite, bixbyite, jacobsite, spessartine) to produce Mn-rich amphiboles, Mnrich pyroxenes, Mn-rich mica, piemontite, and manganophyllite. Dellaventuraite occurs as anhedral grains, the color of which varies from pink to red, depending on Mn content. It is brittle, H = 5, Dcalc = 3.184 g/cm 3 , has a pale pink streak, vitreous luster, and does notfluoresce in ultraviolet light; it has perfect cleavage on {110} and conchoidal fracture. In transmitted plane-polarized light, dellaventuraite is strongly pleochroic, X = pale mauve-brown, Y ~ Z = dark red-brown; Y ^ a = 20° (in β obtuse), Z = b, with absorption X < Y ~ Z. It is biaxial positive, η α = 1.688 ± 0.003, η β = 1.692 ± 0.005, η γ = 1.721 ± 0.003, 2V (obs) = 49 ± 3°, 2V (calc) = 41°. Dellaventuraite is monoclinic, space group C2/m, a = 9.808(1), b = 17.840(2), c = 5.2848(5) Å, γ = 104.653(1)°, V = 894.6(2) Å 3 , Z = 2. The strongest ten X-ray diffraction lines in the powder pattern are [d(I,hkl)]: 2.697(10,151), 2.542(9,.202), 3.127(8,310), 3.378(7,131), 2.154(7,261), 1.434 (7,.661), 4.450(6,021), 8.459(5,110), 2.727(5,.331), 2.328(5,.351). Analysis by a combination of electron microprobe, SIMS and crystal-structure refinement gives SiO 2 = 54.22, Al 2 O 3 = 0.81, TiO 2 = 5.45, Fe 2 O 3 = 6.44, Mn 2 O 3 = 7.57, ZnO = 0.12, NiO = 0.16, MgO = 8.26, Li 2 O = 1.53, CaO = 1.85, Na 2 O = 8.12, K 2 O = 2.12, H 2 O = 0.80, Cr, V, F, Cl not detected, sum 97.41 wt%. The formula unit, calculated on the basis of 24(O,OH,F) is (K 0.40 Na 0.61 )(Na 1.71 Ca 0.29 ) (Mg 1.81 Zn 0.01 Ni 0.02 -Li 0.90 Fe 3+ 0.71 Mn 3+ 0.85Ti 4+ 0.60Al 0.10 )(Si 7.96 Al 0.04 )O 22 [(OH) 0.80 O 1.20 ]; the ideal end-member composition NaNa 2 (MgMn 3+ 2 LiTi0 4+ )Si 8 O 22 O 2 . The crystal structure of dellaventuraite was refined to an R index of 3.8% using MoKα X-ray intensity data. The M1 site is occupied by Ti 4+ , Mn 4+ , and Mg in approximately equal amounts, the M2 site is occupied primarily by Mg and Fe 3+ , and M3 is occupied by Li with minor Mg and Mn 2+ . Local bond-valence considerations suggest that O 2- at O3 is linked to Ti 4+ Mg or Mn 3+ Mn 3+ at the adjacent M1 sites, and that OH at O 3 is linked to MgMg at the adjacent M1 sites.

Electron microprobe and micro-FTIR (mFTIR) analyses of a quartz crystal from a hydrothermal vein reveal zoning in Al and H concentrations. The Al concentration ranges from 27 to 468 Al/10 6 Si, and the H concentration ranges correspondingly from 49 to 266 H/10 6 Si. The zoning profile reveals a positive correlation between Al and H concentrations. At low Al concentration (<100 Al/10 6 Si), the H/Al ratio is ~1.0 and the infrared spectra show strong bands due to Al-OH and very weak bands due to Li-OH. These results indicate that most of the charge imbalance resulting from Al 3+ substituting for Si 4+ is compensated by H. At an Al concentration >100 Al/10 6 Si, the ratio of H/Al drops to ~0.5 and the infrared spectra show absorption bands due to both Al-OH and Li-OH species. No other alkali ions were detected by microprobe analysis. These results suggest that a combination of H and Li are providing charge compensation in the more Al-rich zones. Replacement of Si 4+ by Al 3+ + H + ± Li + is the principal mechanism for the incorporation of these trace elements into the quartz structure.

The Ti content of biotite can serve as a geothermometer for graphitic, peraluminous metapelites that contain ilmenite or rutile and have equilibrated at roughly 4-6 kbar. The relationship between Ti- coutent, temperature, and Mg/(Mg + Fe) value was calibrated empirically using an extensive natural biotite data set (529 samples) from western Maine and south-central Massachusetts in combination with the petrogenetic grid of Spear et al. (1999). The calculated Ti-saturation surface is curved such that for a given Mg/(Mg + Fe) value, Ti concentration increases as a function of temperature in a nonlinear' fashion, and for a given temperature Ti concentrations decrease with an increase in Mg/(Mg + Fe). The fit to the Ti-saturation surface can be reformulated as the geothermometric expression: T = {[In(Ti) - a - c(X Mg ) 3 ]/b} 0.333 , in which T is temperature in degrees Celsius, Ti is the number of atoms per formula unit (apfu) normalized on the basis of 22 O atoms, X Mg is Mg/(Mg + Fe), a = -2.3594, b = 4.6482 x 1O -9 and c = -1.7283. The calibration range for this expression is X Mg = 0.275-1.000, Ti = 0.04-0.60 apfu, and T = 480-800 0 C. Precision of the Ti-in-biotite geothermometer is estimated to be ±24 0 C at the lower temperature range and improves to ±12 °C at higher temperatures. Application of the Ti-in-biotite geothermometer to ilmenite- or rutile-bearing, graphitic, peraluminous metapelites equilibrated at 3-6 kbar is generally consistent with independent temperature determinations, but with some deviations that represent local reequilibration. Consequently, the Ti systematics in biotite can also serve as the basis of a very sensitive indicator of chemical equilibrium, or lack thereof. Applica­tion of the geothermometer to metapelites not containing the requisite mineral assemblages can lead to minor-to-significant errors in estimated temperatures. Biotite Ti-substitution mechanisms are controlled by several factors. Based on the biotite calibra­tion data set, magnesian biotites (X Mg > 0.65) incorporate Ti in accordance with the exchange vector TiAl 2 R -1 Si -2 , where R is the sum of the divalent cations Mg + Fe + Mn. This substitution mechanism is primarily a response to misfit of the octahedral and tetrahedral layers in magnesian biotites. Inter­mediate biotites (X Mg <0.65), particularly at higher temperatures, exhibit enhanced Ti concentrations, most consistent with the Ti-deprotonation TiO 2 R -1 (OH) -2 , exchange vector. Dominance of Ti-deprotonation substitution is largely a function of reduction of H 2 O activity at higher metamorphic grades. Supplementary biotite data from metaluminous amphibolites and mafic granulites, metamorphosed isothermally with variable H 2 O activities, reveal that Iow-Al biotite incorporates significantly higher concentrations of Ti relative to peraluminous biotite as a result of a combination of the exchange vec­tors Ti0 2 R -1 (0H) -2 and RSiAl -2 , substituting in roughly an 8:1 ratio.

Interest in hydrotalcite-like compounds has grown due to their role in controlling the mobility of aqueous metals in the environment as well as their use as catalysts, catalyst precursors, and specialty chemicals. Although these materials have been studied in a number of contexts, little is known of their thermodynamic properties. Here we present a complete thermochemical study of hydrotalcite- like compounds of the MgO-Al 2 O 3 -CO 2 -H 2 O system. Using high-temperature oxide-melt solution calorimetry, we determined the enthalpies of formation at 298 K from the elements (Afor the compounds: Mg 0.69 Al 0.31 (OH) 2.013 (C0 3 ) 0.15· 0.30H 2 0, Mg 0.74 Al 0.26 (OH) 2 (CO 3 ) 0 . 13· 0.39H 2 O, Mg 0.67 Al 0.33 (OH) 2 (CO 3 ) 0.16· 0.69H 2 O, and Mg 0.66 Al 0.34 (OH) 2 (CO 3 ) 0.17 -0.70H 2 O to be -1171.55 ± 1.81, -1165.98 ± 2.06, -1284.65 ± 1.97, and -1292.07 ± 2.05 kJ/mol, respectively. We also present the heats of formation of these materials from the single-cation hydroxides [Mg(OH) 2 and Al(OH) 3 ] and carbonates (MgCO 3 ) and water (ΔH f scc ); they are energetically stable by 10-20 kJ/mol. Using low-temperature adiabatic heat-capacity measurements we determined the third-law entropy (S°) for the compound Mg 0.74 Al 0.26 (OH) 2 (CO 3 ) 0.13 -0.39H 2 O [85.58 ± 0.17 J/(mol-K)], neglecting any configurational contributions. From our experimental data for Mg 0.74 Al 0.26 (OH) 2 (CO 3 ) 0.43· 0.39H 2 0, we calculated the free energy at 298 K (ΔG 0 f = -1043.08 ± 2.07 kJ/mol). Our thermodynamic studies also provide insight into the state of the interlayer water in hydrotalcite, namely the water of hydration appears to exist in a state intermediate in thermodynamic properties between that of ice and liquid water.

Although many formulations of plagioclase + liquid equilibria have been calibrated in the last decade, few models specifically address the issue of temperature (T) prediction. Moreover, for those that do, T error is not addressed, greatly limiting their use as geothermometers. Several recent models of plagioclase-liquid equilibria are thus tested for their ability to recover T from their calibration data, and predict T from experiments not used for calibration. The models of Sugawara (2001) and Ghiorso et al. (1995, 2002) outperform earlier calibrations. These models perform reasonably well at T > 1100 °C, though recovery and prediction of T is less precise for hydrous compositions. In addition, these models cannot be integrated with geo-hygrometers, or other mineral-melt thermometers and barometers; the following expression predicts T with up to 40% greater precision: Because these thermometers are pressure (P) sensitive, a temperature-sensitive barometer was also developed: In these models, T is in Kelvins and P is in kbar. An pl and Ab pl are the fractions of anorthite and albite in plagioclase, calculated as cation fractions: An = CaO/(CaO + NaO 0.5 + KO 0.5 ) and Ab = NaO 0.5 / (CaO+NaO 0.5 +KO 0.5 ). Terms such as Alliq refer to the anhydrous cation fraction of Al in the liquid; H 2 O in Equation 1 is in units of wt%. Errors on these models are comparable to those for clinopyroxene thermobarometers: In Equation 1, R = 0.99 and the standard error of estimate (SEE) is 23 K; for Equation 2, R = 0.94 and the SEE is 1.8 kbar. The models successfully recover mean pressures for experimental data that are not used for calibration, and are furthermore able to recover near-1-atm P estimates for volcanic rocks from Kilauea, Hawaii, which are thought to have crystallized at or very near Earth's surface.

The standard-state thermodynamic properties of 14 Å Fe-amesite [Fe 4 Al 4 Si 2 O 10 (OH) 8 ] were calcu­lated from the reequilibration experiments of Pana et al. (2005) and natural data bearing on the Fe-Mg partitioning between chlorite and chloritoid over a wide range of pressure (P). temperature (7), and rock composition. The combination of these data with the three-site mixing model and thermodynamic properties of daphnite proposed by Vidal et al. (2001) led us to reappraise the evolution of the Margules parameter W G AlFe with pressure and temperature. The new data [H 0 f Fe-Am = -7607460 J/mol, .S 0 Fe-Am = 514.8 J/(mol·K), V 0 Fe-Am = 20.90 J/bar, W G AlFe = 1200 - 31T + 0.7(P - 1)] are compatible with all the natural data and reequilibration experiments, and in fair agreement with the results of synthesis. They are used to calculate the compositional evolution of chlorite with T. P, or α O2 at different bulk-system compositions. The calculated phase relations and chlorite compositions are compared with previous experiments conducted in the FeO-Al 2 O 3 -SiO 2 -H 2 O system. The addition of the Fe-amesite end-member increases the number of equilibria that can be calculated for any natural assemblage involving chlorite. Two independent equilibria are obtained for natural (Fe-Mg)-chlorite-quartz or diaspore assemblages. It is therefore possible to estimate metamorphic conditions for high-variance assemblages from the composition of chlorite. We discuss the reliability of such P-T estimates using natural chlorites oc­curring in low-pressure and low-temperature or medium- to high-pressure and temperature samples. Assuming that the proposed model and thermodynamic parameters are correct, these various natural data suggest that X Fe 3+ [= Fe 3+ /(Fe 2+ + Fe 3+ )] in metamorphic chlorites is low (<0.1), whereas it can be as high as 0.3 in Si- and Mg-rich chlorites formed at low -P-T conditions.

Iron chlorite with compositions intermediate between the two end-members daphnite [Fe 5 Al 2 Si 3 O 10 (OH) 8 ] and Fe-amesite [Fe 4 Al 4 Si 2 O 10 (OH) 8 ] were synthesized from gels, under f O₂ conditions defined by the solid oxygen buffer Fe-FeO. The unit-cell parameters and volume of chlorite with Si-content ranging from 2.3 to 2.7 were calculated. A least-squares fit of these data yields V 0 Fe-ames = 213.06 cm 3 and V 0 daph = 216.50 cm 3 . The molar volume of daphnite is similar to that estimated by Vidal et al. (2001), but the volume difference between Fe-amesite and daphnite is too low. The experimental data were also fitted for reasonable values of V 0 Daph - V 0 Fe-ames and V FeMg-1 , with linear (ideal) or non-ideal volume models involving a positive excess volume. With these models we obtain V 0 Daph between 216 cm 3 and 217.49 cm 3 , and V 0 Fe-ames between 209 and 211.35 cm 3 . Equilibration experiments involving chlorite with almandine-hercynite/fayalite or chloritoidhercynite/ fayalite provide data on the chlorite composition as a function of T and P at temperatures between 420 and 520 °C and pressures between 3 and 20 kbar, at f O₂ buffered by the assemblage Fe- FeO. Initial Si-rich and Si-poor chlorite compositions converged in most cases toward an equilibrium composition during the experiments. The results show that the Si-content of chlorite is sensitive to temperature for the various divariant assemblages. The most definitive results, obtained for the assemblage chlorite-almandine-fayalite, were used to estimate H 0 fFe-amesite and the Al-Fe Margules parameter for the various sets of daphnite and Fe-amesite molar volumes constrained from the synthetic chlorites. The results indicate that H 0 fFe-amesite = -7616 ± 3 kJ and W G AlFe ~ -10 kJ.

X-ray diffraction, infrared spectroscopy, and X-ray absorption (XANES and EXAFS) spectroscopy were combined to characterize poorly crystalline Mn-rich silicate scale deposited from brine at a geothermal field in Indonesia. The precipitate has a vitreous pink-amber appearance, a nearly pure SiO 2 -MnO-H 2 O chemical composition, and a bulk Mn/Si atomic ratio of 0.63. X-ray microfluorescence indicated that the sample consists of Mn-free and Mn-containing silica domains, whose Mn/Si ratio (1.2 ± 0.4) ranges between those of 2:1 and 1:1 phyllosilicates. The XRD pattern is characterized by a broad scattering band spanning from ~6 to ~2.5 Å with weak modulations at 5.3, 3.5, and 2.72 Å, and a faint band at 1.605 Å. A surrogate was synthesized in an O 2 -free atmosphere by mixing and aging at 150 °C an MnCl 2 and a sodium meta-silicate solution for 3 hours. Its XRD pattern was similar to the scale sample, but the two reflections at 2.72 and 1.605 Å were enhanced and the former was asymmetrical as in randomly stacked layered compounds. Diffraction data are consistent with a mixture of amorphous silica and a poorly ordered manganoan sheet silicate with a domain size of 40.50 Å for the scale and 50.60 Å for the surrogate material. The divalent oxidation state of manganese was confirmed by XANES spectroscopy, and the presumed existence of trioctahedral [Mn 2+ ] 3 (O,OH) clusters having a clay-like local structure was corroborated by the existence of a weak vibration band at 600-650 cm -1 in infrared spectra. The connectivity of Mn octahedra was examined using EXAFS spectroscopy by comparing the local structure of Mn in the scale sample and in a large series of Mn compounds having different kinds of Mn octahedra and Si tetrahedra linkages. The manganoan scale has a polyhedral local structure resembling that of sheet silicates, in which metal octahedra are joined along edges and share corners with ditrigonal SiO 4 rings. Similar short-range ordering has been described in Fe- and Al- silicate scales, but the Mn-silicate scale has a lower domain size owing to the greater misfit between the lateral dimensions of the Mn and Si sheets. The strain induced by the shrinkage of the Mn sheet to fit the Si sheet(s) is alleviated by the nanometer size of the two-dimensional hydrous silicate domains.

A sequence of progressively more realistic geometric crystal chemical models for TOT layers in mica is developed, starting from the usual main uniform distortions (octahedral flattening, tetrahedral rotation, octahedral counter-rotation) toward additional features as they are shown to be required. These additional features include tetrahedral basal flattening and apical bond adjustment, geometric meso-octahedral sheets (having unequal M1 and M2/M3 site bond lengths), and geometric heterooctahedral sheets (having unequal M1, M2, and M3 site bond lengths). A crystal chemical model for the unit cell of a 1M polytype with C2/m space group is developed from geometric homo-octahedral sheets (having equal M1, M2, and M3 site bond lengths) and is described using a minimal number of independent crystal chemical parameters: octahedral, tetrahedral basal, tetrahedral apical, and interlayer metal-anion bond lengths, and flattening angles of octahedral and tetrahedral sheets. The monoclinic lattice parameters (a, b, c, and β) and the tetrahedral rotation angle (α) follow from these assumed parameters. These models are designed to allow analyses (that are reported elsewhere) of both structural and lattice-parameter refinement data in terms of deviations from various predictions based on specified sets of crystal chemical assumptions. Fractional atomic coordinates are derived in terms of the atomic positions for the 1M unit cell of C2/m symmetry for each known homogeneous mica polytype with highest space group symmetry (polytype, space group = 2M 1 , C2/c; 2M 2 , C2/c; 2O, Ccmm; 3T, P3 1 12). These coordinates allow a structural analysis of diffraction data for different stacking polytype structures using the same 1M-type TOT layer as a modular unit.

Based on powder X-ray diffraction (pXRD) measurements of many mica solid solution series of synthetic powder samples (between Mg, Co, Ni, and Fe end-members, with different degrees of oxidation, vacancy, and Al/Si contents, and including an OH/F substitution series; 75 samples indexed as 1M polytype, space group C2/m) and on 175 previously published structural refinements of 1M micas (refined in space group C2/m) comprising 37 synthetic and 138 natural single-crystal samples, we show that the relation (c/a) cos β* = 1/3 between the a, c, and β monoclinic lattice parameters (β* = 180E - β), holds exactly (within experimental error) for all the synthetic powder samples whereas it does not hold in general for the natural or synthetic single crystal samples. This relation is not imposed by the C2/m space group. It is predicted to hold for geometric homo-octahedral sheets (having equal M1 and M2 site bond-lengths) and predicted not to hold in general for geometric meso-octahedral sheets (having unequal M1 and M2 site bond lengths). We conclude that synthetic powder samples (usually having particle diameters in the range 0.1.20 μm) are geometric homo-octahedral whereas single crystals (typical size of 0.05 × 0.05 × 0.01 mm or larger) are geometric meso-octahedral, with either M1 > M2 (most common, 153 samples out of 175) or M1 < M2. The geometric meso-octahedral character of the single crystals is corroborated by several parameters of the structure refinements. The synthetic powders presumably appear not to have had the time to equilibrate into a minimal free energy state, possibly because average diffusion hoping times occurring in those syntheses (compared to synthesis times) are larger than under synthesis conditions that lead to single-crystals (e.g., fluxing agents used in single-crystal syntheses, geological conditions of natural samples), whereas all the single crystal samples have to some extent exploited the free energy advantage of sorting smaller and larger cations into distinct crystallographic sites in a geometric meso-octahedral sheet structure. These results suggest further experimental work in which the conditions for and kinetics of octahedral M1/M2 chemical ordering could be studied.

An iron sulfate of nominal composition Fe 2 (SO 4 ) 3 (H 2 O) 5 has been synthesized and its structure determined and refined by high resolution powder diffraction using synchrotron radiation. The structure consists of corrugated slabs in which iron octahedra are linked by sulfate tetrahedra in the monoclinic space group P2 1 /m with lattice parameters a = 10.711(1), b = 11.085(1), and c = 5.5747(5) Å, β = 98.853(3)°. We compare these results with the type specimen of lausenite from Jerome, Arizona, which has monoclinic lattice parameters a = 10.679(2), b = 11.053(3), and c = 5.567(1) Å, β = 98.89(1)°. Weight loss experiments show that it is currently a pentahydrate, despite earlier reports that lausenite is a hexahydrate. We argue that our synthetic material provides a structure determination for the type specimen of lausenite.

The purpose of this study was to replicate experimentally the growth of new rims around highly anorthitic plagioclase “core”. phenocrysts, analogous to the incorporation of xenocrysts into a silicic magma body through magma mixing. Aniakchak rhyodacite forms the bulk starting composition, and phase-equilibria experiments constrained the pre-eruption magma conditions to be ~110 MPa and 870.880 °C. The experimental runs were seeded with Great Sitkin anorthite (An 91-95 ) crystals. New rim growth of An 28-38 plagioclase occurred at rates between 3.5 (±0.3) × 10.10 to 60.6 (±20.0) × 10.10 cm/s at pressures and temperatures from 50 to 150 MPa and 825 to 880 °C. The values in parentheses are ±1σ standard deviation. Microlite crystallization (An 27-41 ) occurred in all experiments within the plagioclase stability field, and their growth rates varied from 4.4 (±1.3) × 10.10 to 65.7 (±10.1) × 10.10 cm/s. The rim and microlite growth rates are similar to one another within each experiment, and microlite number density (NV) is correlated approximately inversely with rim growth rates. Microlite crystallinities increased from 4.2 to 49.7 vol% as a function of increasing ΔT eff up to 95 °C. The results indicate growth-dominated crystallization at low ΔT eff , and nucleation dominated crystallization at high ΔT eff , in agreement with previous studies. Assuming the experiments apply to nature, the rim growth rates can provide a minimum estimate on how fast magma mixing can occur. Rims that are 10 to 100 μm wide can grow in ~10 days to 4 months, recording fast mixing timescales as long as eruption occurs shortly after mixing. The growth rate estimates presented here generally agree with those derived from sodic rims growing around anorthite cores after mixing between basalt and andesite prior to the 1996 eruption of Karymsky volcano, Kamchatka.

The optically active defects in as-grown, high-pressure high-temperature-treated (HPHT), borondoped, and synthetic diamonds (SD) grown with a nitrogen-getter, as well as of natural diamonds (ND), were characterized by absorption and luminescence spectroscopies using different excitation sources. The laser-excited photoluminescence (PL) spectra of SDs show numerous sharp lines characteristic for nickel-related centers, whereas NDs yield mainly broad PL bands. The emission from the nickel-related defects in NIR range increases and the maxima of the bands shift to lower energies with increasing temperature. Under UV and electron beam excitation, the yellow synthetic diamonds display green luminescence patterns along octahedral directions. The UV-excited PL spectra of the yellow SDs show a green band associated with nickel-related optical species. NDs display broad bands centered at ca. 450 nm that are related to complex nitrogen-related aggregates formed in the mantle in a long period of geological time. The cathodoluminescence (CL) spectra of SDs reveal many nickel-related and simple nitrogen-vacancy defects. Differences among as-grown, HPHT, and boron-doped diamonds can be observed. The CL bands of NDs are partly formed or caused by complicated nitrogen aggregates, like N3 and N4 centers. The CL band of NDs shifted from blue to green with increasing irradiation time. The deep-UV (DUV) excited time resolved PL spectra of NDs showed fast-decaying donor-acceptor pair recombination (DAPR) bands at around 410 nm. With longer delay and gate times, the broad band shifted to ca. 435 nm. All synthetic diamonds display luminescence from nickel-related defects centered at ca. 480 and 530 nm at room temperature (RT) and 77 K, respectively, and all SDs had a sharp luminescence band at 694 nm due to a Cr 3+ impurity in corundum inclusions.

We determined near-infrared (NIR) molar absorptivities of hydroxyl group (4500 cm -1 ) and molecular water (5230 cm -1 ) in rhyolitic glasses at room temperature and at high temperature, 400.600 °C. Five rhyolitic glasses with 0.5.2.8 wt% total water were analyzed by FT-IR microspectroscopy at room temperature and at 50 °C intervals in the range of 400 to 600 °C using a heating stage. The total water contents in the rhyolitic glasses were measured by Karl-Fischer titration. Based on these data, the linear molar absorptivities of the 4500 and 5230 cm -1 bands at room temperature were determined to be 1.42 ± 0.12 and 1.75 ± 0.08 L/mol⋅cm, respectively. The integral molar absorptivities obtained are 285 ± 24 and 239 ± 11 L/mol⋅cm 2 for the 4500 and 5230 cm -1 bands, respectively. These values can be used to determine the concentrations of hydroxyl group and molecular water in rhyolitic glass with 0.5.2.8 wt% total water at room temperature. The experimental results at high temperature show that the linear and integral molar absorptivities of the 4500 cm -1 band at 400.600 °C generally remain within 6% of the values at room temperature. On the other hand, the linear and integral molar absorptivities of the 5230 cm -1 band systematically decrease with increasing temperature in the ranges of 400 to 600 °C. These high-temperature molar absorptivities are useful for evaluating the water speciation in rhyolitic glass by in situ high-temperature IR microspectroscopy

The two low-temperature phase transitions of lawsonite have been studied using single-crystal X-ray diffraction from 86 to 318 K and a single-crystal high-frequency continuous-wave resonance technique from 323 to 102 K. While recently published data of the variations of strains, birefringence, and IR line widths are consistent with the (271 K) Cmcm-Pmcn transition being simply tricritical, our investigation of critical X-ray reflections and the six diagonal elastic constants of lawsonite reveals, consistently, a more complex crossover pattern in the temperature range of 205.225 K. Below 205 K the overall pattern is again in good agreement with a tricritical solution of the Cmcm-Pmcn transition and a second-order behavior of the (120 K) Pmcn-P2 1 cn transition. The structure determination from single-crystal X-ray data at 215 K reveals a possible orientational disorder of some of the hydroxyl groups in the Pmcn phase. From this and a recent strain analysis of deuterated and hydrogenated lawsonite we conclude that down to 205 K the Cmcm-Pmcn transition is driven by a displacive component, as observed in strain and birefringence data, plus an order/disorder component or dynamical effects associated with proton ordering. Below 205 K only the displacive component plays a role, and the (120 K) Pmcn-P2 1 cn transition is driven by a single order parameter. The remarkable elastic softening of C 66 ahead of the Cmcm-Pmcn transition indicates another orthorhombic-monoclinic transition, which is suppressed on cooling through the low-temperature phase sequence Cmcm-Pmcn-P2 1 cn, but can be observed on applying pressure to the mineral.

We have carried out in-situ X-ray diffraction experiments on high-pressure transformations of a Ca- and Fe- rich pyroxene (Ca 1.03 Mg 0.61 Fe 0.23 Al 0.14 Si 2 O 6 ) to investigate the stability of Ca 0.5 (Mg, Fe, Al) 0.5 SiO 3 perovskite (CM-perovskite) in a multi component system at about 32 GPa and up to 1900 °C. We observed that cubic CM-perovskite was formed at about 1300 °C and decomposed into cubic Ca-perovskites and orthorhombic Mg-perovskites and stishovite at 1800 °C when using a glass starting material. In another experiment using a crystalline pyroxene starting material, two cubic perovskites; Ca-perovskite and CM-perovskite, and orthorhombic Mg-perovskite formed simultaneously during the initial stage of the transformation. However, the cubic CM-perovskite subsequently decomposed into Mg- and Ca-perovskites and stishovite at 1200 °C. These results indicate that the assembly of cubic Ca-perovskite, orthorhombic Mg-perovskite and stishovite is stable and cubic CM-perovskite is a metastable phase at around 32 GPa and temperatures over 1000 °C in this system. Chemical analyses of product phases showed that Mg, Fe, and Al were preferentially partitioned into Mg-perovskite and the compositions of Ca-perovskite were close to pure CaSiO 3 . The present study shows that CM-perovskite nucleates during the initial stage of Ca(Mg, Fe, Al)Si 2 O 6 pyroxene transformation. Therefore, cold subducting slabs and impacted meteorites are the possible places in which CM-perovskite could exist. The Ca-rich glassy phase in a shocked chondrite (Tomioka and Kimura 2003) might have formed by vitrification of a metastable CM-perovskite-like phase.

Unusually large δ 18 O heterogeneities (≥4‰) within single crystals are reported in garnets from dioritic migmatites in the Pyrenees. These heterogeneities, together with contrasting Ca and P zoning, allow the identification of different growth zones. Garnet cores with high δ 18 O values (12-14‰) are relatively poor in Ca (7.9 mol% Grs) and rich in P (400-900 ppm P 2 O 5 ). In contrast, garnet rims with lower δ 18 O values (7-12 ‰) are richer in Ca (12-14% Grs) and poorer in P (100-200 ppm). These growth zones can be ascribed to a metamorphic event followed by crustal partial melting and contamination by magmas from the mantle. High δ 18 O intra-crystalline contrasts result from mineral growth in an open magmatic system involving the interaction of partial melts with distinct δ 18 O signatures. At the garnet core-rim interface, compositional profiles in major divalent cations are consistent with the relaxation of an initial sharp step in Ca, Fe, and Mg by Ca ↔ (Fe, Mg) interdiffusion. At the same interface, an O-isotope profile is documented. The analogy of Ca and O isotope profiles suggests that the δ 18 O distribution may also result from a diffusion process. In this particular case (temperature, garnet composition, oxygen fugacity), O diffusion appears to be of the same order of magnitude as Ca ↔ (Fe, Mg) interdiffusion. Considering a duration of 10 Ma for the plutono-metamorphic event in the Pyrenees, Ca and O diffusivities in the range 10.22 m2/s (at 850 °C) are retrieved from the measured profiles. Like Ca, O diffusion in garnet at magmatic temperatures (850.900 °C) is both slow enough to preserve large δ 18 O heterogeneities and fast enough to generate relaxation profiles.

Spinel [(Mg, Fe 2+ )(Cr, Al, Fe 3+ ) 2 O 4 ] with an inhomogeneous texture was observed in a chromitite section from the Iwanai-dake complex, Hokkaido, Japan, which mainly consists of highly depleted Alpine-type peridotite. The inhomogeneous spinel identified under reflected-light microscopy has speckled and symplectic textures in the chromitite layer adjacent to dunite. On the other hand, spinel in the surrounding dunite and harzburgite is homogeneous. The inhomogeneous spinel comprises two different phases: an MgAl-rich spinel and an Fe-rich spinel with wide variation of Cr content. The compositional relationships of contiguous phases correspond to the solvus at 600 °C, which suggest that the inhomogeneous spinel was formed by an unmixing process due to the miscibility gap between Al-rich and Fe 3+ -rich phases. The original spinel composition in the chromitite layer, estimated from bulk spinel composition, is intermediate between magnesioferrite (MgFe 2 3+ O 4 ) and magnetite (Fe 2+ Fe 2 3+ O 4 ). Such an Fe 3+ -rich spinel composition indicates that the chromitite was formed under highly oxidized conditions at magmatic temperatures. A highly oxidized hydrous silicate melt, such as minette or absarokite, generated by melting of hydrous mantle, possibly reacted with peridotite in the mantle wedge and crystallized the Fe 3+ -rich spinel.

Pink, Mn-bearing “oxy-rossmanite” from a pegmatite in a quarry near Eibenstein an der Thaya, Lower Austria, has been characterized by crystal structure determination, chemical analyses (EMPA, SIMS), and optical absorption and infrared spectroscopy. Crystal structure refinements in combination with the chemical analyses give the optimized formulae X(⃞ 0.53 Na 0.46 Ca 0.01 ) Y(Al 2.37 Li 0.33 Mn 2+ 0.25 Fe 2+ 0.04 Ti 4+ 0.01 ) Z Al 6 T (Si 5.47 Al 0.28 B 0.25 )O 18 (BO 3 ) 3 V[(OH)2 .85 O 0.15 ] W [O 0.86 (OH) 0.10 F 0.04 ], with a = 15.8031(3), c = 7.0877(3) Å, and R = 0.017 for the sample with 2.05 wt% MnO, and X(⃞ 0.53 Na 0.46 Ca 0.01 ) Y (Al 2.35 Li 0.32 Mn 2+ 0.28 Fe 2+ 0.04 Ti 4+ 0.01 ) Z Al 6 T (Si 5.51 Al 0.25 B 0.24 )O 18 (BO 3 ) 3 V [(OH) 2.80 O 0.20 ] W [O 0.86 (OH) 0.10 F 0.04 ] for a sample with a = 15.8171(3), c = 7.0935(2) Å, R = 0.017, and 2.19 wt% MnO. Although the structure refinements show significant amounts of [4] B, the <T-O> bond-lengths (~1.620 Å) mask the incorporation of [4] B because of the incorporation of [4] Al. The <T-O> distances, calculated using the optimized T site occupancies, are consistent with the measured distances. This “oxy-rossmanite” shows that it is possible to have significant amounts of [4] B and [4] Al in an Al-rich tourmaline. The “oxy-rossmanite” from Eibenstein has the highest known Al content of all natural tourmalines (~47 wt% Al 2 O 3 ; ~8.6 apfu Al). The nearinfrared spectrum confirms both that hydroxyl groups are present in the Eibenstein tourmaline and that they are present at a lower concentration than commonly found in other lithian tourmalines. The integrated intensity (850 cm -2 ) of the OH bands in the single-crystal spectrum of “oxy-rossmanite” from Eibenstein is distinctly lower than for other Li-bearing tourmaline samples (970-1260 cm -2 ) with OH contents >3.0 pfu. These samples fall on the V site = 3 (OH) line in the figure defining covariance of the relationship between the bond-angle distortion (σ oct 2 ) of the Z O 6 octahedron and the <Y-O> distance. On a bond-angle distortion-<Y-O> distance diagram “oxy-rossmanite” from Eibenstein lies between the tourmalines that contain 3 (OH) at the V site, and natural buergerite, which contains 0.3 (OH) and 2.7 O at the V site. No H could be found at the O1 site by refinement, and the spherical electron density in the difference-Fourier map around the O1 site supports the conclusion that this site is mainly occupied by O. The pink color comes from the band at 555 nm that is associated with Mn 3+ produced by natural irradiation of Mn 2+ . This is the first time a tourmaline is described that has a composition that falls in the field of the previously proposed hypothetical species “oxy-rossmanite”.

The lattice energies for a variety of compounds that can be classified as double salts are calculated by summing the lattice energies of the constituent simple salts. A comparison with the lattice energies obtained from the Born-Haber or other thermodynamic cycles shows that the simple salt approximation reproduces these values generally to within 1.2%, even for compounds that have considerable covalent character. Application of this method to the calculation of the lattice energies of silicates, using the sum of the lattice energies of the constituent oxides are, on average, within 0.2% of the value calculated from the experimental enthalpies of formation. The implications of the simple salt approximation for the thermodynamics of geochemically important processes are discussed.

The activity of alkali metal oxides can be controlled in one-atmosphere wire-loop experiments at high temperature by suspending a crucible containing alkali silicate melt beneath the samples. The method has been applied to measuring the activity coefficient of NaO 0.5 in a series of CMAS-NaO 0.5 melts relative to that in the anorthite-diopside eutectic composition at 1400 °C, using a reservoir of NaO 0.5 -SiO 2 melt. The results show that this relative activity coeficient decreases strongly with SiO 2 , increases with CaO and MgO, but is insensitive to AlO 0.5 This latter behavior is inconsistent with “quasi-crystalline” models of melt thermodynamics that hypothesize Na-Al species.

The solubility of rutile in H 2 O has been measured at 1000-1100 °C, 1.2 GPa. The data indicate that solubility is very low over the investigated range, with a maximum of 4.7 millimol/kg H2O at 1100 °C, 2 GPa. The data were fit with the equation log m Ti = 4.892.10470/T + 0.1923P, where m Ti is Ti molality, T is in Kelvins, and P in GPa. When compared to previous results, the new data indicate substantially lower solubility, opposite pressure dependence, and thermodynamic properties of the reaction rutile = TiO 2,aq that are now consistent with other oxide hydrolysis reactions. Calculations of Ti transport during mantle metasomatism by H 2 O in subduction zone environments predict much lower Ti mobility at all conditions. These results offer strong support for models of Ti retention in eclogites during slab devolatilization, and require that examples of significant Ti mass transfer be explained by complexing agents in solution, most likely aluminosilicate complexes.

A series of synthetic pyrope-grossular garnets (Mg 3 Al 2 Si 3 O 12 -Ca 3 Al 2 Si 3 O 12 ) were investigated by powder X-ray synchrotron radiation at 5 K to determine their microscopic structural strain, which may be responsible for the observed excess enthalpy of mixing for this binary. This substitutional solid solution provides an excellent system for investigating microscopic-macroscopic relationships and the physical nature behind non-ideal thermodynamic mixing behavior in silicates, because of the measurable nonidealities shown by its enthalpy and volume of mixing. An analysis of the X-ray refection profiles, based on theoretical considerations of X-ray line broadening, permits for the first time a direct experimental determination of crystallite size and the root-mean-square structural strain for a mineral solid solution. The measured microscopic strain shows positive and asymmetric deviations from linearity across the join with the largest excess in pyrope-rich compositions. There is a good correlation between the structural strain and the macroscopic enthalpy of mixing behavior for pyrope-grossular garnets as measured by calorimetry.

An ~1:1 mixture of ferrihydrite and nanocrystalline akaganeite (β-FeOOH; 10.15 nm) was incubated with Shewanella putrefaciens (strain CN32) under anoxic conditions with lactate as an electron donor and anthraquinone-2,6-disulfonate (AQDS) as an electron shuttle. The incubation was carried out in a 1,4-piperazinediethanesulfonic acid (PIPES)-buffered medium, without PO 4 3- at circumneutral pH. Iron reduction was measured as a function of time (as determined by 0.5 N HCl extraction), and solids were characterized by X-ray diffraction (XRD), electron microscopy, and Mössbauer spectroscopy. The biogenic reduction of Fe 3+ was rapid; with 60% of the total Fe (Fe TOT ) reduced in one day. Only an additional 10% of Fe TOT was reduced over the next three years. A fine-grained (~10 nm), cation-excess (CE) magnetite with an Fe 2+ /Fe TOT ratio of 0.5-0.6 was the sole biogenic product after one day of incubation. The CE magnetite was unstable and partially transformed to micrometer-sized ferrous hydroxy carbonate [FHC; Fe 2 (OH) 2 CO 3 (s)], a rosasite-type mineral, with time. Ferrous hydroxy carbonate dominated the mineral composition of the three year incubated sample. The Fe 2+ /Fe TOT ratio of the residual CE magnetite after three years of incubation was lower than the day 1 sample and was close to that of the stoichiometric magnetite (0.33). To the best of our knowledge, this is the first report of biogenic FHC, and was only reported twice in literature but in a very different context. Ferrous hydroxy carbonate appeared to form by slow reaction of microbially produced carbonate with Fe 2+ -excess magnetite. The FHC may be an overlooked mineral phase that explains the infrequent occurrence of fine-grained, biogenic magnetite in anoxic sediments.