Band 83, Heft 9-10

The high pressure phase transitions and melting of the mantle mineral MgSiO 3 with the perovskite structure were investigated using molecular dynamics (MD) simulations of a large system of atoms on a parallel computer. The simulations reveal an orthorhombic to cubic transition accompanied by a sharp increase in diffusion of the O atoms. The phase transition and melting temperature depend sensitively on the level of defects in the solid. At pressures of the Earth’s lower mantle, the transition is found to occur at temperatures substantially higher than the mantle temperatures. Therefore, any seismic discontinuity in the lower mantle may not be related to a phase transition of the perovskite structure, but instead may be due to chemical changes at that depth, in contrast to currently accepted mineralogical models assuming chemical homogeneity of the lower mantle.

We have estimated the water content of Mars’ interior by using analyzed water contents of kaersutite inclusions from shergottites nakhlites chassignites (SNC) meteorites in conjunction with an experimentally-derived crystal-chemical model of kaersutite amphibole. This model predicts quantitatively the relationships between iron oxidation and hydrogen deficiency in the kaersutite. The H 2 O content of the magma from which the kaersutites in SNC meteorites could have crystallized is in the 100-1000 ppm range. That H 2 O content leads to an estimated water content of 1-35 ppm for a Martian mantle that could have been the source rock for such magmas.

An atomistic computer simulation study was undertaken to address the role of Al in the substitution of ferric iron into magnesium silicate perovskite, MgSiO 3 . Calculated substitution and vacancy energies at 0 K were used to consider two different substitution mechanisms: (a) Fe 3+ and Al independently charge balanced by vacancies or by a similar cation in another site, or (b) Fe 3+ and Al charge balancing each other. Our results show that it is preferential by 1 to 2 eV for Fe 3+ to substitute into an Mg site charge balanced by an Al substitution into an Si site. This is increasingly favorable if the Fe 3+ and Al substitute into adjacent sites. The coupled substitution occurs because it is energetically unfavorable for Al to enter the dodecahedral site.

Calcic amphiboles were synthesized from a natural mid-ocean ridge basalt (MORB) in 39 experiments representing 24 sets of pressure-temperature (P-T) conditions ranging from 650-950 °C, 0.8-2.2 GPa, at ƒ O₂ controlled by the fayalite-magnetite-quartz (FMQ) buffer, and P aqueous fluid = P total . Experiments lasted up to 1630 h at low temperatures; in all cases, synthesized hornblendes were coarse-grained (5-7 × 10-15 μm) and chemically homogeneous. Over the investigated pressure range, Ca-amphibole coexisting with phases rich in Al and Ti gradually changes composition from sodic-calcic, Si-rich at low temperatures to calcic, Si-poor at high temperatures: it is barroisite at 650 °C, edenite at 700 °C, and pargasite at 800-950 °C. Electron microprobe data were combined with 41 comparable analyses from the literature for Ca-amphiboles synthesized from MORBs at intermediate ƒ O₂ in order to erect a petrogenetic grid for the experimental range 0.0-2.2 GPa, 450-1050 °C. Isopleths for Al 2 O 3 in Ca-amphibole exhibit markedly negative P-T slopes, indicating increasing Al 2 O 3 contents with both P and T. In contrast, TiO 2 isopleths are nearly independent of P, demonstrating that TiO 2 in Ca-amphibole increases almost exclusively as a function of T. For natural metabasaltic assemblages that contain coexisting Al-rich and Tirich phases, and closely approached chemical equilibrium under crustal or uppermost mantle conditions, this semiquantitative petrogenetic grid allows the simultaneous assignment of attendant P and T employing Ca-amphibole Al 2 O 3 and TiO 2 contents. However, during slow cooling, natural Ca-amphiboles may exsolve TiO 2 as rutile, titanite, and/or ilmenite, but in general do not redistribute Al 2 O 3 , so this thermobarometer must be applied with caution to inhomogeneous specimens.

The intergranular fluid involved in solid-solid reactions is tacitly assumed to be a melt or a (C-O-H-S-Cl-F)-bearing phase. We have studied the system olivine+metal using diffusion couple experiments, in situ reaction progress monitoring using Knudsen-cell mass spectrometry, and thermodynamic-kinetic analysis to show that a dry vapor phase coexisting with solids (silicates, oxides, metals) has all the characteristics of a classical petrologic ‘‘intergranular fluid,’’ and it is a viable transport agent for major rock-forming elements such as Mg, Fe, or Si in many petrologic situations. Some of the major conclusions of the work are: (1) ignoring the vapor phase leads to incorrect estimation of degrees of freedom and consequently, incorrect interpretations of mineral assemblages and zonation; (2) normally refractory elements such as Mg may in some cases be more volatile than O 2 ; and (3) reaction modeling using free-energy minimization allows the main parameters controlling reaction progress, pathway, and products (assemblage, abundance of phases, and composition) to be identified. These parameters include: available reactive surface area; volume of the reaction system; diffusion rates in the product solid; temperature; and relative rates of reaction to transport (in/out of the system). Components other than those appearing explicitly in the mass-balance equations (e.g., ƒ O₂ in the olivine+metal system) O 2 may play an important role. Transport of Mg in the vapor phase away from local reaction sites explains the compositional zoning of olivine around FeNi-metal inclusions and simultaneously provides a mechanism for the growth of at least some of the fayalite-rich rims in Allende and other meteorites of the CV3-class. Similar considerations may play a role in terrestrial problems where metal and silicate coexist, e.g., the primitive terrestrial magma ocean and the ‘‘D’’ layer.

Problems associated with intermittent and variable degrees of sample blackening are often reported for studies involving the preparation of CO 2 -bearing silicate glasses in piston- cylinder apparatus. This phenomenon is generally attributed to H infiltration, which leads to the reduction of CO 2 and the precipitation of graphite with the concomitant formation of water. In this study we demonstrate that carbon diffusion into platinum capsules may be a common cause of blackened glasses and this process may be detected using fourier transform infrared spectroscopy (FTIR) to identify the presence of CO without elevated H 2 O contents. The simulated infiltration of 12 C from a graphite furnace into a 13 C -bearing sample is illustrated using secondary ion mass spectroscopy (SIMS) and micro- FTIR analysis. Careful FTIR monitoring of variable sample reduction has helped to identify the precautions required to reduce C (and H) infiltration in solid media assemblies and it appears that physical barriers can be more important than the chemical buffers traditionally employed.

The hydrothermal diamond-anvil cell (HDAC) was employed as a pressurized fluid inclusion heating stage to determine temperatures of phase transitions in synthetic fluid inclusions in quartz. Using this technique, the common problem of decrepitation or stretching of inclusions having high internal pressures was eliminated. Homogenization temperatures of pure H 2 O synthetic inclusions determined in the HDAC are inversely related to the confining pressure exerted on the sample, suggesting a decrease in inclusion volume with increasing confining pressure. The very good reproducibility and reversibility of these experiments indicate that volume changes during heating under confining pressure in the HDAC are elastic in nature. However, results of microthermometric experiments in the HDAC indicate that the change in homogenization temperature is significantly larger than would be predicted by the equations of state for quartz and water. This difference reflects an additional component of volume change related to stress within the quartz host, causing displacement of the inclusion walls into the inclusion cavity, as predicted by theoretical models describing the behavior of inclusions in minerals. Liquid-vapor homogenization temperatures [Th(L-V)] and halite dissolution temperatures (Tm halite) were determined for synthetic fluid inclusions in the ternary H 2 O-NaCl- CO 2 . The measured homogenization temperatures were regressed as a function of confining pressure at Th(L-V). The intersection of the Th(L-V)-confining pressure curve with an independently obtained P-T curve for the solvus of the same ternary composition provides the ‘‘correct’’ homogenization temperature, that is, the homogenization temperature corresponding to a confining pressure equal to the pressure on the solvus at that same temperature. This method for determining corrected Th(L-V) is based on the assumption that the effect of elastic stress on Th(L-V) approaches zero as the confining pressure approaches the internal pressure in the inclusion at homogenization. The Th(L-V) values at these intersection points were used to calculate lines of constant homogenization temperature (iso-Th lines). For a composition of H 2 O + 40 wt% NaCl + 10 mol% CO 2 , the iso-Th slopes decrease from about 53 bar/°C at Th(L-V) = 500 °C to 8.5 bar/°C for Th(L-V) = 650 °C. The halite dissolution temperature (in the presence of liquid and vapor) averages 342 °C (range ±6 °C) without recognizable pressure dependence. The slopes of iso-Th lines for a composition of H 2 O + 20 wt% NaCl + 20 mol% CO 2 decrease from approximately 23 bar/°C at Th(L-V) = 475 °C to 6 bar/°C for Th(L-V) = 600 °C. Using the HDAC technique, the high-pressure portion of the halite liquidus was redetermined for an H 2 O-NaCl solution containing 40 wt% NaCl. These new measurements confirm the data of Bodnar (1994), which were obtained using a gas-flow heating stage at 1 atm confining pressure. This indicates that Bodnar’s (1994) observation of an essentially pressure independent liquidus at pressures ≳2 kbar is real and not a result of inclusion stretching

Experiments reported herein document heterogeneous crystal nucleation on bubbles in supercooled lithium disilicate melt. Crystalline lithium disilicate (Li 2 Si 2 O 5 ) nucleated and grew on small bubbles (∼1 μm) with a one-to-one correspondence between the number of bubbles and crystals (ranging from <10 2 to ∼10 5 bubbles/mm 3 ). Crystals grew on large bubbles (>100 μm) only in samples fused in N 2 , suggesting a chemical control on nucleating efficiency. Bubbles ∼1 μm in diameter served as nucleation sites for polycrystalline lithium disilicate spherulites; bubbles smaller than ∼1 μm served as nucleation sites for the more common ellipsoidal crystalline form. This difference in behavior might be due to the additional surface area available for crystal nucleation on the 1 μm bubbles. Our findings suggest that superliquidus thermal history can influence crystal nucleation via bubble formation induced by supersaturation, and has implications for both natural samples and experimental studies. Heterogeneous crystal nucleation on bubbles may serve as an efficient nucleation mechanism in natural degassing magmas and may aid in the formation of fine-grained groundmasses common to many volcanic rocks. Furthermore, we have documented a new mechanism of spherulite formation in highly supercooled silicate melt, similar to conditions thought to exist during devitrification of natural glasses. The ability of crystals to nucleate on bubbles can be exploited in the production of commercial glass-ceramic materials.

The shear viscosities of melts in the system Na-Fe-Si-O-F-Cl were determined over a wide range of temperatures (400-1200 °C) at 1 atm pressure in air. The compositions are based on the addition of Fe 2 O 3 , FeCl 3 , and FeF 3 to a base melt composition corresponding to sodium disilicate (Na 2 S i 2O 5 ). Viscosities were determined using concentric cylinder and micropenetration methods and measurements span the range of 10 0.5 to 10 11 Pa·s. The chemical compositions of these melts were analyzed after the viscometry determinations. The iron is fully oxidized under the conditions of the viscometry. Although F and especially Cl are volatile elements in silicate melts, levels of Cl and F up to over 3 and 4 wt%, respectively, were stabilized in these melts, assisted presumably by the presence of Fe 3+ . Although some volatilization occurred during the original synthesis of these samples, none occurred during viscometry. The anionic substitutions Cl 2 O -1 and F 2 O -1 have very different influences on the viscosity. The F 2 O -1 substitution causes a drastic decrease in viscosity over the entire investigated range whereas the Cl 2 O -1 substitution causes a much smaller decrease in viscosity in the high viscosity range and a slight increase in viscosity in the low viscosity range. As a consequence, minor to major element abundance of Cl in strongly peralkaline undersaturated volcanic rocks are not likely to significantly influence melt viscosity.

25 Mg NMR spectra for several silicate and aluminosilicate melts were obtained from 1000-1470 °C. The peaks are initially very broad, but narrow with increasing temperature to near 500 Hz at the highest temperatures. The peak positions for most of the melts do not shift noticeably with temperature in the range studied, except for a sodium magnesium silicate composition that was previously studied by Fiske and Stebbins (1994). This material showed a decrease in frequency of the peak position by about 4 ppm between 1150-1360 °C in both this and the previous study, consistent with an increase in the average size of the site. The chemical shifts vary with composition as well, ranging from 31 ppm for a potassium sodium magnesium silicate melt to 22 ppm for diopside melt (CaMgSi 2 O 6 ) at 1400 °C. Compositions with higher field strength cations have lower frequency chemical shifts, which correspond to larger coordination numbers and bond lengths for Mg 2+ . All of the peak positions obtained fall to slightly higher frequency than the range for sixfoldcoordinated Mg in crystals and well below the fourfold-coordinated range, indicating that the Mg is in fivefold to sixfold coordination in the melts. Spin-lattice relaxation times show that measurements are on the high-temperature side of the T 1 minima, and a simple expression for quadrupolar relaxation can be used to obtain correlation times for the motion responsible for the relaxation. The correlation times obtained in this manner are very similar to the correlation time τ shear obtained from viscosity measurements, implying that the Mg motion is strongly coupled to the network motion at these temperatures. Line widths also scale with T 1 in this temperature range, leading to the conclusion that the viscosity is the fundamental limit to observing the 25 Mg signal in the melt.

Unit-cell parameters of zoisite, Ca 2 Al 3 Si 3 O 12 (OH), have been measured at simultaneously high pressures and temperatures (up to 6.1 GPa and 800 °C) in a Walker-style multi-anvil apparatus at the synchrotron radiation source at Daresbury Laboratory, U.K. Measurements were made in a series of heating cycles at increasing loads. Sample pressure, measured using an internal NaCl standard, increased during heating. Cell parameters vary smoothly with pressure and temperature; individual expansivities and compressibilities vary in the order c > b > a. Isothermal bulk moduli were calculated from the volumes measured at 30, 200, 400, 600, and 800 °C by fitting the Murnaghan equation of state to each isothermal data set. This assumes K′ = 4. Ambient-pressure volumes calculated from previous measurements of thermal expansivity of zoisite were included in the Murnaghan fits. A linear fit of the bulk moduli with temperature gave values for the bulk modulus at 298 K, K 298 = 125(3) GPa, and its variation with temperature, ∂K T /∂T = -0.029(6) GPa K -1 . K 298 is slightly higher than the recent value for a single crystal in a diamond-anvil cell, indicating a lower maximum pressure stability of zoisite than would be calculated using that value. Our data allow zoisite volumes to be calculated at P-T conditions relevant to the Earth and show that, in a typical subduction zone, zoisite becomes more dense as subduction proceeds, helping to stabilize it to high pressures.

Multianvil experiments were conducted at 1400 to 1600 °C on olivine and peridotite starting compositions to determine the partitioning of Ti, Al, Cr, Ni, Ca, and Na between coexisting olivine and wadsleyite. All of these elements occur as minor amounts in mantle olivine. Our experiments indicate that all, except Ca, partition preferentially into wadsleyite relative to olivine. The order of preference for wadsleyite is Ni<Na<Cr<Ti<Al, with D tr wad/ol of about 2 for Ni, 3 for Na, and between 5 and 8 for Cr, Ti, and Al. We observe a tr strong negative correlation between the Si and Cr (+Al) contents of wadsleyite, indicating a coupled substitution of 2Cr 3+ for Mg 2+ + Si 4+ . Modeling the influence of the trace elements on the olivine-wadsleyite transformation in the mantle indicates broadening effects on the order of 1-3 km, much smaller than that predicted to arise from mantle concentrations of H 2 O. Therefore, effects of these trace elements on the properties of the 410 km seismic discontinuity are considered negligible. The maximum solubility of TiO 2 in wadsleyite (about 0.6%) is consistent with the suggestion that olivines containing about 1 vol% FeTiO 3 could be inverted wadsleyite.

Local structure analysis of Al-containing magnesium silicate perovskite has been carried out with X-ray absorption spectra recorded at the Mg, Al, and Si K-edges using the SA32 beam-line of SuperAco (Orsay, France). The Al-XAFS spectrum of (MgSi) 0.85 Al 0.3 O 3 perovskite (synthesized in a multi-anvil apparatus) cannot be explained by assuming that Al 3+ occurs in octahedral or dodecahedral sites only. This conclusion is based on comparison between Al-spectrum and those recorded at the Mg and Si K-edges for the same structure, general trends found for Al-spectra in various atomic sites, and ab-initio calculations using the FEFF-6 code. Thus, Al appears to be partitioned between both octahedral and dodecahedral perovskite sites. However, the structural accommodations needed to stabilize Al 3+ cation in such different sites are not straightforward. Also, Mg K-edge spectra in enstatite and perovskite were compared with those previously reported at the Fe K-edge for the same structures, confirming that these two elements are located in the same polyhedra in both structures, and thus that Fe 2+ enters the dodecahedra of the silicate perovskite.

Ab initio, molecular orbital calculations for two different Hartree-Fock basis levels were performed on clusters in the system Al-O-H, and tested by comparing derived vibrational frequencies to the measured values for aluminum oxides and aluminum oxyhydroxide minerals. Models were chosen to reflect surface groups that may be present on aluminous minerals such as α-Al 2 O 3 (corundum) and Al(OH) 3 (gibbsite). Protonation and deprotonation reactions on bridging and terminal oxygen or hydroxyl sites were modeled to investigate the effects of solution pH on the structure of surface groups. Relative deprotonation energies, including solvation effects, were calculated for each site using the self-consistent isodensity polarized continuum model of Keith and Frisch (1994), and then used to predict the order in which each particular surface group protonates.

Pyrite has a poor {001} cleavage. Unlike most other minerals with a rocksalt-type structure, pyrite typically fractures conchoidally, demonstrating that parting surfaces are not constrained to the {001} crystallographic plane. Cleavage along {001} require rupture of only Fe-S bonds, but pyrite consists of both Fe-S and S-S bonds. Analysis of bond energies indicates that S-S bonds are the weaker bonds and they are likely to be ruptured when pyrite is fractured. With each ruptured S-S bond, two mononuclear species (formally S 1- ) are produced, one bound to one fracture surface and the second to the opposite fracture surface. This monomer is reduced to S 2- (monosulfide) during relaxation through oxidation of surface Fe 2+ ions to Fe 3+ . These surface relaxation processes explain the surface states observed in S(2p) and Fe(2p 3/2 ) X-ray photoelectron spectra (XPS) of pyrite. The S(2p) XPS spectrum is interpreted to include bulk disulfide contributions at 162.6 eV and two surface state contributions at 162.0 and 161.3 eV. The monosulfide (S 2- ) emission is near 161.3 eV, as observed in S(2p) spectra of pyrrhotite, and the 162 eV peak is interpreted to result from the surface-most sulfur atom of surface disulfide ions. The Fe(2p 3/2 ) XPS spectrum includes three contributions, a bulk Fe 2+ emission near 707 eV and emissions from two Fe surface states. One surface state is interpreted to be Fe 2+ surface ions. Their coordination is changed from octahedral before fracture to square pyramidal after fracture. The consequent stabilization of the antibonding Fe d 2 z orbital yields unpaired electrons in the valence band resulting in multiplet peak structure in the Fe(2p 3/2 ) spectrum. Similarly, each surface Fe 3+ ion, having contributed a non-bonding 3d electron to the valence band (bonding orbital), contains unpaired 3d electrons, resulting in multiplet splitting of its Fe(2p 3/2 ) signal. The high-energy tail observed in the Fe(2p 3/2 ) spectrum of pyrite is the product of emissions from both surface states with Fe 2+ multiplet peaks centered near 708 eV and the surface Fe 3+ multiplets spanning the binding energies from 708.75 to about 712 eV.

Atomic-resolution transmission electron micrographs show that nanocrystalline TiO 2 coarsens by oriented attachment and growth under hydrothermal conditions. In addition to forming homogeneous single crystals, attachment at anatase surfaces leads to twinning on {112} and intergrowths on (001) and {001}. Brookite, a polytype of anatase, occurs at some {112} twin surfaces. Alternating two octahedra-wide structural slabs in brookite are shared with the two adjacent anatase twin domains. Because {112} anatase twin interfaces contain one unit cell of brookite, we propose that brookite may nucleate at twin planes and grow at the expense of anatase. Alternatively, anatase-brookite interfaces may form by oriented attachment of primary brookite and anatase {112} crystallites. In this case, three unit cell-wide lamellae of brookite are interpreted as remnants of larger crystals that partly converted to anatase by propagation of the anatase-brookite interface. Which phase is stable is unclear over this particle size range, and products of random thermal fluctuations may be preserved by quenching. Regardless of reaction direction, polytypic interconversion of anatase and brookite essentially involves displacement of Ti (by c/4 brookite) into adjacent octahedral sites in one of the pair of two octahedra-wide structural slabs. The results have broad relevance for nucleation and growth models as they suggest that twinning and polytypism in macroscopic crystals can originate at oriented interfaces between primary nanocrystalline particles early in their crystallization history.

Radiogenic impurities of 400 to 800 ppm U and Th in titanite, CaTiSiO 5 , lead to moderate radiation damage (≈1.5 × 10 18 α-decay events/g) and therefore to partial amorphization (≈30%). Powder X-ray diffraction on such damaged titanite from the Cardiff locality in Canada shows that two modifications of the crystalline material coexist. Both modifications are structurally β phase but differ systematically in their lattice parameters and also in their chemical composition. One modification exhibits strong particle size broadening in X-ray diffraction patterns, whereas it is almost unstrained with respect to fully annealed titanite. The other modification shows large strain broadening and increased specific volume (about 3%) due to a high concentration of defects. The unstrained modification consists of small nucleation centers in the damaged material, and it grows when the sample is annealed. At annealing temperatures above 823 K, this modification dominates rapidly and replaces the strained titanite. The results of Rietveld refinement of the annealed samples and of the time evolution of isothermal annealing studies are discussed. The analysis of volume strain and of structural strain resulting from the peak profiles suggests a temperature-dependent activation energy for the recrystallization process, with E A ≈ 380 kJ/mol at T > 873 K and E A ≈ 500 kJ/mol at temperatures 773 K < T < 873 K.

The temperature dependence of the cation distribution in synthetic hercynite (FeAl 2 O 4 ) has been determined using in-situ time-of-flight neutron powder diffraction. The sample was synthesized from the oxides under controlled oxygen fugacity and then quenched in air. Neutron diffraction patterns were then collected under vacuum on heating from room temperature to 1150 °C, and the cation distribution was determined directly from site occupancies obtained by Rietveld refinement. The degree of inversion, x, decreased from 0.135(4) at room temperature to 0.112(4) at 600 °C. Thereafter the degree of inversion increases smoothly with increasing temperature, reaching a value of 0.219(5) at 1150 °C. The decrease in x on heating to 600 °C is a kinetic phenomenon caused by the system moving toward its equilibrium degree of order from the relatively disordered state maintained after quenching from the synthesis temperature. The equilibrium ordering behavior between 600 and 1150 °C has been analyzed using both the O’Neill-Navrotsky and Landau thermodynamic models. Although the data could be fitted with both models over the temperature range of the measurements, Landau theory predicts the incorrect curvature of the equilibrium x-T curve, leading to a significant discrepancy in the calculated behavior when the model is extrapolated outside the calibrated temperature range. The correct x-T curvature is predicted by the O’Neill-Navrotsky model, and values of the model coefficients α = 31.3 ± 1.1 kJ/mol and β = 19.7 ± 3.4 kJ/mol were obtained by least-squares fitting to the equilibrium data. This confirms the results of a previous study using quenched material, which suggested that the sign of the β coefficient in FeAl 2 O 4 is opposite to that found in other 2-3 oxide spinels.

Small syngenetic exhalative Fe-Mn deposits embedded in Triassic marbles of the Suretta, Starlera, and Schams nappes (Eastern Swiss Alps) were subjected to a Tertiary regional metamorphism under blueschist- to greenschist-facies conditions. In one of the deposits (at Fianel, Val Ferrera), this polyphase metamorphism led to the formation of quartz+dolomite veinlets containing beryl, scheelite-powellite, paraniite-(Y), monazite- (Ce), fluorapatite, bergslagite, fluor-roméite, and antimonian betafite; these veinlets crosscut dolomite breccia lenses that are embedded in hematite-quartz-carbonate ores. Scheelitepowellite displays a continuous range of compositions between 28 and 70 mol% CaMoO 4 ; its As 2 O 5 contents range from 0.73 to 3.96 wt%, and are positively correlated with the Y 2 O 3 contents that vary between 0.33 and 2.47 wt%. The scheelite-powellite grains display a two-stage chemical zoning: stage A generally produced a core and a rim that, relative to the core, is richer in W, As, and Y. During the second stage (stage B), W-rich scheelitepowellite replaces stage-A grains along fractures and rims. Crystals of paraniite-(Y), ideally (CaWO 4 ) 2 ·YAsO 4 , occur as small inclusions (=1 μm) in stage-B scheelite-powellite. The Fianel deposit is only the second locality where paraniite-(Y) has been reported. The paraniite-(Y) from Fianel displays, like the type material, no polysomatic stacking fault in the scheelite-YAsO 4 layering. At Fianel, paraniite-(Y) is characterized by elevated Mo contents, and seems to have crystallized under influence of W- and LREE-rich fluids during stage B, i.e., during the metasomatic replacement of Y- and As-rich scheelite-powellite produced in stage A.

A hydrous zinc- and sodium-rich hydroxy-chlorosulfate, discovered in a sulfide sample collected by the Deep Sea Recovery Vehicle (DSRV) Alvin in 1984, is identified as gordaite, Zn 4 Na(OH) 6 SO 4 Cl·6H 2 O, recently described as a new mineral species from Antofagasta, Chile. Results of re-examination of the original Alvin dive sample from the Juan de Fuca Ridge, northeastern Pacific Ocean, and additional data on gordaite, including vibrational and luminescence spectroscopy, X-ray diffractometry and thermal analysis, are presented.

Thin sections of breast tissues from patients with possible tumors also contained closely associated mineral deposits. Using scanning electron and backscattered electron microscopy (SEM-BSE) energy and wavelength dispersive chemical analyses, we found the deposits ranged from <1 μm to 50 μm in size and were composed predominantly of calcium phosphate. Semi-quantitative chemical determinations indicate that the mineral exhibits different Ca/P ratios depending on the site of deposition. This high-resolution methodology to chemically analyze precipitates in human tissues is a novel approach that may provide some new insights into mineral deposition in soft tissues. Application of the technique to establish compositional biases in a wide variety of tissues seems warrented. Specifically, additional data on mineral deposits in breast tissues could contribute toward the understanding of the localized changes and to the development of disease at this site.

Algorithmic modifications to the MELTS software package are presented in order that calculations of heterogeneous phase equilibria can be performed in the subsolidus. Methods are presented for: (1) selecting an ‘‘initial guess assemblage’’ that satisfies the bulk composition constraints; (2) detecting saturation of new phases (including liquid) in an assemblage; (3) adding and removing phases from the assemblage without adjusting the system bulk composition; and (4) constraining the assemblage to a fixed f O₂ . These methods have been added to MELTS, allowing it to calculate heterogeneous phase equilibria with or without liquid, closed or open to O, and with fixed intensive variables (P,T), (P,S), (P,H), or (V,T). Applications include fractional melting calculations, metamorphic phase equilibria, and geophysical models of subsolidus regions of the Earth.