Volume 88, Issue 8-9

Experimental solidus determinations for nominally anhydrous fertile peridotites at 4-6 GPa differ by as much as 150°C. These differences have recently been ascribed to experimental methods, specifically, the use of graphite capsules leading to elevated CO 2 contents during the runs. We examine this claim by directly comparing phase relations of fertile peridotite KR4003 encapsulated in rhenium and graphite sealed in Pt, using an extensively calibrated double capsule and thermocouple assembly. Experiments are conducted at 5 GPa and 1750 to 1575 °C using the MA6/8 multianvil apparatus. We found that the phase assemblages recovered from both capsules of a given experiment are largely indistinguishable down to solidus conditions. The solidus is estimated to be 1600 ± 15°C, in good agreement with previous experiments on KR4003, but substantially below most other determinations for garnet peridotite solidi at 5 GPa. We find Opx to be stable at the solidus of KR4003 at 5 GPa, in contrast to earlier findings and relate its stability to Fe 3+ content. Our work further shows that a loss of calibration of the W-Re thermocouple circuit connected through the Re capsule at high temperatures is of the same order as the temperature discrepancy reported previously for fertile peridotite solidi. Revisions to the anhydrous peridotite solidus at high pressures invite reconsideration of potential temperatures of mantle plumes, which could be as much as 150°C lower than commonly assumed

The interfaces between fine-scale exsolution lamellae of hematite and ilmenite from an igneous rock in Rogaland, Norway have been studied by conventional transmission electron microscopy (TEM), high-resolution TEM, and energy-filtered TEM (EFTEM), to investigate the lamellar magnetism hypothesis for the origin of the unusual magnetic properties of this rock. Very fine hematite and ilmenite lamellae, less than 50 nm in length and parallel to (001) of their host, were abundant throughout the hemo-ilmenite sample. Dark-field and EFTEM observations indicated the hematite and ilmenite have very sharp structural and compositional interfaces with their hosts. The interfaces between the coarse hematite and ilmenite lamellae (length >1 μm) and their hosts have some interface dislocations to relieve elastic coherency strain. On the other hand, very fine lamellae (length <50 nm) have no interface dislocations and are perfectly coherent. The interface dislocations for lamellae on the order of 100 nm in length are distributed heterogeneously, and more than 80% of the length of the interfaces seen by TEM is dislocation free. Thus, most of the interfaces in the sample are coherent. These results are in accord with the predictions of Monte Carlo simulations of the exsolution process (Harrison and Becker 2001) and with the hypothesis that coherent and sharp structural and compositional interfaces are the origin of lamellar magnetism (Robinson et al. 2002).

We investigated structural changes during precipitation of HgS from sulfidic solutions using Xray absorption and X-ray diffraction techniques. The results show that initially an unstable low Hg coordination complex forms that is probably chain-like in structure, with one sulfide atom at 2.35 Å and one at 2.97 Å. This is rapidly transformed to a four-coordinate mercury sulfide compound that initially forms as clusters with the local ordering characteristics of cubic metacinnabar. However, during aggregation the black Hg-S precipitate loses its initial longer-range ordering and becomes pseudocubic. As it ages, the pseudocubic structure transforms to a cubic structure, and then to stable crystalline metacinnabar. This study provides clear evidence that the precipitation and formation of metal sulfides is a complex multistage process

Aqueous fluid inclusions have been synthesized within fluorite microfractures (T = 200°C, P = P sat , t < 30 days), quartz microfractures, and quartz overgrowths (T = 400 to 300°C, P = 400 bars, t < 20 days). The experiments were designed to determine the time necessary to form the inclusions, within a minute in a fluorite host, and within a day in a quartz host. The results permit estimation of the time needed to heal microfractures or create overgrowths, which is accomplished by measuring the melting temperature (T m ) of the inclusions in fluorite and the homogenization temperature (T h ) of the inclusions in quartz. For the fluorite experiments, the results show that fluorite/NH 4 Cl solution equilibrium took 82 hours. Moreover, the healing process appears to be an irregular process along the microfracture. Fluorite experiments mimic boiling processes occurring in natural hydrothermal or epithermal systems and show how a boiling fluid may be progressively salted during vaporization and trapped as fluid inclusions. For the experiments involving quartz, the quartz/H 2 O-NaCl solution equilibrium was reached on the first day for synthetic quartz and on the sixth day for natural quartz. This difference is linked to the hydration state of the two types of quartz. The decrease in T h of synthetic fluid inclusions from the core-overgrowth boundary to the external part of the quartz overgrowth shows that the growth of quartz is a progressive and protracted process, which involves a silica oversaturated solution

To elucidate at what pressure and temperature and for what fluid compositions monazite may be induced to form from fluorapatite, the LREE-enriched Durango fluorapatite has been metasomatized experimentally at temperatures of 300, 600, 700, 800, 850, and 900°C and pressures of 500 and 1000 MPa. Fluids used included pure H 2 O, various NaCl, KCl, and CaCl 2 brines (salt/H 2 O = 50/50, 30/70, or 10/90), and either 90/10 CO 2 /H 2 O or 40/60 CO 2 /H 2 O mix. Monazite formed in the fluorapatite + H 2 O, fluorapatite + 40/60 CO2/H 2 O, and the fluorapatite + KCl brine experiments. At 900°C and 1000 MPa, monazite formed both as inclusions within the fluorapatite and externally on its surface. Below 900°C, monazite grew only externally on the fluorapatite, either as euhedral to semi-euhedral crystals or as partial mantles over smaller fluorapatite grains. Monazite, especially at 900°C and 1000 MPa, is compositionally heterogeneous, specifically with respect to the Th content (ThO 2 = 4-38 wt%). Whereas the reactant fluorapatite in the pure H2O experiments remained unzoned at lower temperatures, three coupled zones with different (LREE+Si+Na) abundances developed at 900°C. These zones roughly follow the rim of the fluorapatite enclosing a fourth zone or the core, resembling the original composition. Monazite inclusions formed only in the one zone where the LREE are depleted. In the NaCl brine experiments, the Na replaced Si lost to the solution, which stabilized the LREE, and precluded formation of monazite. Similarly, the high activity of Ca in the CaCl 2 brine caused Ca to replace (LREE+Na) on the Ca site and discouraged the growth of monazite. The fluorapatite recrystallized to a fluor-chlorapatite, which displays oscillatory zoning, specifically with respect to the LREE. The results from this study imply that the presence of monazite inclusions and rim grains associated with fluorapatite (1) can be metasomatically induced; (2) can give insights into the chemistry of the metasomatizing fluids; (3) can provide some information on the grade of the metasomatic overprint; and (4) could indicate the occurrence of one or more metasomatic events

We describe a technique for determining the 3D size, shape, and number density of prismatic microlites in obsidian. The procedure involves collecting a series of optical photomicrographs at successive levels in a transparent thin section with a petrographic microscope and digital camera. These images are combined to form 3D stacks with NIH Image software. The number, position, orientation, and projection length of each microlite in the stack are then determined, and these data are used to calculate the true length and number density. Crystal-size distribution (CSD) based on direct measurements exhibit asymptotic, lognormal, and near-lognormal profiles. These forms, in addition to broad variations in crystal aspect ratio suggest that microlites experienced a range of growth rates. Textural parameters (e.g., total crystal number density and volume) determined from regressions of linear CSDs under the assumption of constant growth rate compare well with directly measured parameters. We compared CSDs based on 3D measurements with those based on 2D measurements of intersection length and area number density. Stereological conversions of 2D data are necessary owing to the cut-section effect and intersection probability problem. We performed corrections with three widely used algorithms and, thus, our comparisons test the accuracy of these correction methods as applied to prismatic microlites in obsidian. CSDs based on 3D measurements are linear over most of the size range. In contrast, conversion programs produce kinked CSDs, with large positive errors in population density at small crystal size. Errors in population density are caused by shape (aspect ratio) variability in the sample population. Conversion programs, which assume a constant shape, overestimate the number of small crystals owing to a large number of intersections along the short crystal dimension. In the real population, these intersections correspond to a wide range of true lengths. Consequently, CSDs constructed from intersection lengths are kinked rather than linear. Kinked and curved CSDs have been interpreted to result from mixing of distinct crystal populations, sharp variations in growth and/or nucleation rate, or from crystal settling. Our results suggest that nonlinear CSDs in some cases may also arise as an artifact of shape variability in the natural population

Transmission electron microscopy (TEM) has been used to investigate dendritic forsterite systematically, in order to understand its morphology in three dimensions. TEM images of olivine dendrites are reported for the first time. Crystals have been obtained by dynamic crystallization in the CaO-MgO-Al 2 O 3 -SiO 2 (CMAS) system in a one-atmosphere vertical furnace. A fast cooling rate (1890°C/h) has been used with degrees of undercooling varying from 156 to 356 ∞C. Different microstructures were observed depending on their location in the dendritic crystal. The external part of the crystal reveals true olivine dendrite propagation, as classically observed in other materials (succinonitrile, alloys). In the inner part of the crystal, this microstructure changes to the formation of single crystalline units with well-defined crystalline faces. The shapes observed are very reproducible. These units are limited by the usual forms of olivine [(010), {021}, {110}, {120}, {101}] and also by less common forms [(001), {130}, {140}] as additional faceting. All these elementary units present the same morphology, the hopper shape that corresponds to the skeletal form of olivine. These units are linked in a peculiar spatial organization giving rise to dendrite branches. It is shown that the [101] preferential direction of growth of dendrites is a mean direction and corresponds to the juxtaposition of these elementary units. In the innermost part of dendritic crystals, a dissolution-recrystallization process of elementary units occurs and a typical textural ripening microstructure is observed. This textural ripening starts early and is coincident with the real dendritic growth (external part of the crystal). Thus, the final appearance of dendritic forsterite crystals mainly results from textural ripening. This study emphasizes that dendritic solidification is a complex phenomenon, and the various microstructures suggest that multiple mechanisms are involved during the formation of so-called dendritic crystals

We consider the fluid-mediated exchange of elements or isotopes between two mineral grains that are physically separated from each other in an analytical model. The process is found to depend on three non-dimensional parameters defined herein as β, γ, and δ. The behavior of a given system depends on only two of these (γ and δ), while the third (β) determines the rate. This allows us to define a reaction mechanism map as a function of γ and δ. These parameters can be calculated from independently measured or estimated thermodynamic, kinetic, and transport properties, thereby enabling a prediction of reaction mechanisms for the first time. It is found that at least six different reaction mechanisms are possible rather than just diffusion or interface control. Some of the more significant observations are that: (1) time scales of equilibration may be substantially greater than the diffusive time scale (x 2 /D); (2) rim compositions of zoned minerals may be out of equilibrium with each other and even with the adjacent fluid in some cases; and (3) solubility of the species of interest in the intergranular fluid plays a very important role in determining reaction behavior. We discuss case studies, selected from the literature, of garnet-biotite geothermometry and speedometry from granulite-facies metamorphic rocks in the light of the model to show that intergranular transport may be inefficient (rather than slow) in some cases. Finally, we consider the implications of these results for geothermometry, geospeedometry, and geochronometry

The stability and chemistry of chromian spinel were determined for dry mantle (spinel lherzolite) melting at pressures between 1.0 and 2.5 GPa and temperatures between 1250 and 1500 ∞C. To find the most suitable sample container, we tested three different possibilities at 1.5 GPa: a Pt capsule, a Re/Pt capsule, and a graphite/Pt capsule. The oxygen fugacity of the run products with Pt, Re/Pt, and graphite/Pt are FMQ + 4 to 5, -2 to 0, and < -10, respectively. We conclude that the Re/Pt capsule is most suitable for melting experiments of dry peridotite at high temperatures and the oxygen fugacity similar to that of the terrestrial upper mantle, which has been estimated to be near FMQ. Using Re/ Pt capsules, batch-melting experiments of a dry spinel lherzolite from Kilborne Hole (KLB-1) were performed. Our experimental results and natural mantle peridotites are significantly different in terms of the stability field and the Cr/(Cr + Al) atomic ratio (= Cr') of chromian spinel. Spinel, which is common in natural mantle peridotite regardless of their bulk chemistry, disappeared at a much lower degree of melting in our experiments. The upper limit of the Cr' of spinel just before its disappearance decreases rapidly from 0.45 to 0.13 with increasing pressure, whereas natural peridotites have a wide range of Cr' from 0.1 to 0.8. Our dry experiments, compared with previous hydrous experiments, indicate that the upper limit of Cr' increases with increasing H 2 O content. These findings suggest that the petrological variation of natural mantle peridotite cannot be formed by simple batch melting but may be the result of more complex melting

High-quality valence band X-ray photoelectron spectroscopy (XPS) of vacuum-fractured pyrite (FeS 2 ) was performed using photons from a high resolution undulator beamline. The spectra, collected at photon energies ranging from 50 to 65 eV, span the Fe 3p ionization threshold and reveal the nature of both final state and surface state contributions to the valence band. The lower valence band (1.5 to 10 eV binding energy) reflects primarily S 3p-like direct emission final state contributions. In contrast, the upper valence band (0 to 1.5 eV binding energy) incorporates both a ligand to metal charge transfer and a Fe 3d direct emission final state contribution, the latter perhaps derived in part from surface Fe atoms (a surface chemical state contribution). The results imply that calculations of the pyrite density of states, based on ground state configuration, and without consideration of final state contributions, should reflect closely the XPS valence band structure; this is confirmed by comparison of calculation and collected spectra. Although generally similar to low resolution valence band spectra previously collected, these high resolution spectra show little resonant enhancement of any valence bands above the Fe 3p threshold. The important implication concerns the state of Fe 3d t2g orbitals; the weakness of the direct emission final state (Fe3d 5+ ) contribution and strength of the charge transfer final state contribution to the valence band is explained if Fe 3d t2g -derived orbitals are mixed with S valence orbitals, a possibility predicted by recent theoretical considerations (Eyert et al. 1998). Direct emission final state contributions are, however, observed in the upper valence band (0.40 to 1.3 eV binding energy) and some of the signal may result from surface Fe 3d non-bonding orbitals produced by fracture of the mineral

Small outcrops of wollastonite- and melilite-bearing pyrometamorphic rocks (paralavas) are found along the Apennine chain in Central Italy at the localities of Colle Fabbri and Ricetto. These rocks have coarse- to fine-grained crystalline, spotted, and glassy textures. The Colle Fabbri rocks have abundant and ubiquitous wollastonite, with plagioclase ± clinopyroxene. Melilite is found only in the most Ca-rich and silica-poor samples. Garnet, perovskite, quartz, leucite, silica-rich glassy mesostasis, and ocelli filled by calcite are present locally. The Ricetto rock is made up of wollastonite and melilite with minor clinopyroxene set in a glassy matrix with a few crystals of restitic quartz. These rocks have variable contents of SiO 2 (43 to 64 wt%) and CaO (37 to 3.7 wt%). The latter decreases with increasing MgO, Al 2 O 3 , Fe 2 O 3tot , TiO 2 , K2O, SiO2, and most trace elements, excluding Sr. Peculiar parageneses (e.g., coexisting melilite and anorthite in the most silica-poor samples, abundance of wollastonite), mineral compositions (gehlenite-rich melilite; Al-, Ti-, and Fe 3+ -rich clinopyroxene; Al-, Ti-rich, but Fe-poor garnet), and major- and trace-element geochemistry suggest that the Colle Fabbri and Ricetto rocks are not ultimately mantle-derived, but are the result of melting, devolatilization, and recrystallization of marly sediments (50-60 to 5-10% calcite, with clay minerals ± quartz), likely due to coal fires. The 87 Sr/ 86 Sr and 143 Nd/ 144 Nd range from 0.70772 to 0.71190 and from 0.51223 to 0.51219, respectively, again suggesting sedimentary protoliths of a mixed nature. Substantial chemical and isotopic differences between these samples and those from the Roman Magmatic Province are highlighted

A systematic transmission electron microscopy (TEM) study was performed on the size distribution of antiphase domains in omphacite from eclogites from the Adula/Cima Lunga nappe. The measured mean antiphase domain size is shown to depend on peak temperature, duration of peak metamorphism, cooling rate, and composition. The systematics of the size distribution are modified by dislocation interaction, recrystallization, and the time of growth during the temperature-timedeformation history of the rock. Based on a new model for the size distribution of antiphase domains, we are able to show the potential and limits to estimating time and peak temperature of metamorphism and the subsequent cooling history of this important rock type

Compressional and shear wave velocities of ringwoodite (γ-Mg 2 SiO 4 ) were measured on a polycrystalline specimen to 12 GPa at room temperature using ultrasonic interferometry techniques. Velocity measurements at ambient conditions yielded Vp = 9.86(3) and Vs = 5.78(2) km/s. Finite strain analysis of the high pressure velocity data yields K = 185(2) GPa, G = 120(1) GPa, K' = 4.5(2), and G' = 1.5(1) for the elastic moduli and their pressure derivatives, respectively. The velocities and elastic moduli at ambient conditions are indistinguishable from aggregate properties of polycrystalline ringwoodite calculated using single crystal elastic constants (Cij). Comparison of the current results on pressure derivatives of bulk and shear moduli with previous acoustic data on iron free ringwoodite to 3 GPa and iron bearing sample to 16 GPa indicated that the discrepancy can be explained by the pressure range of the experiments rather than iron content. Current results suggest that ringwoodite and wadsleyite posses very similar pressure dependence in elastic properties to the transition zone depth. In a pyrolite mantle with 1400°C adiabatic foot temperature, the wadsleyite to ringwoodite phase transition is characterized as a seismic reflector spreading about 20 km in width with a density jump of 2.1% and impedance jumps of 2.4% and 3.1% for P and S waves near 520 km depth, which is consistent with seismic observations in long period data

The step-wise oxidation of the disulfide group during pyrite oxidation by ferric iron was investigated using in situ attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy. Pyrite oxidation by ferric iron was investigated at pH 2.0, in the dark, and with both ultraviolet and visible wavelength illumination. We concluded that the spectra collected during pyrite oxidation are consistent with the existence of more than a single sulfur species at the pyrite surface. Exact assignment of all of the sulfur species was not realistic. However, the assignment of sulfate in an outer-sphere complex and thiosulfate in a monodentate complex was possible. The presence of multiple sulfur species at the pyrite surface directly confirms the step-wise oxidation of the disulfide group, and the presence of sulfate suggests that this species is the surface release group. Photochemical experiments verify an increase in the rate of pyrite oxidation with illumination and suggest that either the mechanism or the rate of sulfur oxidation is affected by illumination

The ganophyllite-group minerals, with general formula (K, Na, Ca)xMn6(Si 9 Al)O 24 (OH) 4 ·nH 2 O (ganophyllite = K, eggletonite = Na, tamaite = Ca), are complex modulated layer silicates that contain 2:1 trioctahedral layers with Mn 2+ -O 6 octahedra. Pervasive superstructures have frustrated the numerous attempts at solution of the atomic arrangement of these modulated layer silicates. An orthorhombic dimorph of tamaite has been discovered and its atomic arrangement solved, elucidating the elusive structure of the ganophyllite-group minerals. The dimorph was discovered in the Val Graveglia mining district in the northern Apennines, approximately 35 km east of Genoa, Italy. The phase crystallizes in space group Pnma, a = 16.8146(6), b = 25.2036(9), c = 13.3866(5) Å. The weak reflections from the 3¥ “superstructure” along a, long observed but never successfully measured in ganophyllite-group minerals, were obtained using a CCD detector and subsequently the atomic arrangement was solved and refined (R = 0.079). The structural modulation in ganophyllite-group minerals results from the misfit between the sheets of Mn 2+ O 6 octahedra and silicate tetrahedra. The atomic arrangement consists of corrugated T-O-T layers, with inverted tetrahedra in the tetrahedral sheets connecting adjacent layers along b. The inverted tetrahedra exist as four-member rings, and incorporate Al, with a maximum Al occupancy of Si 2.00 Al 2.00 . Charge balance for the substituent Al is maintained by adding cations (Ca, K, Na) or [6]Al sufficient to balance the charge lost by the Al ↔ Si substitution in the interlayer tetrahedra. Zeolitic H 2 O molecules also exist in the interlayer channels. Contrary to earlier speculation, the 5.6 Å “subcell” (along a in the orthorhombic dimorph) observed in ganophyllite-group phases is not a true subcell, in that it does not have approximately equivalent atoms at (x, y, z), [~(x + 1/3), ~y, ~z], and [~(x + 2/3), ~y, ~z) for all atoms. Although the majority of the cation scattering (all Mn + 2/3 of the Si atoms) exist in a supercell-subcell relationship, as manifested in very strong h = 3n and very weak (heretofore immeasurable) h ≠ 3n reflections, the lack of such a relationship for all atoms prohibited a successful solution based on previous assumptions of a subcell-supercell relationship

The crystal structure of kassite, ideally Ca[Ti 2 O 4 (OH) 2 ], containing 2 wt% Cr 2 O 3 , from the Saranovskoye chromite deposit, Perm’ district, Northern Urals, has been determined and refined to R1 = 0.06 using single crystal X-ray diffraction data. The crystals have monoclinic symmetry, P21/a, with a = 5.275(1), b = 9.009 (2), c = 9.557 (2) Å, β = 90.43∞. A pronounced sub-structure for the mineral, conforming to space group I2/a, is related to the I2/a structure for lucasite-(Ce), Ce[Ti 2 O 5 (OH)]. It comprises (001) layers of gibbsite-like fused hexagonal rings of edge-shared Ti(O,OH) 6 octahedra with the Ca atoms sandwiched between pairs of opposing rings and displaced from the center of the rings along [010]. Ordering of the protons in chromian kassite lowers the symmetry to P2 1 /a. Kassite, CaTi 2 O 4 (OH) 2 , and cafetite, CaTi 2 O 5 (H 2 O), are identical chemically but significantly different in their crystal structures

Infrared, optical absorption spectroscopy, and Mössbauer spectroscopy were used to investigate the color of jade from New Zealand. Spectroscopic results were supplemented by chemical analyses and petrological examination. Infrared spectra gave a quick identification of the matrix, optical absorption spectra gave information on color in relation to observed absorption bands, and Mössbauer spectra gave the distribution of Fe 2+ and Fe 3+ at the cation sites and also show how the Fe 3+ /Fe 2+ ratio increases due to oxidative weathering. The development of the attractive flecking in the gem-quality jade is due to agglomerations of colloidally dispersed magnetite or chromite that can also lead to the formation of black spots. Darker samples are generally high in total iron, although not all lightly colored (or pale) samples are low in iron-cream or white unweathered nephrite can also contain high iron concentrations. Weathering under the climatic conditions where the samples occur can produce either a brown, hydrated iron oxide, or a whitish outer rind if the acidity is high enough to remove the oxide. In either case the nephrite matrix is unaltered. Two quite rare variations were found and ascribed to: (1) incomplete nephrite formation in samples developed in association with an unusual ultramafic protolithology and (2) the formation of chromian margarite giving rise to a bluish-green (pseudo) jade

Electron paramagnetic resonance (EPR) spectroscopy has been used to study the valence state of silver in “argentiferous” galena samples from the Apuane Alps (Tuscany, Italy) mining district. This method was used to reveal primary metallic silver (Ag0) in galena. Both thermodynamic data and experimental studies suggest that galena and native silver can stably coexist, but have not been reported as a primary (hypogene) assemblage in natural samples. EPR spectroscopy proved to be a suitable tool to solve this problem, because this technique is capable of detecting paramagnetic species down to the ppb level, even in a highly absorbent matrix such as galena. A detailed SEM-EDS investigation could not detect metallic silver (or gold) in galena samples, but did reveal small (few micrometers) inclusions of Ag-bearing phases, in which silver has a formal valence of +1. On the other hand, EPR spectra indicated the presence in galena of pairs and clusters of elemental silver atoms, which may be associated with pairs of metallic gold, or with silver-gold hetero-atomic pairs. Therefore, SEM/EDS and EPR are complementary techniques, revealing the presence of both Ag + and Ag 0 . The Ag(Au) metallic species were apparently deposited on the galena surface during its growth from mineralizing fluids. Their scarcity, and the presence of larger amounts of Ag 1+ phases, suggest that the assemblage galena-metallic silver was stable only under peculiar physical and chemical conditions. The formation of Ag 0 was presumably linked to local and rare chemical fluctuations of the hydrothermal environment, characterized by low activities of S, Sb, Bi (Cu…) and high activity of Ag in the fluids. The occurrence of both homo- and hetero-atomic pairs suggests either different kinetics of pair formation, or possible fluctuations in the composition of the hydrothermal fluids, which alternatively carried Ag or Au species, or both

The effect of pressure on melt viscosity was investigated in the system NaAlSi 3 O 8 -CaMgSi 2 O 6 (Ab-Di) at pressures from 0.1 to 400 MPa using a parallel plate viscometer. The new measurements in the high viscosity range of 10 8.5 -10 11 Pa·s are consistent with previous data obtained at higher temperature (lower viscosity) in that the pressure dependence changes from positive for the polymerized Ab melt to negative for the depolymerized Di melt. However, a pressure independent viscosity is observed at much higher Ab contents at low temperature (70-90 mol%) than at high temperature (<30 mol%, Brearley et al. 1986). Hence, the sign of the pressure dependence of viscosity changes from positive to negative with increasing temperature for intermediate compositions along the join. The apparent activation volume (V α ), which is derived from the pressure derivative of log viscosity using an Arrhenian relationship, decreases systematically with temperature for Di-rich compositions, e.g., for the Di melt from 52.0 ± 2.7 cm 3 /mol at 1020 K to 24.8 ± 6.6 cm 3 /mol at 1057 K. To model the P-T dependence of viscosity, we propose a modification of the commonly used equation based on Adam-Gibbs theory as where A AG , B AG , and C AG are fit parameters and S conf is the configurational entropy. The parameter C AG can be interpreted as the volume change of a structural unit during transition from one stable configuration to another one. The new approach excellently reproduces the experimental viscosity data for Di melts over a wide range of temperature. A modified Vogel-Fulcher-Tamman equation also describes the data with a similar reproducibility, but one additional fit parameter is needed. Experimental viscosity data for Ab melts scatter too much at given P-T to be fitted by a unique model. To calculate viscosities of stoichiometric Ab melt at elevated pressure and temperature, we suggest that the C AG parameter determined in our study be combined with published viscosity equations for ambient pressure

A Raman spectroscopic investigation of the SiO 2 -clathrate melanophlogite was undertaken to investigate the enclathrated molecules. The Raman spectra show the presence of N 2 , CO 2 , and CH 4 molecules based on the observation of their normal stretching modes. An analysis of the spectra, together with single-crystal X-ray results (Gies 1983), demonstrates that the molecules are located in structural cages. Most of the CH 4 is partitioned into the smaller nearly spherical [5 12 ] cage, while CO 2 and N 2 prefer the larger more oblate [5 12 6 2 ] cage. The difference in wavenumber for their stretching modes between room temperature and 4 K is minimal. There are also only small differences between the wavenumbers of the modes of the enclathrated molecules and those they possess in the free, gaseous state. The molecules are orientationally disordered in the cavities and they have weak dispersion interactions with the SiO 2 framework. Although the energetic states of the molecules are only slightly modified by being enclathrated, subtle interactions between molecule and framework are necessary to stabilize the clathrate phase. The properties and behavior of single, quasi-free molecules occurring in the cavities of beryl and cordierite are compared to the situation in melanophlogite

Yellow-brown to pink Mn-rich tourmalines with MnO contents in the range 8-9 wt% MnO (~0.1 wt% FeO) from a recently discovered locality in Austria, near Eibenstein an der Thaya (Lower Austria), have been characterized by crystal structure determination, by chemical analyses (EMPA, SIMS), and by optical absorption spectroscopy. Qualitatively, the optical spectra show that Mn 2+ is present in all regions of the crystals, and that there is more Mn3+ in the pink regions (~8% of the total Mn is Mn 3+ ) than in the yellow-brown regions. A gamma-ray irradiated crystal fragment is distinctly pink compared to the yellow-brown color of the sample before irradiation, but it still has hints of the yellow-brown color, which suggests that the natural pink color in Mn-rich tourmaline from this locality is due to natural irradiation of the initial Mn 2+ . For these Mn-rich and Li-bearing olenite samples, crystal structure refinements in combination with the chemical analyses give the optimized formulae X (Na 0.80 Ca 0.01 ◻ 0.19 ) Y (Al 1.28 Mn 2+ 1.21 Li 0.37 Fe 2+ 0.02 ◻ 0.12 ) Z Al 6 T (Si 5.80 Al 0.20 )B 3 O 27 [(OH) 3.25 F 0.43 O 0.32 ], with a = 15.9466(3) Å, c = 7.1384(3) Å, and R = 0.036 for the sample with ~9 wt% MnO, and X (Na 0.77 Ca 0.03 ◻ 0.20 ) Y (Al 1.23 Mn 2+ 1.14 Li 0.48 Fe 2+ 0.02 Ti 0.01 ◻ 0.12 ) Z Al 6 T (Si 5.83 Al 0.17 )B 3 O 27 [(OH) 3.33 F 0.48 O 0.19 ] for a sample with a = 15.941(1) Å, c = 7.136(1) Å, R = 0.025 and ~8 wt% MnO. The refinements show 1.22-1.25 Al at the Y site. As the Mn content increases, the Li and the F contents decrease. The Li content (0.37-0.48 apfu) is similar to, or lower than, the Li content of olenite (rim-composition) from the type locality, but these Mn-rich tourmalines do not contain [4] B. Like the tourmaline from Eibenstein an der Thaya, synthetic Mn-rich tourmaline (in a Li + Mn-bearing system), containing up to ~0.9 apfu Mn (~6.4 wt% MnO), is aluminous but not Li-rich. This study demonstrates that although a positive correlation exists between Mn and Li (elbaite) in tourmaline samples from some localities, this coupling is not required to promote compatibility of Mn in tourmaline. The a parameter in Mn-rich tourmalines (MnO: ≥3 wt%) is largely a function of the cation occupancy of the Y site (r 2 = 0.97)

Compositionally heterogeneous crystals of olivine occur in thermally metamorphosed serpentinites from Southwest Japan. They have a variation in forsterite (Fo) content up to 10 mol% within a specimen. The chemical heterogeneity is the result of two causes: the intermingling of metamorphic neoblasts with relict crystals of primary olivine, and the compositional variation within the neoblasts themselves. The metamorphic olivine in each specimen shows a bimodal distribution of Fo content with a highly magnesian group (Fo 93-98 , varying among specimens) and a relatively ferroan one (Fo 85-94 ). Textural relationships and the variation of NiO and FeO contents between the two groups of olivine suggest that the heterogeneity results from the local involvement of magnetite and awaruite in dehydration reactions of serpentine at the initial stages of metamorphism

Hematite reduction in the presence of pyrite and CaO led to the formation of anhydrite and magnetite at 30 MPa and 723 K under hydrothermal conditions in a sealed gold container. For the first time, hydrothermal magnetite production was monitored magnetically at the run conditions. Sample magnetic susceptibility was found to follow an Avrami kinetic law with k = 1.5 10 -5 /s and n = 2.07. Application of up to two external field reversals in the course of the magnetite production reaction shows that partial Chemical Remanent Magnetizations (CRM) are recorded by the sample. The intensity of each partial CRM acquired within a given time interval (under a given field intensity) is proportional to the amount of magnetite produced during that time interval. Alternating magnetic field demagnetization failed at separating these partial CRMs, which are believed to superpose in the (V, Hc) diagram of Néel. Additional TEM, SEM, and magnetic characterizations suggest that continuous nucleation and fast magnetite growth (to a maximum size of ca. 10 mm), compared to the field polarity duration, can account for the evolution of the sample CRM with time. This study demonstrates that one should be cautious when using natural CRM apparent polarity to infer timing of the corresponding chemical event

The prediction of viscosity in silicate melts, over the range of conditions found in nature, remains one of the most challenging and elusive goals in Earth Sciences. We present a strategy for fitting non-Arrhenian models [e.g., Vogel-Tammann-Fulcher (VTF) or Adam-Gibbs (AG)] to viscosity data that can be employed toward a full multicomponent model for melt viscosity. Our postulate is that the high-T viscosities of silicate melts converge to a common value. The implications are twofold. First, the number of composition-dependent parameters is reduced by a third. Second, our optimization constrains the experimentally inaccessible, high-T properties of silicate melts. The high-T limits to melt viscosity are constrained by the VTF and AG models to between 10 -4.3±0.74 and 10 -3.2±0.66 Pa·s, respectively, and overlap in the interval 10 -3.86 to 10 -3.56 Pa·s.

This paper describes the first reported occurrence of berlinite (AlPO 4 ) formed entirely under sedimentary conditions. Berlinite appears as grayish or colorless fine crystals in vacuoles and along cracks in heavily compacted, phosphate-rich sediments within the Cioclovina Cave in Romania. The X-ray powder diffraction pattern and the hexagonal unit-cell of the Cioclovina berlinite specimen compare well with other published determinations. The lattice parameters are a = 4.94(4), c = 10.87(1) Å, V = 230.1(3) Å 3 . Microprobe analyses confirmed a nearly ideal formula for berlinite. In situ guano combustion is responsible for the transformation of taranakite and for the dehydration of variscite into berlinite. Berlinite was successfully synthesized from these two minerals at atmospheric pressure and temperatures ranging from 350 to 600°C