Band 93, Heft 8-9

We synthesized hematite spherules whose mineralogic, chemical, and crystallographic properties are strikingly similar to those for the hematite-rich spherules in lag deposits on the surface and embedded in outcrops at Meridiani Planum, Mars. The spherules were synthesized in the laboratory along with hydronium jarosite and minor hydronium alunite from Fe-Al-Mg-S-Cl acid-sulfate solutions under hydrothermal conditions. The reaction sequence was (1) precipitation of hydronium jarosite; (2) jarosite dissolution and precipitation of hematite spherules; and (3) precipitation of hydronium alunite upon depletion of hydronium jarosite. The spherules exhibit a radial growth texture with the crystallographic c axis aligned along the radial direction, so that thermal emission spectra have no hematite emissivity minimum at ~390 cm -1 . Our experiments show that hydrothermal, acid-sulfate solutions are a pathway for formation of jarosite and the hematite spherules at Meridiani Planum, Mars.

The crystalline core of the southern Black Hills, South Dakota, exposes an extensive, low-P-high-T aureole of garnet- to second-sillimanite-zone schists centered on the plutonic core of the 1715 Ma Harney Peak Granite (HPG). This paper demonstrates regional patterns of apparent ages observed for 52 40 Ar/ 39 Ar dates of muscovite and biotite in diverse rocks from across the ~1000 km 2 metamorphic aureole and its plutonic center. About 20 biotite dates, sampled mostly near faults, are influenced by excess 40 Ar and obscure the regional trends. The remaining mica dates reveal radial patterns of apparent younging from outer aureole toward inner granite, with previously unrecognized, elliptical age zones centered on the main HPG pluton and its outliers. The regional pattern of 40 Ar/ 39 Ar cooling ages indicates non-uniform slow cooling of the mid-crust between ~1600-1250 Ma. This scenario of delayed slow cooling from aureole to pluton is consistent with published cooling ages for muscovite (Rb/Sr) and apatite (U/Pb), which range from 1690 to 1550 Ma and from 1700 to ~1500 Ma, respectively. To explain these results, it is likely that ambient pre-granite temperatures of the country rocks were ≥350 °C at the ~10-14 km depth of granite emplacement, as previously proposed, and that the entire complex resided at this depth and cooled slowly from aureole to granite for hundreds of millions of years. Alternatively, or in addition, the HPG and inner aureole were not uplifted until ~1480-1330 Ma, whereupon they finally cooled through ~300-350 °C.

Monazite ages from carbonatites and high-grade assemblages exposed along a significant lineament within the Southern Granulite Terrane of India termed the Kambam fault were obtained in thin section (in situ) using an ion microprobe. X-ray maps for Ce and Th were acquired in larger monazites to decipher the significance of the ages of individual spots within grains. The Kambam carbonatite contains large (millimeter-sized) apatite rimmed by ~10 μm thick bands of monazite. Monazite commonly appears as a lower-Th, late-stage mineral in carbonatites, and bands surrounding apatite are interpreted as products of metasomatism, rather than exsolution. The age of a Kambam carbonatite monazite band is 715 ± 42 Ma (Th-Pb, ± 1σ), but monazite filling cracks within the apatite is ~300 m.y. younger (405 ± 5 Ma). The younger monazite grains are in contact with quartz, a mineral thought to be an indicator of subsolidus alteration in carbonatites. The age of the monazite rim is similar to ages of several carbonatites located 50-400 km further north, and chemical analyses show that this sample displays chemical trends similar to the other complexes (e.g., Y/Ho, Ce/Pb, REE, and HFSE patterns). The mid-Neoproterozoic event is recorded in garnet-bearing assemblages ~20 km west of the Kambam fault (733 ± 15 Ma) and garnet-bearing enclaves within Southern Granulite Terrane charnockites (701 ± 26 Ma; 786 ± 84 Ma). The results show that monazite can crystallize during metasomatism and be useful in deciphering fluid processes occurring at deeper crustal levels. The Kambam fault, which records over 300 million years of monazite growth, should be considered a major boundary in reconstructions of Gondwana.

This paper reports the results of several thousand analyses of the Mg content of calcite from 31 samples from the Ubehebe Peak contact aureole, Death Valley, California. All data reported are from metamorphic calcite formed during mixed volatile-mineral reactions in which dolomite remained in the rock. The Mg content generally increases toward the intrusive contact and bend with increasing temperature, but it varies strongly. Indeed, probability distributions for each sample are near Gaussian, possess a relatively small skewness (-1.72 to 3.32), and a variance that is a multiple of the estimated measuring uncertainty. These findings complicate direct application of the Mg content in calcite for use as an accurate thermometer. The purpose of the study presented in the second part of the paper is to explore the significance of these systematic variations of Mg composition of calcite to aid the interpretation of contact metamorphic temperatures recorded in carbonates. We developed forward models to evaluate the effect of growth zoning, volume diffusion, and the formation of submicroscopic exsolution lamellae (<1 μm) on the measured Mg distribution in individual calcite crystals and compared the modeling results to the field data. Modeled Mg distributions were transformed into histograms by taking into account intersection probabilities and random microprobe analyses. Modeling results reveal that the original prograde Mg zoning in calcite crystal will be reset if the calcite crystal is assumed to grow slowly along a prograde path. Original low-Mg compositions can only be preserved if the entire grain forms over a small temperature interval, as can be expected for infiltration-driven mineral reactions. It is shown that all three mechanisms combined give an adequate model for the Mg-content data. We demonstrate that Mg distributions in calcite grains of the Ubehebe Peak contact aureole are the consequence of rapid crystal growth in combination with diffusion and exsolution.

An approximate low-temperature, metastable phase diagram is drawn for the system (1 - X) Fe 2 O 3 -(X)FeTiO 3 . It is based on published and new magnetic data from nine synthetic samples with bulk compositions in the range 0.6 < X < 1.0. Fields are plotted for (1) the paramagnetic phase (PM); the Fe 2 O 3 -rich ferrimagnetic phase (FM); (2) the FeTiO 3 -rich antiferromagnetic phase (AF); and (3) a re-entrant spin-glass phase (RSG). In addition, two subfields are plotted: (1) FM′, a subfield of the FM-phase, which occurs below a characteristic temperature TK, at which the magnetic susceptibility drops sharply on cooling, and (2) PM′, a subfield of the PM-phase (traditionally called superparamagnetic) forms below a sharp rise in susceptibility at TS, and exhibits measurable dispersion in the magnetic susceptibility at T < TS. The diagram is drawn with a bicritical point, Tλλ′, at X ≈ 0.87, T ≈ 39 K, which is the intersection of second-order magnetic phase boundaries for the paramagnetic → ferrimagnetic [PM(PM′) → FM] transition, TC(X), and the PM(PM′) → AF transition, TN(X). In addition, the RSG phase is plotted as one of four stable phases at Tλλ′, a construction that is not required by the phase rule, but is strongly favored by the physics of competition between the incompatible magnetically ordered structures of the FM- and AF-phases. These phase relations are at such low temperature as to be of little consequence for terrestrial magnetism, however, they may well be essential for interpreting the magnetism of the Moon, Mars, and other cold planets. These phase relations are also essential for the characterization of fine natural and synthetic intergrowths, and for understanding magnetic materials for low-temperature technological applications.

We present in situ microanalyses of Fe 3+ /ΣFe in mantle-derived kaersutites as measured by electron probe microanalysis (EMP) based on the “self absorption induced FeLα peak shift” method. The EMP results are not in agreement with bulk (wet chemistry) data. The heterogeneities revealed for some kaersutite megacrysts, when comparing bulk and EMP Fe 3+ /ΣFe results, cannot explain the differences with the EMP measurements. It is thus proposed that any EMP overestimation of Fe 3+ /ΣFe results from a beam-induced dehydrogenation and a subsequent oxidation of Fe 2+ to Fe 3+ according to the known relation: Fe 2+ + OH- = Fe 3+ + O 2- + 1/2 H 2 . To demonstrate this phenomenon, H losses were measured by secondary ion mass spectrometry (SIMS) after EMP irradiation at different beam currents on two amphiboles with 1.1 and 1.7 wt% H 2 O, respectively. In both amphiboles, H losses were observed under high beam currents (240 and 100 nA). No dehydrogenation is observed under lower beam currents for the 1.1 wt% H 2 O amphibole, but still occurs, down to at least 50 nA, for the amphibole with the greatest H 2 O contents. Amphiboles with low H 2 O contents (below ~0.5 H + ) are less affected by beam damage. For amphiboles with higher H 2 O contents, the electron beam current density should be reduced with consideration given to the resulting high statistical errors.

To gain insight into hydroxyl solubilities and possible hydration mechanisms of mantle silicates, as well as to test the utility of germanium analog models in studies of water-related defects, our present work is focused on the protonation of germanium analogs of silicates. For this purpose Ge-analogs of ringwoodite, anhydrous phase B (anhB), and for the first time, superhydrous phase B (shyB), were synthesized in a piston cylinder device at 2 GPa and 950-1000 °C under water-excess conditions. Electron probe microanalysis (EPMA), transmission electron microscopy (TEM), and X-ray diffraction as well as Raman and infrared (IR) spectroscopy were used to characterize the experimental products. Ge-ringwoodite incorporates from 900 to 2200 ppm H 2 O by weight, which is much less than Smyth et al. (2003) observed for the Si-equivalent synthesized at 22 GPa and 1500 °C, but 200× more than published for γ-Mg 2 GeO 4 by Hertweck and Ingrin (2005). In addition to this discrepancy, the incorporation mechanism of H in Ge-ringwoodite also differs from that of Si-ringwoodite. Ge-anhB, which is currently believed to be anhydrous in the Si-system, contains from 2400 to 5300 ppm water by weight. A hydration model for germanate anhB was constructed based on single-crystal X-ray diffraction analysis and IR spectroscopy, in which OH is incorporated via the hydrogarnet substitution [V Ge ·4(OH) O ] x and via vacant Mg sites [V Mg ·2(OH) O ] x . For Ge-shyB the water concentration and incorporation mechanism obtained in this study are identical to results reported for the silicate phase synthesized at 22 GPa and 1200 °C (Koch-Müller et al. 2005). Thus, germanates are good low-pressure analogs for hydrous mantle silicates in which protonation is controlled by stoichiometry. However, for nominally anhydrous minerals we cannot recommend the use of germanates as high-pressure models in water-related studies. In these Ge-analogs, which are usually synthesized at much lower pressures, i.e., lower water fugacities, OH incorporation seems to differ from the high-pressure silicate equivalents qualitatively and quantitatively, as hydroxyl solubility is governed by other factors such as water fugacity and intrinsic defects.

Andreyivanovite (ideally FeCrP) is another new phosphide species from the Kaidun meteorite, which fell in South Yemen in 1980. Kaidun is a unique breccia containing an unprecedented variety of fragments of different chondritic as well as achondritic lithologies. Andreyivanovite was found as individual grains and linear arrays of grains with a maximum dimension of 8 μm within two masses of Fe-rich serpentine. In one sample, it is associated with Fe-Ni-Cr sulfides and florenskyite (FeTiP). Andreyivanovite is creamy white in reflected light, and its luster is metallic. The average of nine electron microprobe analyses yielded the formula Fe(Cr 0.587 Fe 0.150 V 0.109 Ti 0.081 Ni 0.060 Co 0.002 )P. Examination of single grains of andreyivanovite using Laue patterns collected by in situ synchrotron X-ray diffraction (XRD), and by electron-backscatter diffraction revealed it to be isostructural with florenskyite; we were unable to find single crystals of sufficient quality to perform a complete structure analysis. Andreyivanovite crystallizes in the space group Pnma, and has the anti-PbCl 2 structure. Previously determined cell constants of synthetic material [a = 5.833(1), b = 3.569(1), and c = 6.658(1) Å] were consistent with our XRD work. We used the XPOW program to calculate a powder-XRD pattern; the 5 most intense reflections are d = 2.247 (I = 100), 2.074 (81), 2.258 (46), 1.785 (43), and 1.885 Å (34). Andreyivanovite is the second new phosphide to be described from the Kaidun meteorite. Andreyivanovite could have formed as a result of cooling and crystallization of a melted precursor consisting mainly of Fe-Ni metal enriched in P, Ti, and Cr. Serpentine associated with andreyivanovite would then have formed during aqueous alteration on the parent asteroid. It is also possible that the andreyivanovite could have formed during aqueous alteration; however, artificial FeTiP has been synthesized only during melting experiments, at low oxygen fugacity, and there is no evidence that a hydrothermal genesis is reasonable.

The atomic arrangements of two natural samples of whitlockite, a synthetic whitlockite specimen, a synthetic whitlockite specimen heated at 500 °C, and a synthetic merrillite specimen (formed through dehydrogenation of synthetic whitlockite by heating at 1050 °C for 24 h) have been determined in space group R3c by X-ray diffraction methods; the high-quality structure refinements yielded R < 0.019. Whitlockite, ideally Ca 18 Mg 2 (PO 4 ) 12 [PO 3 (OH)] 2 and merrillite, ideally Ca 18 Na 2 Mg 2 (PO 4 ) 14 , are similar phases that differ by the lack of hydrogen and the concomitant addition of charge-balancing sodium (or calcium) in merrillite. The atomic arrangements of whitlockite and merrillite contain a structural unit consisting of a [(Mg,Fe)(PO 4 ) 6 ] 2 16- complex anion that forms a “bracelet-and-pinwheel” arrangement. The central octahedral cation and the six coordinating phosphate tetrahedra form a pinwheel, and in whitlockite and merrillite the pinwheels are not polymerized; the structural units are linked by interstitial complexes. In unsubstituted merrillite (assuming no Na or REE substituents for Ca), the interstitial complex has a formula of [Ca 19 (PO 4 ) 2 ] 32+ , and in whitlockite, the terrestrial phase in which hydrogen is accommodated, the interstitial unit has the formula [Ca 18 (PO 3 [OH]) 2 ] 32+ , yielding the charge-balancing relationship [H (whit) ↔ Ca 0.5(merr) ] 2 . Whitlockite and merrillite are perhaps the only phases that form a solid solution with terrestrial and extra-terrestrial end-members that differ by structural adjustments that result from the accommodation of hydrogen in the terrestrial phase. The results of the study also suggest that in terrestrial samples of whitlockite, a merrillite component of the solid solution is common, but that extraterrestrial samples of merrillite are devoid of any whitlockite component.

The single-crystal elastic moduli of (Mg 0.987 ,Fe 0.013 )O were measured by Brillouin spectroscopy in a diamond-anvil cell at high pressures to 9 GPa at room temperature. The ambient-pressure singlecrystal elastic moduli are (1) C 11 = 291.2(3.0) GPa; (2) C 12 = 96.1(2.0) GPa; and (3) C 44 = 151.9(2.0) GPa. From the single-crystal moduli, the aggregate elastic moduli are calculated to be adiabatic bulk modulus K S0 = 161.1(3.0) GPa, the Voigt bound of the shear modulus is G V = 130.0(2.0), and the Reuss bound G R = 124.2(2.0) GPa, giving a Voigt-Reuss-Hill average G = 127.1(2.0) GPa. We find that the addition of 1.3 mol% of Fe has a surprisingly large effect on the aggregate shear modulus, decreasing the room-pressure value by 2.4% as compared to Brillouin data for periclase (MgO) measured with the same technique. The adiabatic bulk modulus also decreases by 1.3%, although this decrease is within the mutual uncertainties of the measurements. Our results confirm significant non-linearity in single-crystal elastic moduli C 11 and C 44 and the aggregate shear modulus G of magnesiowüstite in the Mg-rich end. The pressure derivative of the bulk modulus K′ S = 4.2(2), as determined by a third-order finite-strain fit, is about 9% higher than the Brillouin results for the MgO end-member, whereas the pressure dependence of the shear modulus G′ = 2.3(1) is found to be identical to that of periclase. The measurements demonstrate that even a small amount of Fe (1.3 mol%) has a measurable effect on the elastic properties of MgO-FeO solid solutions.

Spinel single crystals (up to 4 mm in size) of eight compositions along the FeFe 2 O 4 -Fe 2 TiO 4 solidsolution join, with more than 75 mol% ulvöspinel, were synthesized by use of a flux-growth method. The crystals were characterized by electron microprobe and Mössbauer spectroscopy. Results demonstrated that these ulvöspinels are statistically stoichiometric. The atomic proportions of Ti 4+ and Fe 2+ progressively increase from 0.75 to 0.94 and from 1.75 to 1.94 apfu, respectively. Concomitantly the Fe 3+ content decreases from 0.49 to 0.13 apfu. Consequently, the cation substitutions are restricted to the ideal classic substitution 2Fe 3+ ↔ Fe 2+ + Ti 4+ . An average equilibrium temperature from coexisting spinel-ilmenite pairs of about 950 °C was estimated using the QUILF95 and Ghiorso-Sack models. In contrast to previous studies based on non-stoichiometric samples sintered at higher temperatures, the present stoichiometric ulvöspinel samples were grown from a melt under moderate temperatures. This fact indicates that the formation of vacancies is related to high-equilibration temperatures or growth mechanisms, whereas natural ulvöspinel may very well be stoichiometric under normal magmatic conditions.

Single crystals of the chabazite-type zeolite mineral willhendersonite, KCaAl 3 Si 3 O 12 ·5H 2 O (from Bellerberg, eastern Eifel district, Germany), were studied by X-ray diffraction methods between 100 and 500 K. The zeolite shows a phase transition from triclinic to rhombohedral symmetry between 350 and 375 K under dry nitrogen and between 450 and 475 K under humid air. Under these conditions, the unit-cell parameters change from P1̅ at 350 K [a, b, c (Å); α, β, γ (°); V (Å 3 ) = 9.210, 9.210, 9.405; 92.75, 92.80, 90.80; 795.8] to R3̅ at 375 K (a = 9.380 Å, α = 91.40°, V = 824.5 Å 3 ), and from P1̅ at 450 K [a, b, c (Å); α, β, γ (°); V (Å 3 ) = 9.215, 9.215, 9.415; 92.65, 92.85, 90.75; 797.5] to R3̅ at 475 K (a = 9.375 Å, α = 91.35°, V = 823.8 Å 3 ), respectively. The crystal structures were refined based on X-ray diffraction data collected at room temperature [P1̅; a, b, c (Å); α, β, γ (°); V (Å 3 ) = 9.248(5), 9.259(5), 9.533(5); 92.313(5), 92.761(5), 89.981(5); 814.7(8)], at 373 K [P1̅; a, b, c (Å); α, β, γ (°); V (Å 3 ) = 9.205(5), 9.231(5), 9.442(5); 92.550(5), 93.086(5), 90.519(5); 800.3(8)], and at 423 K [R3̅, a = 9.411(4) Å, α = 91.48(1)°, V = 832.7(6) Å 3 ]. Upon heating, the elliptical 8-rings of willhendersonite expand to a triangular shape in the rhombohedral structure with upper and lower rings in the double 6-ring (D6R) twisted by 60° to each other corresponding to the center of symmetry in the center of the D6R. The changes in the framework are accompanied by migration of cations, partly assuming unfavorably low coordinations in the high temperature structure due to the loss of H 2 O molecules. Rehydration at room temperature yields the triclinic structure of willhendersonite, although the single crystals become polysynthetically twinned.

The transformation of schwertmannite to goethite and/or hematite in high pH solutions was studied between 60 and 240 °C using synchrotron-based, in-situ energy-dispersive X-ray diffraction (EDXRD). Powder diffraction and electron microscopy indicate that the crystallization of hematite and goethite occurred via intermediate ferrihydrite. At temperatures ≤80 °C goethite was the only crystallization product, while at temperatures >80 °C goethite and hematite crystallized almost simultaneously. At temperatures ≥150 °C a secondary crystallization stage was observed in which goethite transformed to hematite. The activation energies of nucleation for goethite and hematite are 27 ± 1 and 25 ± 1 kJ/mol, respectively, while the activation energies of crystallization are 33 ± 1 and 28 ± 1 kJ/mol. Most of the sulfate was released from the schwertmannite during the early stages of crystallization with <5% of the sulfate remaining associated with the solid phase after crystallization was complete. Sulfate from the initial schwertmannite retarded the dissolution of ferrihydrite, which inhibited the nucleation and the early stages of goethite formation, but did not significantly affect the later stages of goethite crystallization. At high temperatures the presence of sulfate favored the crystallization of hematite over goethite. The activation energy of crystallization for the secondary transformation of goethite to hematite is 103 ± 3 kJ/mol.

Clay samples of hydrothermal origin from several oceanic spreading centers were studied using XRD, microprobe, infrared, and thermal analysis. They are talc, smectite, and mixed-layer talc-smectite (T-S) where the talc layers have several degrees of crystalline order. The smectite is trioctahedral in most cases, but there is also dioctahedral smectite both as mixed-layer and as a separate phase. All specimens contain Fe 3+ , some of them in moderate amounts (up to 17% Fe 2 O 3 ) distributed between the tetrahedral and octahedral sheets (maximum values: IV Fe = 0.32, VI Fe = 0.68, per O 10 [OH] 2 ). Octahedral Fe abundance correlates with the presence of molecular water that is lost in a well-defined dehydration event above 200 °C. This water does not cause layer expansion and is interpreted to be present within the pseudo-hexagonal cavity, next to Fe 3+ ions that generate a local charge imbalance. The presence of octahedral Fe 3+ is accompanied by vacancies in the octahedral sheet to balance the excess positive charge. An infrared band at ~790 cm -1 is assigned to OH bending in the group Fe-Mg-□-OH. Analysis of this band suggests a range of short-range Fe-□ distributions, from random to ordered. Our sequece talc, T-S, trioctahedral smectite is defined by an increasing Al for Si substitution in the tetrahedra and increasing crystal disorder. The presence of Fe also causes crystal defects. This mixed-layer series can be considered as a continuum generated by the combination of chemical and crystal defect variability. Kerolite was used to designate disordered, hydrated talc. We find that there is no clear line delimiting talc from kerolite as a single phase or in mixed-layer minerals and that it is better to use a descriptive term for the latter such as “disordered talc.” Dioctahedral smectite is also a possible end-member of the mixed-layer sequence, which implies an Al + □ for Mg substitution in the octahedral sheet. If T-S consists of polar TOT layers, the existence of dioctahedral smectite in T-S raises the question of the actual composition of the octahedral sheets within polar TOT layers.

This study is part of a broad research project devoted to the “amphibole fibers environmental problem” as related to the proposed excavation of the Susa Valley railway tunnel. In this locality, tunnel excavations are planned through metamorphic formations containing amphibole asbestos minerals, and this may give rise to worker health and public environmental issues. The Susa Valley tremolite shows a marked fibrous character, a small reduction of fiber size under grinding, and a consistent increase of the surface area. From the toxicological point of view, such tremolite fibers have been shown to be very effective in the generation of reactive oxygen species. They exhibit a very high cellular reactivity as a consequence of their morphology, structure, and crystal chemistry. Results of combined electron microprobe analysis, Mössbauer spectroscopy, and parallel-beam X-ray powder diffraction are reported for fibrous tremolite from a serpentine-schist from the “Unità Oceanica della Bassa Val di Susa” collected near Condove, Susa Valley, Italy. Data indicate that Fe 2+ (84% of Fe tot ) is located at both the (M1 + M3) and M2 sites and that Fe 3+ is at M2, in an approximate 3:2:1 ratio, respectively. No evidence of a split M4 site has been observed. The presence of M1+M3 Fe 2+ is confirmed by FTIR spectroscopy to be distributed 70% at M1 and 30% at M3. Both the composition (Ca 1.95 K 0.01 Na 0.05 ) Σ2.01 ( VI Al 0.01 Fe 3+ 0.02 Fe 2+ 0.11 Mg 4.84 Mn 0.02 )Σ 5.00 Si 8.00 O 22 (OH 1.96 F 0.03 Cl 0.01 ) Σ2.00 and the cell volume 907.37(1) Å 3 of the fibers are close to those expected for end-member (Ca/Mg = 2/5) tremolite.

The mineral cheralite [CaTh(PO 4 ) 2 ] allows for the incorporation of tetravalent actinides in monazitebased crystalline phases. Experimental determination of its thermodynamic properties is crucial for defining its stability and subsequent long-term ability to immobilize radionuclides. Low-temperature heat capacity from 0.5 to 300 K, enthalpy increments from 485 to 1565 K, and the enthalpy of formation of cheralite from the oxides were measured and reported on for the first time. At 298.15 K, S° = (201.6 ± 2.6) J/(K·mol), which includes the configurational entropy of Ca and Th mixing, ΔH f ox = -(506.4 ± 9.5) kJ/mol, ΔH f el = -(3872.8 ± 10.2) kJ/mol, ΔG f ox = -(501.6 ± 9.6) kJ/mol, and ΔG f el = -(3635.5 ± 10.2) kJ/mol. In aqueous environments, cheralite is able to form from whitlockite or apatite and thorianite. Under anhydrous conditions, cheralite can form by solid-state reaction only if the resultant product includes very stable Ca salts instead of CaO.

We have used a combination of classical and quantum-mechanical atomistic calculations, together with Monte Carlo simulations, to study order-disorder phenomena in the spinel mineral qandilite, Mg 2 TiO 4 . Using an interatomic potential model akin to those previously used for 2-3 spinels yielded a general increase in energy E as a function of inversion parameter x, and thus incorrectly predicted a normal-spinel ground state, whereas the E(x) behavior as modeled by density-functional theory exhibited a maximum at an intermediate degree of inversion and correctly predicted an inverse-spinel ground state. We therefore used the quantum-mechanical simulations to derive pair interaction parameters (for nearest-neighbor tetrahedral-tetrahedral, octahedral-octahedral, and tetrahedral-octahedral interactions) and chemical potential to use in Monte Carlo simulations of order-disorder in qandilite. The simulated cation distributions compared favorably with those obtained experimentally, although the long-range ordering transition to the tetragonal P4 1 22 phase was not observed when using only nearest-neighbor interactions. However, this transition could be observed following the addition of two extra parameters to the model. The simulations were used to calculate the effect of short- and long-range cation order on the configurational entropy of qandilite as a function of temperature. The calculated entropy of the hightemperature cubic phase was in very good agreement with the experimental value recently determined, supporting the suggestion that there is considerable short-range order in qandilite.

Silica sinters, deposited from alkali chloride waters in the El Tatio geothermal field in northern Chile (22°20'S, 68°01'W), have been characterized by XRD, SEM, TG-DTA, and FTIR. The dominant silica phase is opal-A. Minor contents of opal-A/-CT and opal-CT are also present together with halite (NaCl), sylvite (KCl), and realgar (AsS). Accessory phases include teruggite [Ca 4 MgAs 2 B 12 O 22 (OH) 12 ·12(H 2 O)], sassolite (H 3 BO 3 ), and detrital quartz (SiO 2 ). FWHM values reflect the immature nature of the studied opal-A. DTA heating experiments of opal-A show cristobalite crystallization at ~1000 °C, whereas DTA cooling experiments show the β → α-cristobalite transformation at ~200 °C. The total weight loss is related to changes in the sinter microtextures, mineral phases, and organic matter contents. FTIR spectra show the effects of silica maturation as consequence of the loss of trapped and absorbed water attached to silanols.

We have made a detailed study of the extended X-ray absorption fine spectra (EXAFS) at the K edge of aqueous Y ion and at L 3 edges of aqueous lanthanide ions and thereby elucidated the systematic changes in their hydration structures. Anomalous peaks arising from double-electron excitation (2p, 4d → 5d, 5d) appear in the EXAFS signals of La 3+ -Tb 3+ between 5-7 Å−1. We established a removal process of double-electron excitation from EXAFS spectra. Using that process, we confirmed that the intensity and energy position of the extracted double-electron excitation are comparable to previously reported data. The presence of double-electron excitation engenders a smaller error than the errors estimated in the fitting process. Consequently, double-electron excitation does not seriously affect the determination of the structures of REE 3+ aquo ions in the first coordination sphere. Subsequent EXAFS analyses of hydrated REE 3+ ions suggest that the hydration numbers, the interatomic distances, and the Debye-Waller factors decrease from 9.7, 2.55 Å, and 9.0 × 10 −3 Å 2 for La 3+ to 7.9, 2.31 Å, and 5.7 × 10 −3 Å 2 for Lu 3+ . These parameters change as a sigmoid curve with increasing atomic number. The hydration structures of REE 3+ ions are inferred to change from the nonahydrated structure for La 3+ -Nd 3+ to the octahydrated structure for Tb 3+ -Lu 3+ through intermediate structures for Sm 3+ , Eu 3+ , and Gd 3+ . In addition, the hydration state of Y 3+ closely resembles that of Ho 3+ because the two have almost identical ionic radii.

This paper reports the results of an in situ HP synchrotron X-ray powder diffraction investigation on the natural zeolite gismondine (ideal chemical formula Ca 4 Al 8 Si 8 O 32 ·16H 2 O, space group P2 1 /c). The study was performed from P amb to 7.9 GPa, and upon decompression, using methanol:ethanol:water (16:3:1) mixture (m.e.w.) as a nominally penetrating hydrostatic P-transmitting medium. No complete X-ray amorphization is observed up to the highest investigated pressure, and the original unit-cell parameters are almost completely recovered upon decompression. From 0.6 GPa, the water content is slightly higher than at ambient pressure, as a result of a moderate over-hydration. Moreover, at about 2 GPa, a significant water molecule system re-arrangement occurs, characterized by an ordering of part of the water molecules from four partially occupied sites to only two fully occupied ones. The over-hydration, but not the water ordering, is substantially irreversible upon pressure release. The Rietveld structural refinements of the powder patterns converged successfully up to 2.8 GPa; above this pressure, a phase transition to triclinic symmetry was observed and only the unit-cell parameters were refined. The comparison of the overall cell volume reductions and of the bulk modula of gismondine compressed between P amb and 7.9 GPa in m.e.w. and in silicone oil, reveals that this is the unique zeolite with a higher compressibility in penetrating vs. non-penetrating P-transmitting media. This is ascribed to the re-organization of the water molecule system upon compression in m.e.w., which leaves a larger free volume inside the pores with respect to the phase compressed in silicone oil.

The effects of heat treatment on stilpnomelane from granite pegmatites at Zółkiewka (Lower Silesia, Poland) were studied using 57 Fe Mössbauer spectroscopy. Samples of naturally altered stilpnomelane and chlorite were also studied. The Mössbauer spectrum of an untreated sample was fitted to two Fe 2+ doublets and two Fe 3+ doublets. The Fe 2+ doublets had similar isomer shifts (1.13 and 1.16 mm/s for the octahedral M1 and M2 sites), different quadrupole splitting values (QS = 2.28 and 2.66 mm/s, respectively), and an M1:M2 area ratio of nearly 1:3. For ferric iron, the first doublet was assigned to the combined M1 + M2 + M4 octahedral positions and had IS = 0.41 and QS = 1.18 mm/s. The second doublet was assigned to the M3 site, with parameters IS = 0.36 and QS = 2.07 mm/s. This is characteristic for stilpnomelane. The oxidation of Fe 2+ is fast at low temperatures, with a maximum at 360 °C. Near 1050 °C, the stilpnomelane structure broke down completely, and the spectrum consisted of two sextets and two ferric iron doublets. The predominant phase in this new material is hematite, as represented by the two sextets. The abnormally high fractional Fe 2+ content in the altered stilpnomelane (0.70) as compared to the untreated sample (0.45) indicates that the sample was subjected to local hydrothermal processes. Nearly identical divalent iron contents and hyperfine parameters for the Fe 2+ and Fe 3+ doublets (except Fe 3+ in M3) in the altered stilpnomelane and associated chlorite strongly suggest that chlorite is transformed into stilpnomelane.

Recently, Michel et al. (2007a) have presented a structure for ferrihydrite that we show to be incorrect. We do this by comparing (1) the sample form factor without adjustable parameters from powder X-ray diffraction data using a recently developed method with (2) exactly simulated (Debyesum method) theoretical sample form factors for the recently proposed structure (including vacancy, particle size and shape, and positional disorder effects). Michel et al. (2007a) used pair distribution functions (PDFs) extracted from synchrotron diffraction data fitted with calculated PDFs using adjustable scale and peak shape parameters. The PDF method gives consistent short-range (coordination sphere) correlations but under-emphasizes intermediate-range correlations that represent more stringent constraints on the structure. Main characteristic diffraction peaks of six-line ferrihydrite (lines 2, 3, and 4) are not reproduced by the proposed structural model. We expect our method to offer rigorous tests of proposed structures of any nanocrystalline materials.

The structural variations of synthetic zircon (ZrSiO 4 ) single crystals irradiated at room temperature by 280 keV Pb + ions (with fluences up to 1 × 10 15 ions/cm 2 ) were investigated using infrared (IR) spectroscopy. Like metamict zircon whose crystal structure is damaged and amorphized by naturally occurring α-decay events, the Pb + -irradiated zircon crystals show a dramatic decrease in reflectivity. However, no significant decrease in wavenumbers of the stretching vibrations of SiO 4 tetrahedra in zircon was detected. The Pb + -implanted zircon exhibits new IR bands, indicating irradiation-induced new vibrations or domains, clusters or phases in addition to SiO 2 and ZrO 2 . IR features consistent with those of Pb silicates (with a divalent state, i.e., Pb 2+ ) are also found in the irradiated sample. This finding implies that some of the radiogenic Pb in natural zircon might not actually reside in the zircon lattice or in ZrSiO 4 phases, but form new local domains or clusters. Infrared bands of OH-stretching vibrations were also detected in the irradiated synthetic zircon, which was originally free from OH features prior to the irradiation. These results indicate that H can easily diffuse into the irradiated layer or into irradiated-induced phases to form OH or and hydrous species after the irradiated material is damaged. The type and content of hydrous species vary with irradiation fluences.

The compressibility and crystal structure of a recently discovered post-titanite phase of CaGe 2 O 5 was investigated by single-crystal X-ray diffraction to 8.6 GPa at room temperature in a diamondanvil cell. Unit-cell parameters decrease non-linearly with increasing pressure and do not show any discontinuity in the pressure range investigated. The unit-cell volume decreases by about 4.6% to 8.6 GPa. The P-V data were fit using a third-order Birch-Murnaghan equation of state giving the following coefficients: V 0 = 345.65(4) Å 3 , K T0 = 159(1) GPa, and K′ = 5.0(3). A parameterized form of the same equation of state was used to obtain the axial moduli for a, b, and c. The room-pressure ratios of axial compressibility are 2.64:1.00:1.42, indicating strong compression anisotropy, with b the stiffest direction and a the most compressible one. The crystal-structure data confirm that no change in symmetry occurs at high-pressure. Most of the compression to 8.6 GPa is accommodated by the CaO 8 and GeO 5 polyhedra, with reductions in volume of 5.6 and 4.6%, respectively. The analysis of the individual bond-lengths with pressure is discussed to explain the observed strong axial anisotropy. A comparison with the closely related crystal structure of andalusite shows that the post-titanite phase is less compressible by about 10%. A further comparison with other titanite phases studied at high-pressures allows us to obtain a qualitative model capable of predicting their bulk moduli when unit-cell volume at ambient conditions is known.

A hypersilicic Cl-bearing mica was synthesized at 4 GPa and 1200-1250 °C, close to the solidus of the join diopside-jadeite-KCl, in association with diopside-jadeite pyroxene, K-rich aluminosilicate glass and/or sanidine and (K,Na)Cl. The mica shows a negative correlation between tetrahedral Si and octahedral (Al + Mg), suggesting an Al-celadonitic substitution (Si + VI Al + VI □ = IV Al + VI Mg) and a chemical formula: K 1.01 (Mg 2.45 Al 0.19 □ 0.35 ) Σ=3 (Si 3.5 2Al 0.48 ) Σ=4 O 10 [(OH,O) 1.66 Cl 0.34 )] Σ=2 . The presence of hydroxyl was confirmed by OH stretching modes at 3734 and 3606 cm -1 in the Raman spectra. Singlecrystal X-ray diffraction data provide the unit-cell parameters (space group C2/m, 1M polytype): a = 5.299(4), b = 9.167(3), c = 10.226(3) Å, β =100.06(4)°, V = 489.1(4) Å 3 . The structure refinement shows the presence of vacancies on the octahedral sites (15% for M1 and 6.5% for M2). Chlorine occupies a position about 0.5 Å from O4 with partial occupancy (0.39 apfu). Crystal-chemical mechanisms seem to govern chlorine incorporation in mica, since a large A site is necessary to locate the anion in the structure. A large A site results when the six-tetrahedra ring is hexagonal and the tetrahedral rotation angle α is 0°. Such a geometry is achieved either by increasing the annite component in biotite or by increasing the hypersilicic character of phlogopite through the Al-celadonite substitution. The present Si-rich mica shows a partial dioctahedral character due to the Al-celadonite substitution, which lowers the α angle and expands its stability field at high pressure. High a K₂O conditions, like in potassium-rich brine or potassic carbonatitic melts, increase the Alceladonite component in the phlogopite solid solution, explaining the association of Si-rich micas with inclusions of potassic liquids in kimberlitic diamonds.

We have performed five separate sets of experiments to elucidate the effects of magnetite, ulvöspinelmagnetite solid solution, and pyrrhotite crystallization on the budgets of Au, Cu, and Ag at magmatic conditions. The experiments were done in both hydrous and anhydrous assemblages at temperatures between 800 and 1050 °C, pressures from ambient to 140 MPa, log f O₂ from NNO-0.25 to NNO, and log f S₂ from -1.5 to -3.0. Nernst-type partition coefficients (±1σ) at 800 °C in a water-saturated assemblage are D Ag Mt/melt = 2 × 10 -4 ± 2 × 10 -9 , D Cu Mt/melt = 0.82 ± 0.69, D Cu Usp/melt = 26 ± 17, D Au Usp/melt = 50 ± 31, D Cu Po/melt = 174 ± 25. Nernst-type partition coefficients (± 1σ) at 1050 °C in an anhydrous assemblage are D Cu Po/melt ≥ 200, D Ag Po/melt = 58 ± 8, D Au Po/melt = 120 ± 50. The calculated values for D Au Usp/melt and D Cu Usp/melt indicate that the addition of Ti to magnetite increases significantly the Au- and Cu-scavenging potential of ulvöspinel relative to end-member magnetite. Partition coefficients for Cu and Au between pyrrhotite and melt indicate that a temperature change from 1050 to 800 °C in an anhydrous and hydrous assemblage, respectively, results in no observable change in Cu partitioning. The calculated partition coefficients are used to model the effect of crystal fractionation on the concentrations of Ag, Cu, and Au. Model results suggest that the co-crystallization of magnetite and pyrrhotite sequester no more than 2% Ag, 7% Cu, and 37% Au from the melt over the first 25% solidification. If the melt reaches volatile saturation after 25% crystallization, the presence of end-member magnetite and pyrrhotite do not appear to inhibit the Cu-, Au-, and Ag-ore potential of the magma. Ulvöspinel-magnetite, however, may reduce the Au concentration in the melt by approximately one-third relative to its initial value that decreases the overall Au available to partition into the volatile phase.

Analcime forms in a wide range of geologic environments from alkaline lakes to primary igneous occurrences in Si-undersaturated lavas and pegmatites and is present in many radioactive waste repository settings. To evaluate its hydration state in these environments, calorimetric-hydration heat measurements, equilibrium observations, and thermodynamic modeling were performed. The enthalpy of hydration of analcime determined by isothermal adsorption calorimetry was found to be independent of degree of hydration and relatively insensitive to temperature. Equilibrium hydration states of analcime as a function of temperature and vapor pressure were assessed by isothermal thermogravimetry. In light of the lack of excess enthalpy of mixing in this solution, an ideal solution model was applied to the equilibrium observations along with previously determined heat capacities hydration to retrieve the standard Gibbs energy of hydration from water vapor (-47.29 ± 0.56 kJ/ mol H 2 O), standard enthalpy of hydration (-85.10 ± 1.03 kJ/mol H 2 O), and entropy of hydration (-126.81 ± 1.60 J/mol H 2 O⋅K) at 298.15 K, 1 bar. The standard enthalpy of hydration at 298.15 K, 1 bar regressed from the calorimetric data are consistent with these results (-86.4 ± 1.9 kJ/mol H 2 O). Calculations of the hydration state of analcime as a function of temperature and pressure indicate that analcime is essentially fully hydrated at temperature and pressure conditions attending its formation in geologic and experimental systems. Dehydration of analcime under water-undersaturated conditions (as during heating in a radioactive waste repository) may contribute significantly to thermal budgets in systems where this phase is important.

Sodium-bearing, type A-B carbonate fluorapatites {Ca 10-1.5y Na y □ 0.5y [(PO 4 ) 6-2y (CO 3 ) 2y ][F 2-2x (CO 3 ) x ], with x ≈ y ≈ 0.1; CFAP} have been synthesized at 1150-1300 °C and 1.0 GPa, and investigated by single-crystal X-ray diffraction and FTIR spectroscopy. Typical crystal and compositional data are a = 9.3659(3), c = 6.8901(4) Å, space group P6 3 /m, R = 0.023, R w = 0.021, x = 0.14(3), and 2y = 0.23(1). Crystal-chemical features and FTIR spectra are similar to Na-bearing, type A-B carbonate hydroxylapatites (CHAP) reported recently. However, uptake of carbonate is significantly lower in CFAP than in CHAP for similar conditions of crystal synthesis. Structural accommodation of the substitutions requires local coupling of the Na cation and channel (type A) and phosphate (type B) carbonate ion defects, as in CHAP but now in the approximate ratio 1:1:2. The type B carbonate ion is located on the sloping faces of the substituted phosphate group, and tilted only 3.5° from the mirror plane. The presence of weak electron density in the outer wall of the c-axis channel of apatite and in triangular coordination with one Ca1 and two Ca2 cations appears to represent a trace amount of excess fluoride, the controversial crystal-chemical feature of francolite.

A scapolite sample, Me79.6, from Slyudyanka, Siberia, Russia, has been studied using in situ synchrotron powder X-ray diffraction (XRD) and Rietveld structure refinements on heating from 26 to 900 °C and on cooling to about 300 °C. The structure was modeled and refined in space group I4/m. An accurate room-temperature structure was also obtained by using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinement. From HRPXRD, the cell parameters are a = 12.16711(2), c = 7.575466(5) Å, and V = 1121.461(3) Å 3 ; <T1-O> and <T2-O> are 1.643(1) and 1.672(1), respectively, so the T1 (Al 0.25 Si 0.75 ) and T2 (Al 0.46 Si 0.54 ) sites are partially ordered at room temperature. On heating, the <T-O> distances indicate that the T1 and T2 sites become more Si- and Al-rich, respectively, and therefore, ordering increases unusually with increasing temperature. This increase in Al-Si ordering occurs from 892 to 900 °C. At 900 °C, the T1 site becomes fully ordered with only Si atoms, while the T2 site contains Al 0.51 Si 0.49 and therefore, is fully disordered. On cooling, the sample does not fully revert back to the original partially ordered state. At 300 °C, all the cell parameters are smaller because of the increased Al-Si ordering that is quenched in.

This study formulates the following relationship for identifying randomly interstratified illite/ smectite using the integrated peak widths (β) for the first-order reflections after ethylene-glycolation (EG) and thermal dehydration (TD) treatments: where p i is the proportion of interstratified illite layers. The relationship minimizes the effect of crystallite thickness on the evaluation of p i , which has been a critical source of error in the previous methods using peak position and intensity.