Band 90, Heft 1

The NNW-trending White-Inyo Range represents the eastern continuation of the Sierran volcanic- plutonic arc along the central California-Nevada state line. South of White Mountain Peak, the NE-trending, steeply SE-dipping Barcroft structural break transects the orogenic belt. This high-angle reverse fault downdropped deformed, weakly recrystallized, mid-Mesozoic, mildly alkaline, White Mountain Peak mafic and felsic metavolcanic flows, interlayered volcanogenic metasedimentary strata, and minor subvolcanic plugs against a folded Neoproterozoic-lower Paleozoic quartzite and carbonate metasedimentary section on the south. The Middle Jurassic Barcroft granodioritic pluton later intruded the Barcroft break; the Middle Jurassic Cottonwood, and Late Cretaceous McAfee Creek + Pellisier Flat granitoids also invaded the superjacent section on the NE. Heating of the White Mountain Peak metavolcanic complex by the Barcroft and nearby plutons caused a southeastward progressive increase in metamorphic grade, producing neoblastic biotite, epidote, and hornblende in mafic volcanic rocks + minor hypabyssal stocks, and biotite + epidote ± andalusite in felsic volcanic rocks + volcaniclastic strata. Gradual increases in biotite Ti and plagioclase An contents reflect this SE metamorphic zonation. Utilizing hornblende-actinolite, muscovite-celadonite, and biotite solid solutions, thermobarometry indicates lithostatic pressures of 2-3 kbar during recrystallization, with temperatures ranging from a background value of ~350 °C far from the Barcroft pluton, to >500 °C along the intrusive contact. Physical conditions were similar to those developed in the Neoproterozoic-lower Paleozoic platform strata directly south of the Barcroft granodiorite. The region constitutes a typical example of the P-T evolution of the late Mesozoic Californian crustal margin, where episodic magma emplacement caused widespread contact metamorphism.

Samples of pegmatite quartz from several localities in Norway and Namibia were investigated by cathodoluminescence (CL) microscopy and spectroscopy, electron paramagnetic resonance (EPR) measurements, and trace-element analysis (ICP-MS) to obtain information about their structure and luminescence characteristics. The defect structure and trace-element composition of the pegmatite quartz samples that were studied differ from those of quartz of other origin (hydrothermal, igneous and metamorphic quartz). EPR measurements reveal an almost complete lack of intrinsic lattice defects associated with O or Si vacancies (e.g., E’ center, O 2 3- center), whereas some trace elements (Al, Ti, Ge, Li) are apparently enriched and form paramagnetic centers. The results indicate that there possibly is a redistribution of alkali ions during electron irradiation. The diamagnetic [AlO 4 /M + ] 0 center transforms into the paramagnetic [AlO 4 ] 0 center, while the compensating ions diffuse away and may be captured by the diamagnetic precursor centers of [GeO 4 ] 0 and [TiO 4 ] 0 to form paramagnetic centers ([TiO 4 /Li + ] 0 , [GeO 4 /Li + ] 0 ). These defects result in a specific luminescence behavior, which is very similar for all samples. In general, quartz from pegmatites shows homogeneous CL without growth zoning or internal structures suggesting constant physicochemical conditions during crystal growth. The CL emission is dominated by a transient bluish-green CL, which disappears after 60.100 s of electron irradiation. The two main emission bands centered at 505 nm (2.45 eV) and 390 nm (3.18 eV) are probably related to alkali-compensated, trace-element centers in the quartz structure. Other CL emission bands, which are characteristic features of igneous, metamorphic, or hydrothermal quartz (e.g., at 450 nm = 2.75 eV, 580 nm = 2.14 eV, 650 nm = 1.91 eV) are almost completely lacking. This fact indicates that the defects responsible for these CL emissions are absent in pegmatite quartz.

Using laser-heated diamond-anvil cells, we have observed CaSO4 undergoing phase transitions from its ambient anhydrite structure to the monazite type, and at highest pressure and temperature to crystallize in the barite-type structure. On cooling, the barite structure distorts from an orthorhombic to a monoclinic lattice to produce the AgMnO 4 -type structure. The barite-structured form of CaSO 4 that we encounter at high pressure and temperature has been, in particular, long expected as a highpressure phase of CaSO 4 -anhydrite from systematic trends of similar A II B VI O 4 -type sulfates, selenates, and tellurates, but has not been observed before. Similarly, the monoclinic distortion of the barite structure has itself been proposed as an intermediate phase between anhydrite and barite types through comparison with the phase diagrams of NaBF 4 and NaClO 4 . This result has important consequences for identifying structural trends between different ABO 4 -type phases of Group II sulfates, selenates, tellurates, chromates, molybdates and tungstates that crystallize in anhydrite, zircon, monazite, barite and scheelite-type structures at ambient and high pressures.

The microscopic behavior of the Ca-zeolite yugawaralite has been studied by ab initio molecular dynamics simulations adopting experimental cell parameters obtained at pressures up to ~9 GPa. Pressure-induced volume contraction occurs via rotations of quasi-rigid TO4 tetrahedra that reduce the size of the channels in which the extra-framework species are located. Such rotations are governed by deformation of the coordination polyhedron of Ca, which is made up of water and framework O atoms. Contraction of the Ca-H 2 O distances is favored at moderate pressure; at higher pressure the shortening of Ca-framework O atom distances becomes prevalent. The hydrogen bond network plays a fundamental role in the overall response to pressure. Our results indicate that the high-P-induced deformation of the framework structure is strictly correlated to the extra-framework species that act as “templates” in the compression process.

Metastable coesite is an important pressure indicator for ultrahigh-pressure rocks. However, in many cases coesite does not survive exhumation, but reacts back to quartz. Although it was shown experimentally that incorporation of H in coesite increases with increasing pressure, most coesite relics found in nature are essentially dry (i.e., OH concentrations are below detection limit, <1 wt ppm H 2 O). Thus, does the incorporation of H promote the back-reaction of coesite to quartz during exhumation? The influence of intrinsic OH on the kinetics of the coesite-quartz phase transition was determined using synthetic “dry” coesite with ≈ 10 wt ppm H 2 O and “wet” coesite with ≈ 105 wt ppm H 2 O. TEM analysis of the quenched samples proved the presence and absence of water in the .wet. and .dry. samples, respectively. The kinetics of the coesite-quartz transition was investigated in-situ using the multi-anvil apparatus MAX 80 at the Hamburger Synchrotron Radiation Laboratory (HASYLAB). The transition rates were measured by observing changes in selected diffraction line intensities as a function of time. The transformation and growth rates were derived using Cahn's model of nucleation and growth at grain boundaries. Under the same experimental conditions the transformation rate of the “wet” coesite is more than ten times higher than that of the “dry” coesite. This difference may explain why OH-bearing natural coesite is rare. This study reveals the importance of structurally bound OH for the kinetics of phase transitions of nominally anhydrous minerals.

We have investigated the physical and structural properties of phase D (MgSi 2 O 6 H 2 ) at high pressure by means of a first-principles calculation method. Phase D is important as a dense hydrous magnesium silicate with high stability under pressure, and as one of the most likely candidates for a water reservoir in the Earth’s lower mantle. The calculated compression behavior of phase D is in very good agreement with experimental results. We found a distinct but continuous change from asymmetric to symmetric hydrogen bonding in phase D at 40 GPa. This pressure-induced hydrogen bond symmetrization has a significant effect on the compression behavior of phase D. The bulk modulus increases by about 20% with this structural change. This behavior of pressure-induced hydrogen bond symmetrization is very similar to that previously reported by us for δ-AlOOH. The transition is reversible and second-order, and thus the high-pressure state is probably unquenchable.

The OH-stretching modes of kaolinite group minerals, namely kaolinite, dickite, and nacrite, are investigated within the density functional theory framework. Good agreement between theoretical and experimentally derived structures is obtained. The total energy of the three polymorphs is found to be similar within 1 kJ/mol. After a review of existing experimental data, infrared and Raman spectra of the three polymorphs are computed. While interpreting the spectra, special attention is given to the role played by the macroscopic geometry of the system, explaining the differences experimentally observed between infrared and Raman spectra. In dickite and nacrite, a significant interlayer coupling is observed for the modes polarized along a direction nearly parallel to c*. The assignment of OHstretching bands is given in a consistent way and is compared with previous suggestions.

The incorporation of hydrogen into wadsleyite (β-Mg 2 SiO 4 ) was investigated using polarized FTIR spectroscopy and X-ray diffraction on oriented single crystals. The experiments were carried out with a new suite of samples containing between ~100 and ~10 000 ppm H 2 O by weight (wt ppm), encompassing the H-contents most relevant to Earth's potentially hydrous mantle transition zone. Attempts to synthesize anhydrous wadsleyite resulted in water contents of no less than ~50 wt ppm H 2 O. An empirical relation between the b/a axial ratio against estimated wt ppm concentrations of H 2 O in wadsleyite (CH 2 O) was determined: (b/a) = 2.008(1) + 1.25(3) × 10 -6 · CH 2 O Polarized infrared absorption spectra were measured in the three orthogonal sections perpendicular to the major axes of the optical indicatrix ellipsoid and are used in concert with results from new structure refinements to place constraints on the main absorbers in the structure. All of the main bands in the O-H stretching region of the FTIR can be explained by protonation of O1, the anomalous non-silicate oxygen site. We assign the band at 3614 cm -1 to a bent hydrogen bond O1···O1 (2.887 Å) along the M3 edge in the a-c plane. The band at 3581 cm -1 is assigned to a bent hydrogen bond on O1···O3 (3.016 Å) of the M3 edge in the b-c plane. The absorption bands at 3360, 3326, and 3317 cm -1 are best explained by hydrogen bonds on O1···O4 (3.092 Å) and O1···O4 (2.795 Å) along the M3 and M2 edges with possible splitting of one band due to vacancy ordering, but we cannot rule out contributions from three other (O1···O3) edges. The broad absorption feature at 3000 cm -1 is unambiguously assigned to the O4···O4 (2.720 Å) tetrahedral edge of the Si 2 O 7 group pointing along the [100] vector. On hydration to ~1 wt% H 2 O, M-site vacancies are observed exclusively at M3. A systematic shortening of several (interpreted) hydrogen bonded O···O M-site edges is attributed to reduced O-O repulsive forces on protonation in the vicinity of an M-site vacancy.

The X-ray diffraction method previously developed for the determination of the amount and distribution of fixed NH 4 + in illite-smectite has been modified to include the effects of layer thickness fluctuations of K-saturated and heated smectite and the effects of mean thickness of coherent scattering domains (CSDs). X-ray diffraction patterns and 002 and 005 reflection profiles are calculated for K-saturated and dehydrated NH 4 + -bearing I-S representing models having different distributions of K and NH 4 + over mica-like interlayers, different proportions of interlayer types, different mean thicknesses of CSDs, and different degrees of thickness fluctuations for K-saturated and heated smectite layers. The diffraction criteria for identification of these models are discussed. The amount of fixed NH 4 + can be determined accurately from the position of the 005 reflection. Diffraction methods have low sensitivity to different distributions of fixed K and NH 4 over mica-like interlayers in NH 4 -bearing I-S containing a high amount of expandable interlayers and a low amount of fixed NH 4 . However, the interstratified nature of NH 4 -bearing illites or I-S can be determined unambiguously in two limited cases: first, when the structures have a low (<20%) content of expandable layers (WS) and, secondly, when NH 4 /(NH 4 + K) ≥ 0.20 in the mica-like interlayers, even for WS > 0.20. The method is applied to I-S from western Greenland Cretaceous oil source rocks heated by intrusions. The samples contain I-T-S consisting of 13.33% tobelite layers. Two groups of samples are identified. One includes I-T-S structures in which dehydrated K-smectite layers have no significant thickness fluctuations. For these samples different broadening of 002 and 005 reflections is due only to interstratification of the 9.98 and 10.33 Å layers. In the other group a satisfactory agreement between the experimental and calculated positions and profiles of the 002 and 005 reflections is achieved only when thickness fluctuations for the K-smectite layers are taken into account.

Near-atomic resolution TEM imaging has been successfully applied to determine the stacking defect structures in kaolin minerals, especially kaolinite. The specimen studied is from the middle stage of the depth-related kaolinite-to-dickite transformation in a sandstone reservoir. Several high-quality images were recorded on films in which the tetrahedral and octahedral positions in the kaolinite unit layers are clearly resolved, despite the obstacle of electron radiation damage. Electron diffraction and high-resolution imaging of dickite showed that few stacking defects exist in this polytype. On the other hand, kaolinite crystals contain a high density of stacking defects. These defects or stacking disorders are formed by a mixture of two kinds of lateral interlayer shifts, t 1 (approximately −a/3) and t 2 (-a/3 + b/3), between adjacent layers. Disorder due to the coexistence of B and C layers (dickite-like stacking sequence) was never observed. These results provide not only an unambiguous settlement for the long -standing controversy of the defect structures in kaolinite, but also a new clue to understanding the kaolinite-to-dickite transformation mechanism.

Cation partition among coexisting minerals has been widely applied to eclogite thermometry, but an accurate estimation of Fe 3+ content compared to total Fe is crucial in obtaining reasonable temperatures for petrologic studies. Room-temperature Mössbauer spectroscopy was measured for garnet-omphacite pairs in high-pressure (HP) and ultrahigh-pressure (UHP) eclogites from the Dabie terrane in east-central China. The results show very low Fe 3+ /ΣFe ratios of 0.026 to 0.082 in garnet but high Fe 3+ /ΣFe ratios of 0.240 to 0.689 in omphacite. The hyperfine parameters of minerals record the HP-UHP conditions that the eclogites experienced. Fe 2+ in clinopyroxenes with low Na + Ca contents in their M2 sites shows pressure-induced occupation in M1 site. The quadrupole splitting of Fe 2+ in HP-UHP garnets (3.61 to 3.77 mm/s) and omphacites (2.77 to 3.06 mm/s) are among the highest values ever reported, indicating effectively pressure-regulated polyhedral sites. After the Fe 3+ was corrected, Fe 2+ -Mg partitioning not only significantly narrow the ranges relative to those without Fe 3+ correction, but also yield temperatures about 8 to 370 ºC lower than the uncorrected temperatures for the same garnet-pyroxene pairs. The recalculated temperatures are constrained to narrow ranges of 477 to 647 °C for quartz-bearing eclogites and 624 to 843 °C for coesite-bearing eclogites. These maximum values provide close proxies to peak metamorphic temperatures provided that the retrograde exchange of Fe-Mg cations by diffusion between minerals during exhumation is taken into account.

Allanite and epidote occur in the parent rocks of weathered regolith at the Coweeta Hydrologic Laboratory in North Carolina and exhibit different responses to weathering. Petrographically, epidote and allanite are identical at Coweeta, and only with additional analytical techniques (e.g., EDS or LAICP- MS) can the two be distinguished. Allanite is more abundant in unweathered bedrock but weathers readily below the weathering front. In contrast, the much less abundant epidote persists into the solum with only minimal evidence of weathering. The rapid dissolution of allanite is likely influenced by its metamict nature. Both epidote and allanite at Coweeta dissolve by interface-controlled dissolution kinetics, evidenced by etch pits on grain surfaces. Etch pits appear to be either large “negative crystals,” or small, shallow, and elongated. The incipient stage of allanite weathering is characterized by Al mobility and Fe precipitation as goethite. During the initial stage of allanite weathering, carbonate precipitates, but with progressive weathering the carbonate is dissolved. Based on electron microprobe analyses and point-count data of the Ca-bearing phases in the Coweeta bedrock, accessory (<1%) allanite contains a minimum of approximately 25% of the total bedrock Ca by volume, whereas garnet and plagioclase contain 5% and 70%, respectively. Although allanite and epidote are Ca-hosts, only allanite is present in significant quantities, and is weathering sufficiently rapidly, to serve as an important source of Ca in pore and stream waters at Coweeta. Allanite weathering should, therefore, be evaluated in weathering studies of crystalline silicate bedrock, especially where other lines of evidence indicate the need to invoke additional Ca sources.

The composition and structure of a new phase crystallizing in high-pressure experiments in the system CaO-Ca(OH) 2 -CaF 2 -SiO 2 are described. The composition is on the join Ca 2 SiO 3 (OH) 2 -Ca 2 SiO 3 F 2 , with the relative amounts of OH and F near 50/50 from electron probe microanalysis, or near the ideal midpoint composition Ca 2 SiO 3 OHF. The symmetry is monoclinic, space group P2 1 /c, with a = 5.8111(7), b = 10.6050(13), and c = 6.6968(8) Å, and β = 102.025(3)°, V = 403.65(8) Å 3 , Z = 4. In the structure isolated SiO 3 OH tetrahedral groups occupy narrow linear channels whose axes lie along a. The channels are lined by two types of eight-coordinated CaO 5 OHF 2 polyhedra that share edges and faces. The axes of the channels lie on a nearly hexagonal array in the b-c plane, leading to a c p /b ratio of 0.618, close to the ideal ratio of 0.577 for a hexagonal array. However, there is no simple packing rule for either the cation or anion array. Although the channels are arranged in a nearly hexagonal fashion, the overall symmetry of the structure is not hexagonal, but instead has 5-rings of calcium polyhedra around the channels. The calcium atoms lie on a 5-3-5-3 net of triangles and pentagons that can be compared to the α-U 3 O 8 net.

Laser-ablation ICP-MS data of hydrothermal vein quartz with zonation in scanning electron microscope cathodoluminescence reveal two groups of trace elements, one that co-varies in concentration with luminosity and another that remains uniform throughout. Bright luminosity correlates with a high total abundance of trace elements in early quartz, including Al (up to 410 μg/g), Ti (up to 240 μg/g), K (up to 330 μg/g), Li (up to 8 μg/g), Na (up to 36 μg/g), and Fe (up to 20 μg/g). Up to 20 times lower concentrations of these elements are associated with dark luminosity in late quartz. Concentrations of P (~21 μg/g), Ga (~0.3 μg/g), Ge (~1.3 μg/g), Sn (~1.5 μg/g), Cu (~0.3 μg/g), and Ag (~0.1 μg/g) demonstrate no relationship with luminosity. Charge balance cannot be achieved for bright luminescent quartz unless the presence of up to 6 μg/g H + (not analyzed by LA-ICPMS) or interstitial Al 3+ is invoked. Interestingly, the inferred ~6 μg/g H + remains constant for quartz containing more than 10 μmol total trace elements and might represent the solubility of H + in hydrothermal quartz at temperatures between 450 and 700 °C. LA-ICPMS results indicate that the fluid chemistry determines the quartz trace-element pattern, which may serve as a monitor for the chemical environment from which quartz crystallized. Ratios of Na/Al and Ti/Al are uniform, whereas K/Al evolves toward lower values with decreasing luminosity. Combined evidence, including quartz vein textures, silica solubility data, and estimates of the temperature of quartz growth, suggest that early quartz crystallized fast but late quartz grew more slowly at 350 to 425 °C. It is speculated that the higher the quartz growth rate is, the more trace elements are incorporated into quartz, besides the temperature dependence known from the literature. Clearly, growth rate, temperature and fluid chemistry are important parameters affecting luminosity; however, their relative importance may vary from case to case.

A quantitative relation between the linear attenuation coefficient (LAC) obtained by Synchrotron Radiation X-ray computed tomography (observed LAC) and the theoretically calculated LAC (theoretical LAC) of standard materials (minerals and metals) has been obtained for an X-ray microtomographic system at BL20B2 of SPring-8, Japan. This system, called SP-μCT, uses highly monochromatized and well-collimated X-ray beams produced by a synchrotron radiation source. Three-dimensional images were obtained for samples 0.4.4 mm in size at X-ray energies of 15.35 keV with a voxel size of 5.83 × 5.83 × 5.83 μm 3 . A histogram of the observed LAC for each sample was well-fitted by a Gaussian curve except for heavy metals whose X-ray transmittance was insufficient. The contrast resolution of CT images is best (within 5% of the LAC value) at LACs of about 10.30 cm -1 . A garnet schist was imaged with SP-μCT to verify the observed-theoretical LAC relation for minerals contained in a rock sample. The result was consistent with the relation obtained for the standards. The CT and back-scattered electron images of the rock sample were compared. The present results put restrictions on discrimination of mineral phases and estimation of chemical compositions (e.g., Mg/Fe ratio) of certain minerals forming solid solutions based on CT values. The present quantitative relationship between observed and theoretical LACs enables us to obtain an absolute elemental concentration map by imaging just above and below the X-ray absorption edge energy of the element (subtraction method).

Bacterial Mn(II) oxidization by spores of Bacillus, sp. strain SG-1 has been systematically probed over the time scale 0.22 to 77 days under in-situ conditions and at differing Mn(II) concentrations. Three complementary techniques, K-edge X-ray absorption near-edge spectroscopy (XANES), X-ray emission spectroscopy (XES), and in-situ synchrotron radiation-based X-ray diffraction (SR-XRD), have been utilized to examine time-dependent changes in Mn oxidation state, local-, and long-range structure in amorphous, crystalline, cell-bound, and solute Mn species. The primary solid biogenic product of Mn(II) oxidation is an X-ray amorphous oxide similar to δ-MnO 2 , which has a Mn oxidation state between 3.7 and 4.0. Reaction of Mn(II) with the primary biogenic oxide results in the production of abiotic secondary products, feitknechtite or a 10 Å Na phyllomanganate. The identity of the secondary product depends upon the Mn(II) concentration as described by thermodynamic relations. A decrease in the dissolved Mn(II) concentration is followed by mineralogic transformation of the secondary products. Thus, Mn(II) appears to act as a reductant toward the biogenic oxide and to control the stability of secondary reaction products. Mineralogic changes similar to these are likely to be commonplace in natural settings where bacterial Mn(II) oxidation is occurring and may liberate sorbed metal ions or alter the rates of important Mn oxide surface-mediated processes such as the degradation of organic molecules. It is plausible that microbes may exploit such mineral transformation reactions to indirectly control specific chemical conditions in the vicinity of the cell.

The equilibrium intracrystalline distribution of Mn and Mg between the M1 and M2 sites of a Mn-rich/Fe-free orthopyroxene (donpeacorite) was investigated by means of annealing experiments at temperatures between 980 and 800 °C and single-crystal X-ray diffraction. The data show that Mn, as does Fe 2+ in Fe-Mg orthopyroxene, preferentially orders at the M2 site. However, comparison of the distribution coefficient k D(Mn-Mg) determined in this study with k D+ measured for Fe-Mg orthopyroxene shows that Mn has a much stronger preference for the M2 site relative to Fe 2+ . This result implies that the practice to partition Fe 2+ + Mn = Fe* as one species, typically implemented to determine the quenched-site occupancies in Fe-rich/Mn-poor orthopyroxene, should be abandoned and that Mn should be considered totally ordered at M2. The partitioning method, i.e., Fe vs. Fe*, has implications for the determination of cooling rates from the observed ordering state of orthopyroxene, particularly for Fe-poor compositions (Fs < 0.16).

The crystal structures of chalcostibite CuSbS 2 (orthorhombic, space group Pnma, a = 6.018(1), b = 3.7958(6), and c = 14.495(7) Å, V = 331.1(1) Å 3 , Z = 4, R1 = 0.040, wR2 = 0.155 for 533 reflections) and emplectite CuBiS 2 (orthorhombic, space group Pnma, a = 6.134(1), b = 3.9111(8), and c = 14.548(8) Å, V = 348.8(2) Å 3 , Z = 4, R1 = 0.037, wR2 = 0.112 for 492 reflections) were redetermined using a four-circle diffractometer and graphite-monochromatized MoKα radiation. These two crystal structures are composed of MS 5 square pyramids (M = Sb and Bi) and nearly regular CuS 4 tetrahedra. The five M-S bond distances in the SbS 5 square pyramid in chalcostibite are always shorter than corresponding distances in the BiS 5 square pyramid in emplectite because the Sb atom is smaller than the Bi atom. The a cell parameter increases appreciably from chalcostibite to emplectite not only because of increasing M-S bond distances in the MS 5 square pyramid, but also because of increasing Cu-S2-Cu bond angles along a. The increase in the b cell parameter is caused mainly by increasing M-S bond distances along b. In contrast, the slight increase of the c cell parameter is largely brought about by decreasing Cu-S2-Cu bond angles ascribed to weakened stereochemical activity of Bi 6s 2 lone-pair electrons. The anisotropic change of unit-cell parameters from chalcostibite to emplectite is strongly associated with the positions of the lone-pair electrons in the unit cell.

This paper concerns the hydration and swelling behavior of four synthetic sodium saponite samples with general formula Na x (Si 4-x ,Al x )(Mg 3 )O 10 (OH) 2 , where x = 0.4, 0.5, 0.6, and 0.7. The combination of gravimetric water adsorption measurements and X-ray diffraction experiments under controlled water pressure allows us to analyze the influence of layer charge on swelling mechanisms. When the layer charge increases, swelling occurs for lower values of the chemical potential, i.e., at lower relative pressure. In parallel, as the cohesion between layers increases with increasing layer charge, a greater amount of water is needed to initiate swelling for higher-charged samples. In this last case, due to the proximity of interlayer cations, the transition between the one layer and two-layer hydrate occurs before the sodium cations are surrounded by a complete hydration sphere. This study also shows that the “one-layer” or “two-layer” hydrates cannot be considered as defined states as they appear to be modified with both layer charge and relative pressure.

Dehydroxylation and incorporation of CO 2 in mica (sericite-2M 1 ) during high-temperature annealing has been investigated in detail using infrared (IR) spectroscopy. The dehydroxylation is characterized by a dramatic change of the OH - ion spectrum and the appearance or development of different extra species (CO 2 , CO 3 2- , and OH - ). A significant decrease in the K-O bond strength and a complex distortion of the silicon-oxygen tetrahedra were also observed. The general similarities between the spectra of the untreated and dehydroxylated sericite show that the layered framework is somewhat preserved in sericite dehydroxylate. The results suggest that the incorporation of CO 2 into the structures of the heated materials is associated with or enhanced by the dehydroxylation process, although the two processes have different reaction mechanisms. The CO 2 contents in heat-treated sericite show a correlation with the loss rate of hydroxyl. A weak O-H stretching absorption band near 3480 cm -1 developed during dehydroxylation and it disappeared above 1100 °C. The results suggest that the developed additional O-H stretching signals and the thermally induced CO 2 do not structurally stabilize each other, as upon further heating the two different types of components vanished at different temperatures.

Exsolution rods of relict coesite together with quartz were identified in omphacite in eclogites from western Tianshan, China. They are oriented along the c-axis of the host clinopyroxenes and have grain size up to 30 μm long and 2.3 μm wide. Raman spectra of exsolved lamellae yield consistent but weak bands at 521, 270, 181, 151, and 118 cm -1 , typical for coesite, in addition to those of quartz and the host omphacite. Such occurrences together with textured observations suggest a two-stage evolution of SiO 2 exsolution rods in omphacite. Lamellae of coesite were apparently exsolved from supersilicic omphacite at P ≈ 5.0 GPa and the transformation from coesite to quartz occurred during retrograde metamorphism.

We report ab initio electronic structure calculations that directly relate given local chemical and distortion environments to corresponding hyperfine electric field gradients (EFGs) in 57 Fe Mössbauer spectroscopy, thereby giving needed interpretive power to the technique in characterizing VI Fe 2+ environments in minerals. Changes of the EFG with various distortions were investigated on model clusters, including the bare octahedra FeO 6 10- and Fe(OH) 6 4- , and various seven-octahedra sections of an octahedral sheet, through self-consistent charge Xα ab initio calculations. Distortions examined for all clusters were flattening, counter-rotation, and bond scaling, as well as changes in neighbor bond lengths and the identity and ordering of neighbor cations for the seven-octahedra clusters. The evolution of the EFG with distortion was derived at T = 0 K and T = 300 K as a function of the distortion parameters. We find that the percent change in the EFG over the range of distortion parameters found in 1M trioctahedral micas is greatest with flattening for the clusters compared, suggesting that flattening is the most important structural distortion in determining the EFG. The EFGs for the sevenoctahedra cluster as a function of flattening were compared for thirteen configurations of Mg 2+ and Al 3+ cations in the first nearest-neighbor octahedra. The percent change in the EFG for flattening and cation substitution was found to be of similar magnitude. In comparing EFG vs. flattening curves with measured quadrupole splitting distributions (QSDs), the magnitudes of EFGs in the theoretical curves agree well with experiment. The sharp high quadrupole splitting edge is explained by the presence of a maximum in the EFG vs. flattening curve. These model calculations are a necessary first step in establishing a firmer link between local structural distortions in minerals and measured QSDs.

The electronic environment of the Fe nuclei in two silicate perovskite samples, Fe 0.05 Mg 0.95 SiO 3 (Pv05) and Fe 0.1 Mg 0.9 SiO 3 (Pv10), have been measured to 120 GPa and 75 GPa, respectively, at room temperature using diamond anvil cells and synchrotron Mössbauer spectroscopy (SMS). Such investigations of extremely small and dilute 57 Fe-bearing samples have become possible through the development of SMS. Our results are explained in the framework of the “three-doublet” model, which assumes two Fe 2+ -like sites and one Fe 3+ -like site that are well distinguishable by the hyperfine fields at the location of the Fe nuclei. At low pressures, Fe 3+ /ΣFe is about 0.40 for both samples. Our results show that at pressures extending into the lowermost mantle the fraction of Fe 3+ remains essentially unchanged, indicating that pressure alone does not alter the valence states of iron in (Mg,Fe)SiO 3 perovskite. The quadrupole splittings of all Fe sites first increase with increasing pressure, which suggests an increasingly distorted (noncubic) local iron environment. Above pressures of 40 GPa for Pv10 and 80 GPa for Pv05, the quadrupole splittings are relatively constant, suggesting an increasing resistance of the lattice against further distortion. Around 70 GPa, a change in the volume dependence of the isomer shift could be indicative of the endpoint of a continuous transition of Fe 3+ from a highspin to a low-spin state.

New melting experiments in a 0.5 inch piston-cylinder apparatus from 0.5 to 3.0 GPa on compositions along the diopside-wollastonite join show that the eutectic liquid composition, which varies with pressure, does not change with time in experiments with barium carbonate or with talc/pyrex pressure media. Mapping out the trace of the diopside-wollastonite eutectic curve in P-T space with both piston-in and piston-out experiments reveals high-temperature pressure intensification in piston-in experiments with barium carbonate or talc/pyrex assemblies, in contrast to frictional loss of pressure at lower temperatures. A new pressure correction algorithm, derived by multiple regression analysis, incorporates both effects. When applied to a barium carbonate experiment at the intersection of the CMAS spinel-lherzolite to garnet-lherzolite transition (SGT) with the model peridotite solidus, the model pressure increases from 2.25 to 2.82 GPa. When a similar type of correction is applied to piston- out experiments run in talc/pyrex assemblies, the pressure of the intersection of the SGT and the CMAS solidus decreases from 3.0 to ~2.9 GPa, in agreement with the barium carbonate experiments from this study and with experiments from other studies run with salt/pyrex assemblies.

Magnesioferrite spinel, MgFe 2 O 4 , was synthesized at 900 °C from equimolar amounts of reagentgrade oxides, MgO and Fe 2 O 3 , and quenched in air. The structural behavior of magnesioferrite was determined from in situ synchrotron X-ray powder-diffraction data [λ = 0.92225(4) Å] at room pressure and temperatures from 28 to 982 °C on heating and cooling. The a unit-cell parameter increases linearly on heating, but deviates to give a discontinuity at 581 °C. Above 581 °C and on cooling from 982 °C, the a parameter varies linearly. The a parameter at 28 °C before heating [8.39704(5) Å] and after cooling to 47 °C [8.39514(4) Å] is different because the cation order frozen in the structure is not the same. Cation order, analyzed in terms of the inversion parameter, x, { iv [Mg 1-x Fe x ] vi [Mg x/2 Fe 1-x/2 ] 2 O 4 }, and the order parameter, Q = 1 - (3/2) x, show no change on heating until the temperature is high enough to cause exchange of Mg 2+ and Fe 3+ cations between the octahedral and tetrahedral sites. This activation barrier is overcome at 581 °C, where the sample achieves the maximum ordered state on heating [X max = 0.867(4)] and begins to move toward equilibrium. This relaxation is toward a more ordered configuration and is a kinetically controlled process. Above 581 °C, the cations continuously disorder along the equilibrium pathway to the maximum temperature studied [T max = 982 °C, x = 0.769(3)] and reverse along the equilibrium pathway on cooling. At TB, the maximum equilibrium order is frozen in, and maintained to room temperature, where X max = 0.895(4). O'Neill-Navrotsky, Landau, and Ginzburg-Landau models give good descriptions of the ordering process in MgFe 2 O 4 . Simultaneous differential scanning calorimetry (DSC) and thermogravimetry (TG) data were obtained using a Netzsch STA 449C simultaneous TG-DSC instrument. The DSC curve for MgFe 2 O 4 contains an irreversible exothermic peak at about 550 °C = T relax in the first heating experiment, and the energy change associated with this peak is -162 J/g (= -32 KJ/mol), and corresponds to cation relaxation. From Rietveld refinements, T relax ≈ 581 °C. The T Curie ≈ 360 °C was obtained from TG experiments carried out in a magnetic field.

An asymmetric binary Margules formulation for excess Gibbs energy, enthalpy, and entropy is adequate to describe the 1 bar halite-sylvite solvus in NaCl-KCl (Thompson and Waldbaum 1969). However, a binary Margules formulation is not adequate to describe excess volumes of single-phase chlorides in P-V-T-X from ambient P-T to ~20 kbar and 700 °C. Excess volumes across NaCl-KCl increase with temperature, decrease with pressure, and show systematic deficits in the consolute region. These patterns can be explained by the importance of a third component.vacancy defects that relieve the lattice stresses of K-Na size mismatch. New, two-phase observations in P-V-T-X allow delineation of the excess Gibbs energies to high pressures where the excess enthalpies and entropies do not depend on T at each P, but show significant variation between 1 bar and ~20 kbar. The volume, entropy, and enthalpy of K-Na mixing become more ideal at high pressure. But the solvus expands with pressure because entropy approaches ideality faster than enthalpy and, therefore, Gibbs energy of K-Na mixing becomes less ideal with pressure. The consolute temperature rises about 80 °C in 17 kbar, with little change in consolute composition. The binary Margules equation of state provides a prediction of the rise of the solvus that is impressively convergent with the new observations. This convergence is especially impressive given the clear inadequacies of the binary excess volume formulation on which the prediction is based.

The crystal structures of synthetic hallimondite and synthetic parsonsite have been refined by fullmatrix least-squares techniques to agreement indices (hallimondite, parsonsite) wR 2 of 5.5, and 7.6% for all data, and R1 of 2.7 and 3.4%, calculated for 3391 and 3181 unique observed reflections (|F o | ≥ 4σ F ), respectively. Hallimondite is triclinic, space group P1̅, Z = 2, a = 7.1153(8), b = 10.4780(12), c = 6.8571(8) Å, α = 101.178(3)°, β = 95.711(3)°, γ = 86.651(3)°, V = 498.64(3) Å 3 , and is isostructural with parsonsite, triclinic, space group P1 . , Z = 2, a = 6.8432(5), b = 10.4105(7), c = 6.6718(4) Å, α = 101.418(1)°, β = 98.347(2)°, γ = 86.264(2)°, V = 460.64(5) Å 3 . In both structures, hexavalent uranium occurs as a uranyl pentagonal bipyramid. The uranyl polyhedra share an edge, forming dimers that are linked by edge- and vertex-sharing with arsenate or phosphate tetrahedra to form chains along [001]. Two symmetrically distinct Pb positions connect the chains. In hallimondite, a partially occupied oxygen atom is located in the cavity between the uranyl arsenate chains and Pb positions, and is attributed to an H 2 O group. The crystal of synthetic parsonsite investigated does not have appreciable electron density at this position, but its structural cavity is large enough to contain H 2 O. The presence of H 2 O in synthetic hallimondite, and its absence in synthetic parsonsite, are supported by the results of FTIR spectroscopy. In conjunction with thermogravimetric results from the literature, we suggest that the formula of parsonsite should be considered Pb 2 [(UO 2 )(PO 4 ) 2 ](H 2 O) n , and hallimondite, Pb 2 [(UO 2 )(AsO 4 ) 2 ](H 2 O) n , with 0 ≤ n ≤ 0.5 in each case.

The temperature-dependent evolution of the crystal structure of natural tetranatrolite (Mt. Saint- Hilaire, approximate formula Na 5.85 Ca 1.90 Al 9.25 Si 10.75 O 40 ·11H 2 O) was investigated using monochromatic synchrotron X-ray powder diffraction and Rietveld analysis. The room-temperature structural model reveals characteristic Al/Si and Na/Ca disordering over the framework tetrahedral and nonframework cation sites, respectively. Water molecules at the OW4 and OW5 sites along the elliptical channels surround the nonframework cations with full and partial occupancies, respectively, similar to what was observed in previous single crystal studies. As the temperature increases up to 300 °C, the partially occupied OW5 site is gradually dehydrated whereas the fully occupied OW4 site and the disordered Na/Ca site remain fully occupied. Upon complete dehydration of the OW5 site at 300 °C, another phase appears with ~1.8% expansion and ~6.7% reduction of the a- and c-axis parameters, respectively, leading to an overall volume reduction of ~3.3%. In this new phase, the Na and Ca atoms migrate to occupy two closely separated sites along the channels, and 80% of the OW4 water is lost with the remaining water molecules occupying a site close to the previously empty OW5 site. The material decomposes upon full dehydration near 400 °C and becomes X-ray amorphous. The temperature-dependent variations of the T-O-T angles and the chain rotation angle are indicative of the framework relaxation occurring during the selective dehydration and subsequent cation-water migration phase transition.

The origin and stability of paranatrolite (approximate formula Na 16-x Ca x Al 16+x Si 24-x O 80 ·24H 2 O), a naturally occurring zeolite with the natrolite topology, has long been debated, with its detailed structure unknown. When taken from an aqueous environment and exposed to the atmosphere, paranatrolite is reported to irreversibly lose water and transform to gonnardite/tetranatrolite, Na 16-x Ca x Al 16+x Si 24-x O 80 ·nH 2 O. Since the latter has a disordered Al/Si distribution over the framework tetrahedral sites, it is believed the same is true for paranatrolite. Natrolite itself (Na 16 Al 16 Si 24 O 80 ·16H 2 O) has Al/Si ordering, and as recently shown, undergoes a reversible volume expansion (~2.5%) due to pressure-induced hydration (PIH) above 1.2 GPa to a superhydrated phase (Na 16 Al 16 Si 24 O 80 ·32H 2 O). During this process, an intermediate phase with an even larger volume expansion of ~7.0% has been detected in a narrow pressure range near 1.0 GPa. We report here that this intermediate phase has a unit-cell compatible with the one reported for paranatrolite at ambient conditions with the same 24 water molecules per formula unit and propose that it is paranatrolite with an ordered Al/Si distribution. An unusual watersodium chain is observed in the ordered paranatrolite structure: a sevenfold coordination of sodium cations provided by alternating two water bridges along the expanded elliptical channels. The density of the ordered paranatrolite is lower than those of the 16 and 32 water phases, with its channel openings far more circular than in the low- and high-pressure analogs. The atomistic details of the ordered paranatrolite provide a structural model for the naturally occurring paranatrolite and a complete understanding of this intriguing pressure-volume-hydration mechanism in natrolite, demonstrating the unique role of pressure in controlling the chemistry of microporous materials.

The addition of ZnS particles to suspensions of ferrihydrite promotes the formation of the more crystalline phases goethite [α-FeO(OH)] and hematite (α-Fe 2 O 3 ) at pH ~ 2.3 and 85 °C. This previously undocumented effect appears to arise from surface-mediated reduction of Fe 3+ species to Fe 2+ with associated dissolution of ZnS.

A phase transition of MgGeO 3 perovskite was examined at high-pressure and -temperature using synchrotron X-ray diffraction measurements. The results demonstrate that it transforms to a CaIrO 3 - type post-perovskite phase above 63 GPa at 1800 K. The density increase is 1.5% at the transition pressure. These observations confirm that MgGeO 3 is a low-pressure analogue to MgSiO 3 , for which a similar phase transition was recently found above 125 GPa and 2500 K. The unit-cell parameters of MgGeO 3 post-perovskite phase obtained at 300 K during decompression from 79 to 6 GPa show that the b-axis is significantly more compressible than are the a- and c-axes, which could be due to the GeO 6 -octahedral sheet stacking structure along b . The bulk modulus was determined to be K 0 = 192(±5) GPa with a fixed pressure derivative of the bulk modulus, K’, of 4.

OH stretching vibration modes for F-rich natural topaz (F-topaz) and for fully hydrated topaz (topaz-OH) synthesized at high pressure, were observed using IR and Raman spectroscopies at pressures up to 30.4 GPa and 17.3 GPa, respectively. In F-topaz, the pressure derivative of the frequency of the OH stretching band observed at 3650 cm -1 at ambient pressure was 0.91(3) cm -1 /GPa, which was consistent with the value recently reported by Bradbury and Williams (2003). On the other hand, in topaz-OH, the pressure derivatives of the bands initially at 3599 and 3522 cm -1 were -5.2(2) and -2.56(6) cm -1 /GPa, respectively. This contrasting behavior between the two forms of topaz at high pressures suggests that the OH substitution for F in topaz affects the hydrogen-bonding behavior under high pressure.