Volume 90, Issue 7

This paper presents several new diagrams that document previously unappreciated patterns and predictability in systematic mineralogy. They illustrate, for example, that sulfide minerals of single cations contain only intermediate to soft cations (cations with at least some outer-shell electrons). On the other hand, hard cations (those with no outer-shell electrons) only enter into a few sulfide minerals, all of which also contain intermediate cations. Among the simple chloride, fluoride, oxide, and carbide minerals of hard cations, ionic potential (charge/radius) of cations is proportional to that of anions (e.g., from KCl to SiC). Among the simple nitrate, sulfate, carbonate, phosphate, and silicate minerals of hard cations, ionic potential of inter-radical cations is inversely proportional to the ionic potential of the cations in the oxysalt radical (e.g., from KNO 3 to ZrSiO 4 ). Across both of these sets of minerals, non-simple minerals (H 2 O- and OH - -bearing minerals or minerals with extra O 2- ) in each group typically involve cations of higher ionic potential than those of the simple minerals. The result is predictability and pattern in the compositions of minerals.

Using X-ray photoelectron spectroscopy, the oxidation/alteration of massive coexisting pentlandite [(Fe,Ni) 9 S] and pyrrhotite (Fe 1-x S) from Voiseys Bay has been studied in aqueous solution at pH 9.3. The Fe 2p, Ni 2p, S 2p, and O1s spectra were obtained for each mineral from polished surfaces, and from surfaces reacted with the solutions for up to 180 minutes. Both minerals oxidize rapidly to give mostly a relatively thin layer (tens of angstroms) of Fe(III) oxyhydroxide (probably FeOOH). In addition, violarite (FeNi 2 S 4 ), Ni(OH) 2 , and NiSO 4 are found in the pentlandite oxidized overlayer, and Fe(III)-S and polysulfide species are found in the overlayers of both minerals. A protocol was developed to fit quantitatively all spectra in a consistent fashion. Fe 2p 3/2 spectra were fitted with two Fe(II)-S peaks depending upon Fe coordination (four and six) in pentlandite, pyrrhotite, and violarite. An Fe(III)-S multiplet, an Fe(III) oxyhydroxide multiplet, as well as Ni Auger peaks, were also required to fit the Fe 2p spectra. The Ni 2p 3/2 spectra for pentlandite were fit with two Ni(II)-S peaks from the original pentlandite or violarite (four and six coordination), as well as Ni(OH) 2 and NiSO 4 peaks. The S 2p spectra for both minerals were fit with four and five coordinate S peaks from the original minerals and violarite, as well as polysulfides and sulfate. Finally, the O 1s spectra of both minerals were fit with peaks from oxide, hydroxide, and adsorbed water. The pyrrhotite surface oxidizes more rapidly than the pentlandite surface. For example, after five minutes of reaction the extent of oxidation of pyrrhotite to give an FeOOH overlayer is similar to that of pentlandite after thirty minutes of reaction. In both minerals, iron diffuses from the bulk to the surface and is oxidized. Pentlandite shows small amounts of Ni(OH) 2 and NiSO 4 in the Fe(lII) oxyhydroxide- dominated overlayer even after 180 minutes of reaction. These observations of the oxidation and alteration of the pentlandite surface are entirely consistent with thermodynamic calculations of the relative stabilities of pentlandite, violarite, Ni(OH) 2 , NiSO 4 , and FeOOH. The first stage of reaction is characterized by formation of violarite and FeOOH on pentlandite. With continued oxidation, this assemblage yields NiSO 4 plus FeOOH plus violarite, which is eventually converted to Ni(OH) 2 plus violarite plus FeOOH. Finally, after the violarite is consumed, the NiSO 4 plus FeOOH combination occurs as the thermodynamically stable assemblage.

Using X-ray photoelectron spectroscopy, the oxidation/alteration of massive coexisting pentlandite (Pn) and pyrrhotite (Po) from Voisey's Bay has been studied in aqueous solution at pH 9.3, with the addition of the xanthate collector potassium amyl xanthate (KAX) at varying solution O 2 concentrations. As in the accompanying XPS study, the oxidation/alteration of the two mineral surfaces is monitored by the Fe 2p, Ni 2p, S 2p, and O 1s XPS spectra. The major surface product on all surfaces is FeOOH. However, the amount of FeOOH depends greatly on the presence of KAX, the presence of ground Pn/ Po of high surface area, and the O 2 solution concentration. At air-saturated O 2 solution concentrations of 8.9 ppm, the amount of FeOOH on both surfaces decreases in the presence of KAX compared to the situation without KAX. In the presence of the Pn/Po powder, the oxidation of Pn decreases still more, suggesting that the powder scavenges O 2 , and that the oxidation of Pn is more sensitive to O 2 concentration than the oxidation of Po. When the O 2 concentration is decreased drastically to <0.01 ppm (by Ar purging) in the presence of KAX, both minerals are relatively free of FeOOH: indeed, both minerals show less FeOOH than the unreacted sample, which implies that the mineral surfaces are being .cleaned. under these conditions. The degree of oxidation of both surfaces increases with increase in the O 2 solution concentration; but at about 0.03 ppm O 2 the pyrrhotite surface is heavily oxidized, while the pentlandite surface remains largely unoxidized, and less oxidized than the initial unreacted sample. This result strongly indicates that the flotation separation of Pn and Po could be enhanced at low O 2 concentrations.

Richteritic amphiboles in which tetrahedral Si was substituted for Ge were synthesized using internally heated gas vessels at 795~905 °C and 720~756 MPa. There is complete solid-solution between IV Si and IV Ge richterite. The materials were characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), analytical transmission electron microscopy (AN-TEM), electron microprobe analysis (EMPA), X-ray diffraction (XRD) Rietveld structure refinement, and Fourier-transform infrared spectroscopy (FTIR). X-ray diffraction data for the richterites indicate that, with increasing Ge replacement for Si, all cell parameters (= a, b, c, and β) increase linearly and the rotation angle of the double chains increases. Refinement of the Ge and Si contents at the tetrahedral sites indicate that the Ge content at the T2 site is greater than at the T1 site for amphiboles of intermediate composition. Deuteration experiments were also made for the purpose of FTIR analysis. Infrared OH/OD-stretching bands attributed to the configurations (MgMgMg)-OH/ OD- A Na(K) and (MgMgMg)-OH/OD- A ⃞ (⃞ = vacancy) were observed. The frequency of the former bands decreases linearly with increasing Ge content, while the frequency and the intensity of the latter band decreases with increasing Ge content. Both sets of OH/OD-stretching bands show a continuous or one-mode change along the compositional join without any identifiable fine structure, indicating a lack of any short-range ordering within the tetrahedral double chain. The Si-O (at 1200~800 cm -1 ) and Ge-O (at 950~700 cm -1 ) stretching bands show similar continuous down-frequency shifts, and interactions of their modes are very small. The chain deformation bands of Si•Si-O, Si•Ge-O, and Ge•Ge-O are observed at 770~650, 660~590, and 590~510 cm -1 , respectively, with the frequency range of their absorption bands becoming narrower with increasing Ge content. A weak and broad OH librational band appears at 600 cm -1 in Si-richterite. With increasing Ge for Si substitution this band shifts upward in frequency, becoming centered at 650 cm -1 in Ge-richterite, which is the opposite behavior to the downward frequency shift of the OH/OD-stretching vibrations. The most notable aspect of this study is the continuous changes that are observed in the structure (cell dimensions, bond distances) and infrared spectra of richterite with replacement of Si by Ge. The only long-range ordering effect that was clearly observed was the preference of Ge over Si at the tetrahedral T2 site for intermediate compositions. Evidence for short-range ordering that can be observed in the OH-stretching region of the Ge-analogue of talc was not observed in Ge-richterite.

We present ab initio calculations of ideal shear strengths (ISS) in forsterite at zero temperature using pseudopotential density functional theory within the generalized gradient approximation. A localized rigid-body shear is imposed on a given plane of an infinite defect-free crystal. The energy increase associated with this shear (called the generalized stacking fault energy) gives access to the ISS. The goal of this study is to assess the influence of crystal chemistry on the intrinsic resistance of plastic shear of a mineral like forsterite. ISS have been calculated for plastic shear along [100], [010], and [001] in various potential glide planes of forsterite. We show that the [001] slip, which corresponds experimentally to an easy glide at low temperature, exhibits the lowest energy barrier. The [010] glide is precluded because it involves very unfavorable atom impingements.

Mössbauer spectra of Fe-substituted 3/2-mullite (10.3 wt% Fe 2 O 3 ) have been measured from room temperature up to 1200 °C under nitrogen atmosphere. A Voigt-type fit to the spectral line shape is proposed that is based on the assumption of three non-equivalent Fe 3+ environments A, B, and C in the mullite structure. The Fe 3+ subspectra display quadrupole splittings of about 1.1 (A), 0.7 (B), and 1.3 mm/s (C) where the isomer shifts at room temperature are about 0.3 mm/s for subspectra A and B and about 0.1 mm/s for site C. Subspectra A and B are attributed to two different Fe 3+ octahedral sites in mullite coordination, experiencing slightly different quadrupolar interactions; the third site is assigned to ions in tetrahedral coordination. Subspectrum A is assigned to the iron-oxygen octahedra having TO 4 tetrahedra as next nearest neighbors, while subspectrum B may correspond to iron-oxygen octahedra having TO 4 and T*O 4 tetrahedra with associated oxygen vacancies in their vicinity. The quadrupolar splittings of the three subspectra are found to be almost independent of temperature. The temperature dependences of the isomer shifts conform closely to those expected for a harmonic solid at the high-temperature limit. The Mössbauer study gives no support for any temperature-induced redistribution of Fe in mullite.

The effects of temperature and pressure on water solubility in wadsleyite in the system MgO-SiO 2 - H 2 O were investigated. Experiments were carried out using a 1000 ton multi-anvil press. One series of experiments was performed at a fixed pressure of 15 GPa and at various temperatures and in a second series the temperature was fixed at 1200 °C and pressure was varied from 13 to 18 GPa. The starting material was a mixture of oxides and hydroxides equivalent to the composition Mg 2 SiO 4 + 5 wt% H 2 O. Run products consisted of wadsleyite, quenched hydrous melt, and minor amounts of clinoenstatite. The water content of wadsleyite was quantified by ion probe. Results show that at 15 GPa, the water solubility in wadsleyite decreases significantly with increasing temperature from ~2.2 wt% H 2 O at 900 °C down to ~0.9 wt% H 2 O at 1400 °C; the corresponding Mg/Si ratios increase from 1.80 to 1.91 over this temperature range. This effect appears to be largely due to changes in the water activity in the coexisting melt. The partition coefficient of water between wadsleyite and coexisting melt is nearly independent of temperature with D water wadsleyite/melt ≈ 0.08. No significant effect of pressure on water solubility was observed at 1200 °C. Our data suggest that the water storage capacity of wadsleyite in the transition zone is much lower than previously suggested. Together with previous results on ringwoodite, our data imply a strong decrease of the water partition coefficient between wadsleyite and ringwoodite with temperature. This decrease could have two important consequences: (1) The width of the 520 km discontinuity may vary strongly as a function of temperature. (2) During cooling of the Earth's mantle since the Hadean, water may have increasingly partitioned from the lower into the upper part of the transition zone.

Oriented quartz needles in clinopyroxene have become one of the diagnostic indicators of ultrahighpressure (UHP) metamorphism. The presence of apparently exsolved quartz is taken as evidence of decompression of a non-stochiometric Ca.Eskola component (Ca 0.5 ⃞ 0.5 AlSi 2 O 6 , CaEs) that is presumed to be stable only at UHP conditions. Eclogite from the Eastern Blue Ridge, North Carolina, contains clinopyroxene (Jd 20 CaTs 5 Ac 5 CaEs 0 Di 65 Hd 5 ) with oriented needles of quartz and calcic amphibole that appear to have exsolved together. The quartz + amphibole intergrowths are surrounded by 1.5 µm haloes of neoformed pyroxene (Jd 10 CaTs 10 Ac 5 CaEs 0 Di 70 Hd 5 ). The modes of quartz, amphibole, and clinopyroxene haloes were determined using BSE images, and reintegrated with the host clinopyroxene. Viewing the quartz and amphibole needles down the c-axis of the pyroxene host provides a better estimate of their proportions than in prismatic sections. Reintegrated pyroxene compositions were nearly identical to the analyzed host pyroxene with no CaEs component. Clinopyroxene with CaEs solid solution has been repeatedly synthesized at UHP conditions. However, examination of the phase equilibria usually cited as evidence for CaEs stability at conditions of ≥25 kbar shows that clinopyroxene with 10 mol% CaEs is stable well within the quartz field, and provides a pressure minimum similar to the albite = jadeite + quartz barometer. Exsolution of quartz and associated amphibole is commonplace in clinopyroxene from the Blue Ridge eclogite that lacks coesite or other evidence for UHP metamorphism. The presence of a diluted (5.10%) CaEs component in clinopyroxene does not require UHP conditions.

The visible and infrared spectral properties of two natural jarosite minerals and a suite of synthetic jarosites and alunite samples are described here. The fundamental stretching and bending vibrations observed in the infrared region for SO 4 2- and OH - are compared with the near-infrared overtones and combinations of these vibrations. Shifts were observed in the SO 4 2- and OH - bands for Al 3+ vs. Fe 3+ at the octahedral sites and K + vs. Na + at the “A„ (frequently monovalent) sites. Crystal-field theory bands were observed for jarosite near 435, 650, and 900.925 nm and were compared to those of iron oxides. Spectral bands near 1.76, 2.17, 2.53, 4.5, 8.10, and 15.24 μm (corresponding to ~5670, 4600, 3970.4150, 2100.2300, 1000.1225, and 420.675 cm -1 , respectively) for alunite and near 0.43, 0.91, 1.85, 2.27, 2.63, 4.9, 8.10, and 15.24 μm (corresponding to ~23 000, 10 990, 5400, 4350.4520, 3800.4150, 1950.2200, 1000.1190, and 440.675 cm -1 , respectively) for jarosite would be most useful for detecting these minerals using remote sensing on Earth or Mars. These minerals are important indicators of alteration processes, and this study contributes toward combined visible/near-infrared and mid-infrared spectral detection of these two alunite-group minerals.

The crystal structure of the rare mineral wakabayashilite was solved by direct methods using intensity data collected from a twinned crystal. This study revealed that, in spite of the strong hexagonal pseudosymmetry, the structure is orthorhombic (space group Pna2 1 ) with a o = 25.262(1), b o = 14.563(1), c o = 6.492(1) Å, and V o = 2388.4(2) Å 3 . The refinement of an anisotropic model led to an R index of 6.08% for 3135 observed reflections [F o > 4σ(F o )] and 8.71% for all 4260 independent reflections. Wakabayashilite is twinned by reticular merohedry [twin plane (110)]. There are two structural units in the structure: (1) [M 6 S 9 ] bundle-like chains running along the [001] axis, consisting of corner-sharing MS 3 trigonal pyramids (M = As, Sb), and (2) rods of As 4 S 5 cage-like molecules, located in the space between three bundles and held together by van der Waals forces. The crystal chemical formula was therefore revised according to the structural results, yielding [(As,Sb) 6 S 9 ][As 4 S 5 ] (Z = 4). Both the intramolecular As-S and As-As bond distances and those within the bundle-like units match closely the values commonly observed in the molecule-formed structures of other arsenic sulfides and those found in orpiment, respectively. To contribute to the understanding of the effects of light exposure on the structure of the arsenic sulfides, the crystal was exposed to filtered polychromatic light. The unit-cell parameters were measured after each exposure. The results showed that the As 4 S 5 molecules do not undergo any transformations, as previously found by exposing uzonite to light.

The structure and equation of state of low albite, NaAlSi 3 O 8 , has been determined using high-pressure single-crystal X-ray diffraction to a maximum pressure of 9.43 GPa. Low albite remains triclinic in space group C1̄ over the entire pressure range and no phase transitions were observed although the evolution of the unit-cell parameters and volume exhibit some unusual features at pressures above 4 GPa. A third-order Birch-Murnaghan equation of state fit to 6 P-V data points up to 3.4 GPa has K 0 = 54.2(7) GPa and K’ = 5.3(6). At higher pressures the volume-pressure curve exhibits d 2 V/dP 2 < 0 and the entire P-V data set can be approximated with a fourth-order Birch-Murnaghan equation of state with K 0 = 52.3(9) GPa, K 0 ’ = 8.8(6), and K 0 ’’ = .2.8(2) GPa -1 . The anisotropy of the compression of albite is typical of that of feldspars with 65% of the volume compression accounted for by the compression of the (100) plane normal. This is due to the closing-up of the crankshaft chains of tetrahedra that are characteristic of the feldspar structure. Single-crystal X-ray intensity data sets collected at 6.4, 8.4, and 9.4 GPa also show that the four-membered rings of tetrahedra within the structure undergo significant shear at high pressures. Changes in the rate of shear of the four-membered rings with pressure are associated with changes in the variation of the unit-cell angles with pressure.

The polymorphic transition from aragonite to calcite is a common phenomenon in nature and laboratory experiment, but its effect on O-isotope fractionation between secondary calcite and water is not known. Low-temperature synthesis experiments were carried out in this study to decipher the behavior of O-isotope fractionation between CaCO 3 and H 2 O during this process. The initial aragonite was synthesized by one- and two-step overgrowth techniques at 50 and 70 °C, respectively, with a difference in the duration of O-isotope exchange between dissolved carbonate and water prior to initial aragonite precipitation. Calcite was then synthesized by in situ polymorphic transition from the precursor aragonite in aqueous solution at the same temperatures. SEM observations show that the phase transition from aragonite to calcite proceeded in the mechanism of dissolution-reprecipitation. Although different approaches were employed to synthesize the starting aragonites, O-isotope analyses of product calcite and coexisting water unanimously yield consistent calcite-water fractionations relative to temperature. This finding demonstrates that calcite derived from polymorphic transition of aragonite can readily achieve O-isotope equilibrium with water regardless of its precursor composition. In particular, not only is the calcite-water fractionation relationship obtained in this study at 50 to 70 °C in agreement with that extrapolated from the literature data by slow precipitation experiments at 0 to 40 °C, but also the experimental calcite-water fractionations available at 25 to 70 °C are in close agreement with theoretically calculated fractionations. These results indicate that O-isotope equilibrium between calcite and water may prevail in calcite formed by the mechanism of dissolution-reprecipitation in aqueous solution despite different reaction paths.

In a previous work (Morgan and London 1996), we proposed an optimized procedure for electron microprobe analysis (EMPA) of rhyolitic glasses using a broad (20 μm diameter), low-current (2 nA) fixed beam. Some important applications for EMPA of glass, such as vitreous inclusions in minerals and experimental run products, require smaller beam diameters that produce greater areal current densities (expressed as nA/μm 2 ). For these situations, we have assessed the effect of areal current density on the migration of Na and its concomitant effects on other elements and their ratios during EMPA of granitic glasses. Anhydrous and hydrous glasses of a haplogranite composition (Ab 38.23 Or 29.31 Qtz 33.37 C 0.10 ) were analyzed at 20 kV accelerating potential, using 2.50 nA beam currents, fixed beam diameters of 2.20 μm, and counting times scaled to yield similar analytical uncertainty at each condition (~2.6% relative for Na 2 O). There is almost no loss of Na (≤1.7.2.7% relative) using a current density of 0.006 nA/μm 2 , minor (7.9%) Na loss for current densities up to 0.1 nA/μm 2 , and increasing Na loss with higher current densities that becomes severe at >0.5 nA/μm 2 (e.g., 48.63% relative loss from hydrous glass at 50 nA and 2 μm during 3.6 s of irradiation). Sodium migration is more pronounced in hydrous glasses than in anhydrous ones, with significant loss from hydrous glass occurring during the first second of irradiation. The migration of Na results in increased concentrations of Al and Si, but little or no change in the concentration of K; if not fully corrected for, these effects produce systematic errors in important elemental ratios. With current densities <0.01 nA/μm 2 , anhydrous glasses or crystalline materials are suitable standards and data correction may not be needed. Significant Na loss using current densities up to ~0.1.0.2 nA/μm2, especially in hydrous glasses, requires data correction or primary standardization utilizing a glass having composition and water content similar to the unknown. Current densities ≥0.5.1.0 nA/μm 2 are not suitable for EMPA of glass because of large and uncertain corrections (~25% to >100% of the Na 2 O value obtained). The correlation of analytical condition (beam current and diameter) with current density and EMPA results provided here allows analysts to select beam conditions that optimize the quality of analyses. When current densities >~ 0.01 nA/μm 2 must be used (e.g., with beam spot sizes <20 μm), the results can lead to improved estimates of the systematic errors due to alkali migration. Natural and some experimental glasses contain a variety of other minor components among which Ca and Fe are important, and so the discussion of analytical methods is extended to more complex compositions. For example, Na migration is accelerated as glass structures become less polymerized than those of simple tectosilicate stoichiometry (e.g., due to increasing alkalinity and/or the presence of fluxing components such as F, Cl, B). Analysis using 20 kV accelerating voltage, as opposed to 15 kV, both slightly decreases Na migration and improves limits of detection and statistical accuracies for minor components such as Fe while providing reasonable beam penetration depths.

The compressional behavior of a natural chromian-clinochlore-Ia polytype has been studied by synchrotron powder X-ray diffraction to 8 GPa at 298 K under hydrostatic conditions. A reversible polytypic transformation occurs at 6 GPa from the Ia-4 to a Ib polytype in which all a-type interlayers are converted to b-type by sheet translation across the interlayer, thus removing the high degree of cation superposition between the brucite-like sheet and the adjacent 2:1 layer that is associated with a-type interlayers. The isothermal bulk modulus for the Ia polytype to 5.9 GPa was determined by fitting pressure-volume data to a second-order Birch-Murnaghan equation of state: K 0 = 78.7 ± 1.4 GPa. This value is close to that of clinochlore-IIb (K 0 = 81 GPa). Elastic moduli for a, b, and d001 obtained from second-order fits to a Birch-Murnaghan equation of state are: Ka 0 = 113(5) GPa, Kb 0 = 91(2) GPa, and K 001,0 = 54(1) GPa. A possible relationship is discussed between the 6 GPa polytypic transformation observed in chromian-clinochlore and the non-polytypic transformation at 9 GPa in clinochlore-IIb reported recently. Both transformations are dominated by contraction of the interlayer, with negligible in-plane compression of the polyhedral sheets. Both may also involve an increase in oxygen close-packing across the interlayer. However, in chromian-clinochlore, the destabilization of a-type interlayers is a further significant structural factor that may cause the transformation to occur at a lower pressure. Both transformations also involve an increase in the compressibility of chlorite. Although the transformation in chromian-clinochlore is influenced by the vertical superpositions of cations across the a-type interlayer, the hydrogen bonding is robust and does not reorganize until a considerable pressure (6 GPa) is reached.

The kinetics of crystal growth have been investigated between 100 and 200 K above the glasstransition temperature for four liquids in the ternary system CaO-Al 2 O 3 -SiO 2 . The observed phases are yoshiokaite, gehlenite, larnite, anorthite, and wollastonite. They crystallize either congruently or incongruently through heterogeneous nucleation on gas-liquid interfaces. In all cases, the growth rate at high degrees of supercooling is independent of time, while its temperature dependence is Arrhenian. When the crystal has the same Al/Si ratio as the parent liquid, the growth rate of congruent and incongruent crystallization shows a 1:1 correlation with the viscosity of the melt. We do not interpret this observation to imply that growth rate is controlled by viscosity but, rather, that both phenomena are controlled by the rate of rearrangement of bonds between oxygen and network-forming cations. When Al/Si varies across the crystal-melt interface, the effects of interdiffusion must also be considered and the temperature dependence of growth rate is no longer correlated directly with that of viscosity, the former being smaller than the latter. Our observations show that, at constant viscosity, the fastest growth rates occur when the Gibbs free energy difference between liquid and crystal is greatest, rather than when crystalline and liquid phases are structurally most similar. A simplified equation that describes crystal growth rates from highly supercooled liquids is proposed.

The elastic properties and thermal behavior of synthetic zincochromite (ZnCr 2 O 4 ) have been studied by combining room-temperature high-pressure (0.0001.21 GPa) synchrotron radiation powder diffraction data with high-temperature (298.1240 K) powder diffraction data. Elastic properties were obtained by fitting two Equations of State (EoS) to the P-V data. A third-order Birch-Murnaghan model, which provides results consistent with those from the Vinet EoS, yields: K 0 = 183.1(±3.5) GPa, K’ = 7.9(±0.6), K’’ = .0.1278 GPa -1 (implied value), at V 0 = 577.8221 Å 3 (fixed). Zincochromite does not exhibit order-disorder reactions at high temperature in the thermal range explored, in agreement with previous studies. The volume thermal expansion was modeled with α V = α 0 + α 1 T + α 2 /T -2, where only the first coefficient was found to be significant [α 0 = 23.0(4) 10 -6 K -1 ]. Above 23 GPa diffraction patterns hint at the onset of a phase transition; the high pressure phase is observed at approximately 30 GPa and exhibits orthorhombic symmetry. The elastic and thermal properties of zincochromite were then used to model by thermodynamic calculations the P-T stability field of ZnCr 2 O 4 with respect to its oxide constituents (Cr 2 O 3 and rocksalt-like ZnO). Spinel is expected to decompose into oxides at about 18 GPa and room temperature, in absence of sluggish kinetics.

Niksergievite, [Ba 1.33 Ca 0.67 Al(CO 3 )(OH) 4 ][Al 2 (AlSi 3 O 10 )(OH) 2 ]·nH 2 O, is a new phyllosilicate closely related to the surite-ferrisurite series. It was found at the .400 m level of the Tekeli Pb-Zn mine, southeast Kazakhstan (44° N, 78° E). The mineral occurs as curved plates 3.5 mm in size forming rosette-like aggregates up to 5 cm across. Associated minerals include calcite, quartz, dolomite, celsian, sphalerite, pyrite, barite, and montmorillonite. Niksergievite is white with a light greenish tint and pearly luster on cleavage planes. The streak is white and the mineral is non-fluorescent. The Mohs hardness is 1.11/2. The cleavage is perfect (mica-like) on {001}. D m = 3.16 g/cm 3 and D x = 3.21 g/cm 3 . The IR spectrum shows the following peaks (* shoulder): 3665*, 3640, 3405, 1630, 1454, 1105*, 1080*, 1035, 1020*, 980*, 960*, 920*, 876, 835*, 750*, 704, 625*, 560*, 535, 474, and 417 cm -1 . Optically, the mineral is colorless, non-pleochroic, biaxial (-), 2V = 0.10°, α = 1.580(2), β = 1.625(2), γ = 1.625(2), and X ~ c. The chemical composition (electron microprobe, CO 2 and H 2 O by TGA) is K 2 O 0.1, CaO 5.1, BaO 27.1, MgO 0.4, FeO 0.2, Al 2 O 3 24.8, SiO 2 28.7, CO 2 6.1, and H 2 O 8.3, with a total of 100.8 wt%. The empirical formula based on (Si + Al + Mg + Fe) = 7 is (Ba 1.27 Ca 0.65 K 0.02 ) 1.92 (Al 3.49 Si 3.42 Mg 0.07 Fe 2+ 0.02 ) 7.00 O 10.00 (CO 3 ) 0.99 (OH) 6.20 · 0.20 H 2 O. The simplified formula is (Ba,Ca) 2 (Al,Si) 7 O 10 (CO 3 )(OH) 6 ·nH 2 O and the proposed structural formula is [Ba 1.33 Ca 0.67 Al(CO 3 )(OH) 4 ][Al 2 (AlSi 3 O 10 )(OH) 2 ]·nH 2 O. The mineral is monoclinic, C2/c, C2, or Cm, a 5.176(3), b 8.989(3), c 16.166(5) Å, β 96.44(6)°, V 747.4(9) Å 3 , Z = 2. The strongest reflections in the X-ray powder diffraction pattern are as follows [d in Å, (I) (hkl)]: 16.1(40)(001), 4.49(90)(020), 3.68(60)(014,1̄13), 2.585(100)(130,2̅01, 1̄31), 2.230(90)(1.34,220), 2.069(80)(043), 1.692(60)(3.11,1̄51,240). It is named in honor of Prof. Nikolai Grigorievich Sergiev (1901.1960) for his contributions to the geology of Kazakhstan.

A systematic crystal-chemical investigation of orthorhombic holmquistites has been done to deter­mine the reasons for their - limited compositional variations. Structural constraints to the relative stability of B Li amphiboles are also suggested by the occurrence of ferro- and feni-ferroclinoholmquistites, and the lack of clinohohnquistite. Detailed crystal-chemical analysis shows: (1) a remarkable constancy in composition, both in terms of charge arrangement and of limited homovalent M1,3 (Mg -1 Fe 2+ ), M2 (Al -1 Fe 3+ ), and 03 (OH -1 F) exchanges, (2) a remarkable constancy in the unit-cell dimensions, with the Fe 3+ content at the M2 site being the only factor affecting the b edge; (3) complete ordering of Li at the M4 site, in contrast with the common partitioning between the M4 and M3 sites in clinoamphiboles, which however couples with partial A-site occupancy; (4) complete ordering of bivalent cations at the M2 site; (5) an inverse relationship between the Fe 2+ and the Fe 3+ contents, which is interpreted as a way to keep the size of the octahedral strip constant; (6) a strong distortion of the octahedral sites, both in terms of angular variance and quadratic elongation. A ⃞ B Li 2 c (Mg 3 Al 2 ) T Si 8 O 22 (OH) 2 is the amphibole composition composed of the smallest possible structural moduli. Crystallization in Puma symmetry, where the two double-chains of tetrahedra can assume different conformations, is probably required by the need to obtain a more suitable [5 + 1]- coordination for B Li, and to shrink the cation-cation distances. This arrangement does not allow for extensive incorporation of larger homovalent substituents, which are hosted via mechanisms implying distortion of the octahedral sites. During this work, a sample with Fe 2+ slightly but significantly higher than Mg was char acterized, and then recognized as a mineral species by the IMA-CNMMN (2004-030). Holotype ferroholmquistite has a = 18.287 (1), b = 17.680 (1), and c = 5.278 (1) Å, and V= 1706.6(1) A 3 . Its crystal-chemical formula is A K 0.01 Na 0.01 B (Li 1.88 Mg 0.08 Na 0.03 Fe 2+ 0.01 ) c (Al 1.89 Fe 2+ 1.07 Mg 1.39 Mn 2+ 0.02 ) T Si 8.00 O 22 (OH 1.97 F 0.03 ). Ferroholmquistite occurs as elongated black to bluish-violet prismatic crystals; it is biaxial negative, with a = 1.628, (3 = 1.646, and y = 1.651 (X = 589 nm), 2V X (calc.) = 55.1°. It is weakly pleochroic, with a = colorless, (3 = pale violet-blue, and y = blue to deep violet; the calculated density is 3.145 g/cm 3 . The holotype specimen belongs to the mineral collection of Renato and Adriana Pagano (Italy), and comes from the Greenbushes pegmatite (Western Australia). The analyzed sample has been deposited at the Museum of the Dipartimento di Scienze della Terra, Università di Pavia (Italy) under the code 2004-01.

Dissakisite-(La), ideally CaLaAl 2 MgSi 3 O 12 (OH), has been found in the Hochwart peridotite, Ulten zone, Italy. The mineral occurs as centimeter-sized black to very dark brown anhedral nodules and smaller grains. Associated minerals are: olivine, spinel, amphiboles, clino- and orthopyroxenes, and minor clinochlore, uraninite, thorite, thorianite, phlogopite, zircon, apatite, calcite, dolomite, pentlandite, and copper sulfides. The streak is gray-greenish and the luster is vitreous. Mohs hardness is 6.5.7; the mineral is brittle with a conchoidal fracture. The cleavage is imperfect on (001). Dissakisite-(La) is monoclinic, space group P2 1 /m. The unit cell dimensions are a = 8.9616(7), b = 5.7265(5), and c = 10.2353(9) Å, β = 115.193(6)°, V = 475.30(7) Å 3 , Z = 2. The strongest X-ray powder diffraction lines are: [d(Å) (I)(hkl)] 2.926(100)(113̅), 2.860(53)(020), 2.553(51)(202), 3.526(49)(211̄), 2.699(44)(120). Electron and ion microprobe analysis of the type sample DISS 5 gave the formula (Ca 1.195 Mn 0.009 Sr 0.010 Na 0.002 Th 0.090 U 0.003 La 0.315 Ce 0.262 Pr 0.019 Nd 0.038 Sm 0.002 Gd 0.001 Er 0.001 ) (Al 1.816 Mg 0.622 Fe 2+ 0.244 Fe 3+ 0.159 Cr 0.148 Ti 0.030 Sc 0.002 V 0.008 Ga 0.001 Ni 0.010 Zn 0.015 ) (Si 2.970 Al 0.022 P 0.008 ) O 11.991 F 0.009 (OH). The La/(La + Ce) ratio is 0.545(16) in the type analysis and 0.543(18) in an average of 70 analyses of the type sample A4310. Ce ≥ La was not observed in any analysis. D meas = 3.79(15) g/cm 3 ; D calc = 3.84 g/cm 3 . Radioactivity is appreciable. The optical properties and Raman spectrometry have also been investigated. The mineral formed by hydration and enrichment in LILE and LREE of a peridotite body, in relation to HP-migmatization of the surrounding gneisses during the Variscan orogeny.

Mazzite-Na, Na 8 Al 8 Si 28 O 72· 30H 2 O, is a new zeolite mineral species occurring in basalt at the bottom of the open pit of the U.S. Borax mine at Boron, California. It crystallizes as white, very thin, flexible fibers up to 2 mm in length, and commonly fills small cavities as satiny mats. The luster is vitreous, and the streak is white. The fineness of the fibers precludes the determination of the hardness and the observation of any cleavage. The observed and calculated densities are 2.16 and 2.18 g/cm 3 , respectively. The mineral is uniaxial positive with n ε = 1.472(3) and n ω = 1.471(3). Electronmicroprobe analyses yielded (in wt%) SiO 2 57.65, Al 2 O 3 14.35, Fe 2 O 3 0.65, MgO 0.22, CaO 0.18, BaO 0.14, Na 2 O 8.07, K 2 O 0.03, and H 2 O 18.70 (TG analysis), which gives the empirical cell contents (Na 7.52 K 0.02 Mg 0.16 Ca 0.09 Ba 0.03 )[Fe 0.24 Al 8.13 Si 27.71 O 72 ]·29.98H 2 O. Mazzite-Na is hexagonal, P6 3 /mmc, a = 18.2343(7), c = 7.6371(2) Å, Z = 1. The strongest seven measured X-ray lines [d in Å(I)(hkl)] are: 9.08(100)(110), 6.86(70)(101), 5.95(70)(210), 4.681(40)(211), 3.787(80)(002), 3.511(40)(112), and 3.150(70)(500). The framework consists of columns of gmelinite cages parallel to c, cross-linked to form two types of channels; one has an elliptical 8-ring cross section and the other has a circular 12-ring cross section. The extra-framework Na cations are located at three different sites. NaI is at the center of the 6-ring between adjacent gmelinite cages and is coordinated by six framework oxygen atoms and two H 2 O molecules. NaII lies on the centerline of the 8-ring channel and is coordinated by four framework oxygen atoms and two H 2 O molecules. NaIII occupies sites along the walls of the 12-ring channel and is coordinated with two framework oxygen atom and four H 2 O molecules. With the naming of this mineral for its Na dominant composition the original mineral is renamed mazzite-Mg, and the name mazzite is raised to series status

Variegated palaeosols, which formed from weathering of clays, silts, and brackish to freshwater limestones, are present in the late Eocene-early Oligocene Solent Group of the Hampshire Basin, southern U.K. The detrital clay mineral suite is dominated by illite and illite-smectite with minor kaolinite and chlorite. In pedogenically modified (palaeosol) and evaporitic lacustrine clay-rich sediments, the proportion of illite in the illite-smectite is greater than in the non-pedogenically modified sediments, and where alteration is most intense, kaolinite and chlorite are absent. The smectite to illite transition has been investigated in the <0.5 μm fraction by XRD analysis (powder and oriented mounts), thermogravimetry (TG), analytical SEM, and chemical analysis of Fe 2+ . Modeling of XRD data reveals that the illite-smectite is a mixture of compositions (overall 60.95% illite), R0, with high rotational stacking disorder. Dehydroxylation occurs mainly at 500 °C, but also at higher temperatures, indicating heterogeneous octahedral cation composition. Analytical SEM and chemical analysis of Fe 2+ indicate that the illite to smectite transition occurs through Fe reduction in octahedral sites leading to increased layer charge, coupled with K fixation. The driving mechanism for what appears to be irreversible Fe 3+ reduction is wetting (reducing) and drying (oxidizing) cycles in gley soil, in which reoxidation of reduced Fe is never complete.

In a recent work, Onac and White (2003) reported on berlinite, but also chlorellestadite and churchite, from the Bivouac Room, Cioclovina Cave, Romania. Our analyses of materials collected from this site failed to identify berlinite, chlorellestadite, or churchite. Due to the nature of the samples collected from the bat guano deposit at Cioclovina, many possible sources of confusion can occur. For example, separates from a mixture of taranakite, quartz, and minor illite could give the same analytical results as those reported by Onac and White (2003) for “berlinite„. For this reason, we consider that the occurrence of berlinite at Cioclovina, as well as the occurrence of other exotic species such as chlorellestadite or churchite, is not well enough substantiated and their report must be regarded with caution. Arguments to support this point of view are problems identified with the analytical techniques (EMPA, XRD), the failure of the experimental synthesis of berlinite in “dry„ conditions at atmospheric pressure, and the geochemical context from Cioclovina.

High pressure optical and near infrared spectra of a single crystal of ringwoodite with composition (Mg 0.90 Fe 0.10 )2SiO 4 were measured to 21.5 GPa. The spectrum at ambient pressure shows a strong band at 12 265 cm -1 with two shoulders at 8678 cm -1 and 17 482 cm -1 . The bands at 12 265 cm -1 and at 8678 cm -1 are due to spin -allowed crystal field transitions of octahedral Fe 2+ , while the band at 17 482 cm -1 is most likely due to Fe 2+ → Fe 3+ charge transfer. The absorption edge due to ligand-to-metal charge transfer occurs close to 30 000 cm -1 . With increasing pressure, both the crystal field and the charge transfer bands shift to higher frequencies. Whereas this is expected for the crystal field bands, this blue shift is surprising for an intervalence charge transfer band. Moreover, neither the crystal field nor the charge transfer bands broaden or intensify significantly with pressure. These results have major implications for radiative heat transfer in the Earth's mantle. It has commonly been assumed that radiative heat transfer is blocked in the mantle, because it was believed that the red shift and the increased intensity of charge transfer bands with pressure would effectively make mantle minerals opaque throughout the near infrared and visible range. Our results demonstrate that this effect does not occur for ringwoodite with a Mg/Fe ratio realistic for the Earth's mantle. Quite to the contrary, the mean free path of photons in ringwoodite actually increases with pressure, because the absorption bands move away from the maximum of the blackbody radiation.

We present a process of organic polyanion-mediated CaCO 3 precipitation similar to biomineralization. Detailed insight into the formation pathway of nano-structured calcite crystals was gained through direct observation with an atomic force microscope (AFM). Essentially depending on a minimum saturation state, a primarily precipitated gelatinous film of polyaspartate calcifies as a transient, apparently amorphous phase, which gradually crystallizes as a calcitic single-crystalline nano-cluster of semicoherent domains. The similarity of this precipitation mechanism with that of biominerals and a striking consistency of materials properties of the synthetic precipitate with sea urchin calcite, investigated as a prime example of biomineralization, qualify our experiment as a functional model of biomineralization. The substantial role of water-soluble organic polyanions in biomineralization is elucidated.

To better understand the relationship between atomic-scale structures and densities of aluminosilicate glasses and liquids, we used 27 Al MAS NMR to determine the speciation of aluminum ions in K 3 AlSi 3 O 9 , Na 3 AlSi 3 O 9 , and Ca 3 Al 2 Si 6 O 18 glasses quenched from melts at 3 to 10 GPa. These data are a first approximation of high-pressure melt structure and illustrate the effects of the type of modifier cation. High field strength modifier cations (e.g., Ca) clearly induce more high-coordinated Al than lower field strength cations (e.g., Na and K). Measured glass densities show that, especially with rapid decompression, a significant portion of the total densification observed in-situ in melts is retained on return to ambient temperature and pressure. Observed increases in Al coordination are well correlated with decreased volume, which suggests that this structural change is a major part of the mechanism for recovered densification of high-pressure melts. Additionally, 23Na MAS NMR, combined with the 27Al MAS spectra and density determinations, reveal that other changes, such as the compression of modifier cation sites and/or decreased network bond angles, must also be significant, especially at low pressure.

A (Ca 0.36 Na 0.56 Mg 0.08 )(Mg 0.73 Si 0.27 )Si 2 O 6 clinopyroxene containing both four- and six-coordinated silicon was synthesized at 15 GPa and 1600 °C and its structure determined with single-crystal X-ray diffraction. Unlike the Na(Mg 0.5 Si 0.5 )Si 2 O 6 clinopyroxene that exhibits an ordered P2/n structure with octahedrally coordinated Mg 2+ and Si 4+ occupying two crystallographically distinct M1 sites (Angel et al. 1988), our sample possesses a C2/c symmetry and shows no detectable ordering between Mg 2+ and Si 4+ in the M1 site. The measured unit-cell parameters are a = 9.5792(13), b = 8.7588(12), c = 5.2610(6) Å, β = 107.199(3)°, and V = 421.7(2) Å 3 . The crystal structure of the clinopyroxene reported in this study is comparable to that for omphacites formed under high temperatures and pressures.