Band 95, Heft 1

Tourmaline grains extracted from rocks within three ultrahigh-pressure (UHP) metamorphic localities have been subjected to a structurally and chemically detailed analysis to test for any systematic behavior related to temperature and pressure. Dravite from Parigi, Dora Maira, Western Alps (peak P-T conditions ~3.7 GPa, 750 °C), has a structural formula of X (Na 0.90 Ca 0.05 K 0.01 ⃞ 0.04 ) Y (Mg 1.78 Al 0.99 Fe 2+ 0.12 Ti 4+ 0.03 ⃞ 0.08 ) Z (Al 5.10 Mg 0.90 )(BO 3 ) 3 T Si 6.00 O 18 V (OH) 3 W [(OH) 0.72 F 0.28 ]. Dravite from Lago di Cignana, Western Alps, Italy (~2.7-2.9 GPa, 600-630 °C), has a formula of X (Na 0.84 Ca 0.09 K 0.01 ⃞ 0.06 ) Y (Mg 1.64 Al 0.79 Fe 2+ 0.48 Mn 2+ 0.06 Ti 4+ 0.02 Ni 0.02 Zn 0.01 ) Z (Al 5.00 Mg 1.00 )(BO 3 ) 3 T (Si 5.98 Al 0.02 )O 18 V (OH) 3 W [(OH) 0.65 F 0.35 ]. “Oxy-schorl” from the Saxonian Erzgebirge, Germany (≥4.5 GPa, 1000 °C), most likely formed during exhumation at >2.9 GPa, 870 °C, has a formula of X(Na 0.86 Ca 0.02 K 0.02 ⃞ 0.10 ) Y (Al 1.63 Fe 2+ 1.23 Ti 4+ 0.11 Mg 0.03 Zn 0.01 ) Z (Al 5.05 Mg 0.95 )(BO 3 ) 3 T (Si 5.96 Al 0.04 )O 18 V (OH) 3 W [O 0.81 F 0.10 (OH) 0.09 ]. There is no structural evidence for significant substitution of [4] Si by [4] Al or [4] B in the UHP tourmaline (<T-O> distances ~1.620 Å), even in high-temperature tourmaline from the Erzgebirge. This is in contrast to high-T-low-P tourmaline, which typically has significant amounts of [4] Al. There is an excellent positive correlation (r 2 = 1.00) between total [6] Al (i.e., Y Al + Z Al) and the determined temperature conditions of tourmaline formation from the different localities. Additionally, there is a negative correlation (r 2 = 0.97) between F content and the temperature conditions of UHP tourmaline formation and between F and Y Al content (r 2 = 1.00) that is best explained by the exchange vector Y AlO(R 2+ F) -1 . This is consistent with the W site (occupied either by F, O, or OH), being part of the YO 6 -polyhedron. Hence, the observed Al-Mg disorder between the Y and Z sites is possibly indirectly dependent on the crystallization temperature.

Given its abundant occurrence at Wieściszowice, SW Poland, we have carried out a revision of the chemical composition and crystal structure of the sulfate mineral slavikite. Slavikite crystallizes in the trigonal space group R3̅. The unit-cell parameters, determined using single-crystal X-ray diffraction (R 1 = 0.0356) at 100 K, are a = 12.1347(6) Å, c = 34.706(3) Å, and V = 4425.9(5) Å 3 . The results of chemical analyses reported in the literature and made on material from Wieściszowice unequivocally show that Na is not an essential component of slavikite, at odds with the generally accepted Süsse formula and model of the crystal structure. Our chemical analyses and structure determination lead us to propose a new, more adequate, formula for slavikite: (H 3 O + ) 3 Mg 6 Fe 15 (SO 4 ) 21 (OH) 18 ·98H 2 O. The crystal structure consists of infinite layers of Fe-hydroxy-sulfate linked with [Mg(H 2 O) 6 ] 2+ octahedra, forming a honeycomb-like structure. These layers are perpendicular to the Z axis and are built up from two types of SO -2 4 tetrahedra and two types of Fe octahedra (Fe1 with O and OH, and Fe 2 with O, OH, and H 2 O ligands attached, respectively). Compared to previous studies, the main skeleton of the slavikite structure, i.e., the layers of Fe 3+ - hydroxy-sulfate, Mg[(H 2 O) 6 ] 2+ octahedra and disordered isolated sulfate ions, remains unchanged. However, on the basis of careful chemical analysis and single-crystal X-ray diffraction studies, we conclude that Na cations are absent from the structure of slavikite and their positions are occupied by disordered protonated water clusters balancing the excess of negative charge in the structure. These protonated water clusters are located at the inversion centers on the 3 axes of symmetry between two [Mg(H 2 O) 6 ] 2+ cations also lying on such axes (but not at the inversion centers). This structure also contains another disordered moiety-an isolated sulfate anion located at the inversion center of the 3 axis. This SO -2 4 anion is disordered in such a way that each oxygen atom partially occupies 5 positions resulting from 5 different orientations of the anion. This complex anion is linked by hydrogen bonds with the O atoms of ordered water molecules. In consequence, the disordered sulfate anion is surrounded by 12 ordered water molecules, thus forming a spherical water environment around the sulfate.

The elastic properties of jarosite were investigated using synchrotron X-ray diffraction coupled with a multi-anvil apparatus at pressures up to 8.1 GPa. With increasing pressure, the c dimension contracts much more rapidly than a, resulting in a large anisotropy in compression. This behavior is consistent with the layered nature of the jarosite structure, in which the (001) [Fe(O,OH) 6 ]/[SO 4 ] sheets are held together via relatively weak K-O and hydrogen bonds. Fitting of the measured unit-cell parameters to the second-order Birch-Murnaghan equation of state yielded a bulk modulus of 55.7 ± 1.4 GPa and zero-pressure linear compressibilities of 3.2 × 10 -3 GPa -1 for the a axis and 13.6 × 10 -3 GPa -1 for the c axis. These parameters represent the first experimental determination of the elastic properties of jarosite.

Chemical, structural, infrared, optical, and Mössbauer spectroscopic data were obtained on tourmalines from gem pockets in the Himalaya mine, San Diego County, California, including a strongly color-zoned crystal. Calcium and Li abundances increase from core to rim, whereas Mn 2+ and F increase, reach a maximum, and then decrease. Upon initiation of crystallization of lepidolite, F contents in tourmaline decrease. The black core is a Mn-bearing “oxy-schorl.” The grayish-yellow, intermediate zone is Mn-rich “fluor-elbaite” that contains a relatively high Mn content with ~6 wt% MnO. The nearly colorless “fluor-elbaite” rim has the highest Li content of all zones. There is an inverse correlation between the lattice parameter a (for values ≥15.84 Å) and the Li content (r 2 = 0.96). Mössbauer studies from the different zones within this crystal show that the Fe 3+ /Fe(total) ratio increases continuously from the Fe-rich core to the Fe-poor near-rim zone, consistent with increasing oxygen fugacity during pegmatite pocket evolution. There is a high positive correlation between lattice parameter a (for values ≥15.84 Å) and (Fe 2+ +Mn 2+ ) content in tourmalines from the elbaite-schorl series (r 2 = 0.99). Values lower than 15.84 Å for a are likely a consequence of greater [4] B contents in samples that usually have a (Fe 2+ +Mn 2+ ) content of <0.1 apfu. Positive correlations between Al at the Y site and [4] B (r 2 = 0.93), and between (Mn 2+ +Fe 2+ ) and [4] Al (r 2 = 0.99) were found in tourmalines from the Himalaya Mine. These correlations indicate that, in the short-range order configurations, Y Al is coupled with [4] B, whereas Mn 2+ and Fe 2+ are coupled with [4] Al. To obtain the most accurate OH data, different analytical methods were used: SIMS, hydrogen manometry, continuous-flow mass spectrometry, and IR overtone spectroscopy. Some elbaites contain a mixed occupation of F, OH, and O at the W site. Based on these data, the assumption OH = 4 - F appears to be valid only for elbaitic tourmalines with FeO+MnO < 8 wt%. In terms of the conditions of formation, whether gel or glass, the transition from low to high viscosity of the pocket-forming medium occurs before primary crystallization within the pockets ceased. At the pocket stage, Li contents of residual hydrosilicate melt were evidently high enough to promote a continuous transition from schorl-foitite at the pegmatite margin to elbaite-rossmanite-liddicoatite in the final stages of consolidation of the pegmatite interior.

Atom location by channeling-enhanced microanalysis (ALCHEMI) was used to determine the occupancy of Al atoms in the T 1o site (t 1o ) of alkali feldspar. Building on the method demonstrated by previous research, analytical electron microscopy proves to be a viable technique for fully characterizing the Al-Si ordering state in the feldspar framework. We applied this method to orthoclase from Itrongay, Madagascar, and to its heated counterpart. Our preliminary results give 0.74 and 0.37 as 2t 1 (=t 1o + t 1m ) and t 1o , respectively, for the original orthoclase, vs. 0.54 and 0.28 for heated orthoclase. Single-crystal X-ray diffraction experiments have been performed, and the results agree with our ALCHEMI measurements. This new method promises to help resolve some complex issues relating ordering paths, for example, twinning in feldspars and domain intergrowth in plagioclase.

The so-called “simple salt approximation” is used to calculate the lattice energies of double salts (compounds that contain two or more different types of ions). Enthalpies and Gibbs energies of dissolution based upon these lattice energies are then used to estimate the solubilities of hydroxyl sulfates, hydroxyl carbonates, hydroxyl nitrates, hydroxyl chlorides, and hydroxyl phosphates. Based on the solubilities, general rules are postulated for the relative stabilities of double salts that have the same cations and anions, but in different stoichiometries. These observations provide an understanding of the relative stabilities of many minerals and guidance for the design of synthetic methods, and have been used to prepare likasite [Cu 3 NO 3 (OH) 5 ·2H 2 O], the zinc analog [Zn 3 SO 4 (OH) 4 ] of antlerite [Cu 3 SO 4 (OH) 4 ], and Cu 5 (SO 4 ) 2 (OH) 6 ·4H 2 O.

The standard Gibbs energy of formation of zircon was constrained by measuring the solubility of silica in H 2 O in equilibrium with zircon and baddeleyite at 800 °C, 12 kbar, by a sensitive weightchange method. Dissolution occurs incongruently according to the reaction: ZrSiO 4 = ZrO 2 + SiO 2(t) zircon baddeleyite where SiO 2(t) is total dissolved silica. Blank runs demonstrated that the effects of ZrO 2 solubility and/or capsule-Pt transfer were near the weighing detection limit, so weight losses or gains could be ascribed quantitatively to SiO 2 solubility. Precise SiO 2(t) concentrations were ensured by use of three types of starting material, by approaching equilibrium from zircon-undersaturation and oversaturation, and by demonstrating time-independence of the measurements. The results yielded a SiO 2 concentration of 0.069 ± 0.002 (1 se) moles per kg H 2 O (m s ), or a mole fraction (X s ) of 1.23 × 10 -3 ± 3.3 × 10 -5 . Two runs on zircon solubility in NaCl-H 2 O solutions at 800 °C and 10 kbar showed silica solubility to decrease by nearly 1% per mol% NaCl. The standard molar Gibbs free energy of formation of zircon from the oxides at a constant P and T is given by: where ZB and Q refer, respectively, to equilibrium with zircon-baddeleyite and quartz, γ s is the activity coefficient of total silica, and the relationship between γ s and X s accounts for aqueous silica activity. Our results yield ΔG ￮ f,ox,zr = -18.5 ± 0.7 kJ/mol at 800 °C, 12 kbar (95% confidence), or a standard apparent Gibbs free energy of formation from the elements of -1918.3 ± 0.7 kJ/mol at 25 °C, 1 bar. Our value is consistent with determinations based on phase equilibrium studies, within reported error limits, but is more precise than most previous values. However, it is less negative than high-temperature determinations by calorimetry and electrochemistry. Our results indicate that solubility measurements at high T and P may be a superior method of free energy determination of other refractory silicate minerals.

The crystal structure of a polycrystalline knorringite, Mg 3 (Cr 0.8 Mg 0.1 Si 0.1 ) 2 (SiO 4 ) 3 , synthesized at 11 GPa and T = 1500 °C in a multi-anvil press, has been refined from high-resolution synchrotron X-ray powder diffraction data. The structure is cubic, space group Ia3̅d, a = 11.5935(1) Å, V = 1558.27(4) Å 3 , D calc = 3.79 g/cm 3 . The structural formula indicates that knorringite is susceptible to majorite substitution at these synthesis conditions. The Cr-O distance, 1.959(7) Å, is similar to that in Cr-bearing pyrope Mg 3 Al 2 (SiO 4 ) 3 . This confirms that the magnitude of the Cr-O distance is not responsible for the difference in crystal field splitting values between green knorringite and red Cr-pyrope. A comparison with the structure of other Cr-garnet end-members shows that the Cr-O distance and the [4] Si-O-Cr angle decrease with decreasing synthesis pressure and with increasing X-cation size.

The most important terrestrial archive of the Quaternary climate is the Chinese Loess Plateau (CLP) distributed over a vast region peripheral to the deserts of Inner Asia. Numerous paleoclimatic studies have examined magnetic, chemical, and isotopic proxies from deep loess-paleosol sections. However, the highly uniform mineralogy of the CLP has made it difficult to track the provenance, transportation, and deposition of dust particles. Here, we report that the micrometer-scale mineralogical heterogeneity of the loess particles has a significant potential in determining high-resolution loess provenance. Scanning and transmission electron microscopy of the Chinese loess reveals that biotite particles commonly have microinclusions of Ba-Sr sulfates displacing the micaceous layers along cleavages. Chemical compositions varied widely from barite to celestine within the same biotite grain. The Ba-rich sulfates precipitated first, followed by an overgrowth of Sr-rich sulfates. The frequency of sulfate-bearing biotite varied greatly over several loess sections, implying a specific source and sedimentation process. Their abundance on the western CLP and sulfur isotopic composition support their derivation from porous sedimentary sequences reacted with hypersaline solutions, e.g., arkosic sandstone, probably located west of the CLP covering the Qilian Mountains, Qaidam Basin, and surrounding mountains of the Tibetan-Qinghai Plateau. The sulfate-bearing biotite is the first microscopic mineral tracer in the CLP, provoking further search for other microscopic heterogeneities of the Chinese loess and their equivalents in the source regions.

Jimthompsonite and chesterite occur as intergrowths within anthophyllite up to a few millimeters across in talc-magnesite schist from the ~3800 Ma Isua Supracrustal Belt, West Greenland. Pyribole lamellae span the entire intergrowth width and have their own crystal faces. The schist is strongly foliated and pyribole intergrowths are aligned with this foliation indicating broad synchronicity among metamorphic deformation, recrystallization, and pyribole formation. Chain-width errors, polytypic disorder, zipper terminations, displacive planar faults, replacement products by sheet silicates, polygon-shaped cavities, cavity-filling materials, and low-angle grain boundaries were revealed by HRTEM. Based on cross-cutting relationships, the order of the formation of these defects was determined. Pyribole intergrowths formed prior to displacive planar faults and cavity-filling materials. Fine sheet silicates replaced pyriboles or filled cracks after the pyribole intergrowth formation. No obvious reaction or replacement products by talc or chlorite were confirmed at the interface between pyriboles and optically recognizable large sheet silicates, suggesting that fine sheet silicates replacing pyriboles formed later than talc and chlorite. Polysomatic and polytypic lamellae may have grown simultaneously because the lamellae cross cut each other and one does not affect the formation of the other. The possible reaction fronts that separate jimthompsonite or chesterite from anthophyllite were not found in [001] HRTEM images of our samples. Instead, narrow jimthompsonite and chesterite lamellae that break termination rules were commonly found. These textural features can be better understood as a simultaneous growth of jimthompsonite, chesterite, and anthophyllite and not as a retrograde reaction product of anthophyllite.

Acicular Mg amphibole in thermal-metamorphic serpentinites from Elba Island, Italy, is affected by polytypic disorder (with the possible coexistence of Pnmn, Pnma, and C2/m structures), chain-width defects, and retrograde hydration reactions. Based on HR-TEM nanostructures, we documented three main hydration stages: (1) formation of chain-multiplicity faults (CMFs); (2) formation of fibrous topotactic talc, elongated parallel to [001] Tlc and resulting from progressive polymerization of CMFs; and (3) pervasive serpentinization. Incipient serpentinization produces isolated inclusions within Mg amphibole, rhombic in shape and parallel to {210} Mg-Am , or elongated parallel to (100) Mg-Am ; the inclusions are filled by not-topotactic, curved (001) Srp layers or chrysotile partial fibers. As the Mg amphibole-to-serpentine replacement proceeds, the isolated serpentine inclusions merge together, producing a sponge-like texture, with interconnected fluid paths, from which serpentine can rapidly grow at the expense of enclosing amphibole. We suggest that whereas the first hydration reactions to CMFs and fibrous talc take place at relatively high T and low fluid/rock ratio (fluid circulation confined to microfractures, cleavage planes, and subgrain boundaries), subsequent serpentinization requires significant changes in physical conditions, with definitely lower T and pervasive fluid circulation.

The proposition has been made and is now gaining popular acceptance that electron energy-loss spectroscopy (EELS) attached to a transmission electron microscope (TEM) can be used to semiquantitatively measure H in minerals-specifically, that there is a pre-peak to the O K edge near 530 eV whose intensity is a measure of H concentration in OH- and H 2 O-bearing minerals. I show here that the O K edges from H-bearing minerals, free of electron-beam damage, lack a peak near 530 eV. Instead, under electron irradiation in the TEM, a transient peak near 530 eV can form in H-bearing as well as anhydrous minerals. The intensity of the transient peak is dependent on total fluence and fluence rate. The origin of the radiation-induced peak at 530 eV is from O 2 liberated during damage by the incident electron beam. In conclusion, there is no evidence for an OH peak near 530 eV from H-bearing minerals.

Turbostratic disorder, consisting in a disorder in which different layers have different rotations with respect to an axis, is commonly found in montmorillonite. The effect of this kind of disorder on diffraction profiles is significant and must be taken into account, especially in quantitative phase analysis. The effect of the turbostratic disorder in textured materials has never been investigated. In the present work, we have developed a strategy to perform quantitative texture analysis on turbostratically disordered Ca-montmorillonite aggregates that were uniaxially compressed. Synchrotron diffraction images were analyzed with a Rietveld method and disordered and ordered models are compared. The method proved to be reliable and ready for further applications.

The anion skeleton of quartz is a distorted body-centered cubic (BCC) arrangement. A hypothetical ideal BCC crystal structure for quartz has been derived and used to locate and describe the unoccupied tetrahedral sites, quantify the distortion of the quartz anion arrangement from ideal BCC, and characterize the role of tetrahedral distortion and O-O interactions in the compression of quartz. Quartz has eight crystallographically nonequivalent tetrahedra, one occupied by silicon and seven unoccupied. These tetrahedra completely fill space, something that cannot be done using only regular tetrahedra. In ideal BCC quartz, the nonequivalent tetrahedra are identical in size and shape with a unique geometry and are referred to as Sommerville tetrahedra. In reality, the unoccupied tetrahedra of quartz are very distorted from both regular and Sommerville tetrahedra. Changes in the unoccupied tetrahedra are responsible for most of the compression in quartz with pressure, as the volume of the Si tetrahedron decreases by <1% over 10.2 GPa, but the volume of the bulk structure decreases by almost 16%. The ideal BCC quartz has been used to quantify the distortion from ideal BCC of the O arrangement in quartz at several pressures up to 10.2 GPa. Distortion decreases by over 60% across this domain. Other parameters have been derived to quantify the distortion of the unoccupied and occupied tetrahedra in quartz from Sommerville tetrahedra, the characteristic tetrahedra of BCC. By all measures, the anion packing in quartz approaches ideal BCC as pressure increases. The compression mechanisms of quartz are compared to those of cristobalite and coesite. Si-O-Si angle-bending controls compression in each of these minerals. The bulk moduli of these minerals are shown to correlate with average nearest intertetrahedral anion distances, consistent with the hypothesis that anion-anion interactions stiffen the Si-O-Si angle as inter-tetrahedral anion distances decrease. The tetrahedral distortion in quartz with pressure is attributed to anion-anion interaction, and is not considered a compression mechanism.

Surface energies are an important factor in determining surface reactivity, sintering, and transformation, yet, until recently, there have been few direct measurements for rock-forming mineral phases. Using calorimetric methodology we developed and first applied to aluminum and iron oxides, we present here the first direct measurement of the surface enthalpy of forsterite, Mg 2 SiO 4 , based on high-temperature oxide melt solution calorimetry and water adsorption calorimetry. The measured surface enthalpies of hydrated and anhydrous Mg 2 SiO 4 particles are 3.37 ± 0.21 and 4.41 ± 0.21 J/m 2 , respectively. The measured water adsorption enthalpies for approximately first and second monolayer coverage are -102 and -79 kJ per mole of H 2 O (liquid water reference state). Both the surface enthalpy and water adsorption enthalpy values agree well with values from atomistic simulations. The relatively high surface enthalpy of forsterite is greater than that of γ-alumina, maghemite (γ-Fe 2 O 3 ), and MgAl 2 O 4 spinel. This suggests that the surface energy of ringwoodite may be lower than that of forsterite, and that surface energy effects may thermodynamically favor the olivine-spinel transition, bringing it to lower pressures for small grains.

The transformation of magnetite to hematite is described and analyzed in three natural samples of banded iron formation, from Quadrilátero Ferrífero, Brazil. In each sample, a particular microstructure related to the transformation process is described. In the first, magnetite crystals are large and euhedral, and they display the beginning of the transformation into hematite. In the second sample, a relict crystal of magnetite was found and the fabric of the transformed hematite was evaluated. In the last sample, the foliation was the main observed structure and the correlations of magnetite and hematite lattices were measured. All the microstructures were analyzed in a scanning electron microscope equipped with a detector for electron backscatter diffraction allowing the complete analysis of crystallographic orientations of hematite and magnetite on a local scale. The results show that the orientations of the basal planes of hematite coincide with the orientations of the octahedral planes of magnetite, indicating that the hematite crystals are a direct product from the magnetite transformation.

A sample of Mg-vermiculite from Santa Olalla (Spain) was studied by X-ray powder diffraction, electron microprobe, and thermo-gravimetry. The 3D structure is described as a disordered stack of two types of 2D building blocks, which are made up of one talc-type layer and one interlayer space containing hydrated Mg 2+ cations. We have succeeded in the refinement of both the atomic positions and occupancies of exchangeable cations and water molecules in the interlayer space of this vermiculite using the program package DIFFaX+. The position of the Mg 2+ cations is the only difference between the two layers. Besides the water molecules associated to the octahedrally coordinated Mg 2+ , we also located water molecules in the interlayer space. The structural analysis confirms that vermiculite is a semi-ordered crystalline material characterized by the existence of a large density of defects due to random ~±b/3 translations along the crystalline [010] direction. In this way, this structure can no longer be described by means of a unit cell repeated in 3D space. Instead, long-range order is only recognized in the a-b plane. The 3D structure is described by means of a recursive method.

Large single crystals of stishovite were successfully synthesized at 11 GPa from a silica solution in water. The potential of both slow cooling and thermal gradient methods were examined. The thermal gradient method provided crystals of 0.8 × 0.8 × 1.3 mm in size grown at 1350 °C and a thermal gradient of 50 °C/mm using stishovite as a silica source. The use of quartz as a source resulted in the appearance of numerous stishovite crystals in the solution interior resulting in diminished space for the growth of large crystals. This can be explained by a significant difference in the solubility of metastable quartz and stishovite in water, estimated to be 85.3 and 5.6 wt% SiO 2 at 1000 °C and 11 GPa, respectively. Crystals up to 0.8 × 1.3 × 1.5 mm were grown by the slow cooling method in the system SiO 2 + 14.7 wt% H 2 O as temperature was decreased from 1600 to 1000 °C with a cooling rate of 2 °C/min. The size of single crystals obtained was large enough to carry out silicon self-diffusion experiments, which were performed at a pressure of 14 GPa and temperatures from 1400 to 1800 °C. The lattice diffusion coefficients along the [110] and [001] directions can be expressed as D [110] (m 2 /s) = 4.10 × 10 -12 exp [-322 (kJ/mol)/RT] and D [001] (m 2 /s) = 5.62 × 10 -12 exp [-334 (kJ/mol)/RT], respectively, where R is the gas constant and T is the absolute temperature.

The density of a peridotite magma was measured up to 2.5 GPa and 2300 K using an X-ray absorption method. The method allowed measurement of the density of a peridotite melt under seven different conditions and clarified the pressure and temperature dependence of the density. A fit of the pressure-density-temperature data to the high-temperature Birch-Murnaghan equation of state yielded the isothermal bulk modulus, K T0 = 24.0 ± 1.3 GPa, its pressure derivative, K 0 ′ = 7.3 ± 0.8, and the derivative of bulk modulus (∂K T /∂T) P = -0.0027 ± 0.0017 GPa/K at 2100 K. The large bulk modulus and its pressure derivative of the peridotite melt compared with that of basaltic melt is consistent with previous results from sink-float experiments.

The crystal structure of a 6C pyrrhotite from Mponeng Mine, South Africa, has been refined using starting atomic parameters postulated by Koto et al. (1975). This pyrrhotite is monoclinic and is described in the non-standard space group Fd, so that the metrically orthorhombic unit-cell shape is preserved. The cell symmetry is monoclinic with dimensions a = 6.897(2), b = 11.954(3), c = 34.521(7), and with β = 90.003(4)°. The structure has been refined with anisotropic displacement parameters to R = 0.029 using 1493 observable reflections with I > 2σ(I) and R = 0.034 for all 1800 reflections. The internal R int is 0.023 for the symmetrically equivalent reflection data. The composition of the crystal, as determined by electron microprobe analysis is Fe 10.91 Ni 0.05 S 12 . The crystal structure resembles that of the 5C pyrrhotite in that the atomic positions of the Fe and S atoms are arranged in a very similar fashion, the only real difference being the arrangement of partially occupied iron sites. The coordination of the iron atoms is octahedral and short Fe-Fe distances along the c-axis are also encountered in this structure. The vacancy distribution is similar to that postulated by Koto et al. (1975) and is characterized by the stacking of two approximately half-occupied sites, followed by an essentially fully occupied layer. This is however a simplification and results in a composition that is too metal-rich. Two other slightly defect sites with occupancies of 0.90 and 0.87 are also present in the structure, and all layers contain both fully occupied and partially occupied sites. Refinement of the occupancies of all these sites gives rise to an atomic distribution that resembles the measured composition most closely, and is refined as Fe 10.99 S 12 . The powder diffraction pattern of this structure is compared to the very similar pattern of the 5C structure. The crystal structure is also given in the more conventional Cc setting so as to be compatible with the available crystallographic software that cannot normally accommodate the Fd space group.

In this study, we examine the effects of reheating on finely exsolved hematite-ilmenite intergrowths from the ~91 Ma Ecstall pluton using reflected light microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). As a result of the emplacement of the younger adjacent ~52 Ma Quottoon pluton, samples closer to a thermal boundary have experienced greater degrees of thermal alteration. Five main microstructural features characterize hematite-ilmenite intergrowths from the Ecstall: (I) exsolution lamellae of hematite and ilmenite; (II) oxidation of ilmenite to form hematite, rutile, and other Ti-rich phases; (III) 20-50 nm magnetite precipitates in hematite; (IV) rutile blitz texture; and (V) exsolution of hematite in rutile. Based on spatial relationships, textures II through V appear to be related to reheating of the Ecstall by the Quottoon, and samples up to ~14 km from the thermal boundary intrusive contact have been affected. We propose a mechanism, similar to that of Kontny and Dietl (2001), in which reheating has driven T-ƒ O₂ conditions across the hematite-magnetite buffer to lower ƒ O₂ resulting in the reduction of hematite. Higher temperatures also enhanced oxidation in ilmenite. The formation of magnetite altered the bulk magnetic properties of these samples, increasing NRM intensity. This study underscores the need to consider a pluton’s post-emplacement thermal history before making tectonic interpretations based on paleomagnetic data.

A new type of Al-rich hydroxylian pseudorutile (HPR), containing 64.6 wt% TiO 2 , 12.1 wt% Al 2 O 3 , 10.9 wt% Fe 2 O 3 , and 12.4 wt% H 2 O, has been identified in detrital heavy mineral concentrates from Central Kalimantan, Indonesia. The unusual translucent orange-colored grains have been characterized using electron microprobe analyses, thermal analyses, 27 Al MAS NMR, and synchrotron XRD studies including the use of differential pair distribution function analysis, to determine how the Al is incorporated. The results support a model in which diaspore-form AlOOH is incorporated into nanopores in the solid hydrogel-like mineral. The HPR matrix structure comprises disordered unit-cell scale intergrowths of diaspore-type and rutile-type structure elements, which have a close dimensional match to the diaspore structure, thus providing a suitable template for epitactic nucleation and growth of the diaspore. The average composition of the grains is [FeTi 6 O 10.8 (OH) 5.4 ]·1.8AlOOH·H 2 O, where the formula within the square brackets represents the HPR matrix, and the AlOOH and H 2 O occupy the intragrain pore volume.

Lapeyreite, ideally Cu 3 O[AsO 3 (OH)] 2 ⋅0.75H 2 O, was found in the old copper mines of Roua (Alpes-Maritimes, France). It is invariably in intimate association with trippkeite. Other associated minerals are olivenite, malachite, gilmarite, cornubite, connellite, theoparacelsite, brochantite, cuprite, native copper, algodonite, and domeykite. Lapeyreite occurs in geodes of cuprite (0.5 mm diameter) as aggregates formed by perfect elongate rectangular crystals (up to 0.2 × 0.05 × 0.01 mm in size), acicular fibrous crystals or powdery masses. The mineral is translucent (transparent in thin fragments), dark pistachio-green. It has a vitreous to adamantine luster and yellowish green streak. The tenacity is brittle and the fracture conchoidal. The rectangular crystals are elongate parallel to [010], flattened on (001), and have a perfect cleavage on {001}, and good cleavage on {100}. All crystals, without exception, are twinned on the (001) plane. The recognizable crystal forms are {100}, {010}, and {001}. In transmitted light, the mineral is pistachio-green, with strong pleochroism: X = light yellow-green, Y = pistachio-green, Z = dark pistachio-green; dispersion: r > v, medium. Lapeyreite is biaxial (+), with n α ~ 1.82, n β ~ 1.85, n γ ~ 1.90 (for λ = 589 nm). 2V meas = 76° (universal stage), 2V calc = 77°. The optical orientation is X ^ c ~ 12°, Y = a, Z = b. The mean chemical composition determined by electron microprobe is (wt%): CuO 46.49, As 2 O 5 45.82, H 2 O (from crystal structure analysis) 6.30, total 98.61. The empirical formula calculated on the basis of nine structural O atoms (excluding molecular water) is Cu 2.96 As 2.01 O 6.99 (OH) 2.01 ⋅0.77H 2 O. Lapeyreite is monoclinic, C2/m, a = 19.158(3), b = 2.9361(6), c = 9.193(2) Å, β = 103.26(1)°, V = 503.32(6) Å 3 , Z = 8/3. The calculated density is 4.385 g/cm 3 (based on the empirical formula). The strongest X-ray powder-diffraction lines are [d(Å) (I) (hkl)]: 7.36 (30) (2̅01), 5.842 (40) (201), 4.476 (35) (002), 3.173 (90) (6̅01), 2.984 (100) (003), 2.883 (30) (6̅02), 2.484 (80) (311), 2.396 (40) (112, 8̅01), and 2.337 (35) (800). The crystal structure of lapeyreite was solved by direct methods (MoKα radiation) and refined on F 2 using all 617 observed reflections to R = 0.069. The structure of lapeyreite is formed by a three-dimensional network of CuO 5 square pyramids and AsO 4 tetrahedra with a water molecule in structural cavities. This structure shows some similarities to that of theoparacelsite. The mineral is named in honor of Laurent Lapeyre, an eminent mineral collector and expert on Roua minerals.

High-pressure and high-temperature phase relation on Mg 0.74 Fe 0.26 -staurolite composition with excess water in the system FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O was experimentally determined at 12-19 kbar and 850-1050 °C, with a focus on the lower-pressure stability limit of magnesian staurolite. The experimental results show that the stability field of staurolite in the system shifts to higher temperatures as compared to that of pure-Mg staurolite. At 950 °C, the lower-pressure stability limit is located between 15 and 14 kbar. This study demonstrates that staurolite breaks down with isothermal decompression by a series of reactions at 950 °C: St → Opx + Crn + Melt and St + Opx → Spr + Melt. The Mg' [=Mg/(Fe+Mg)] of staurolite in the former reaction is 0.7. When staurolite coexists with orthopyroxene in run products, the staurolite Mg' is decreased to 0.6-0.5. Moderate-Mg staurolite (Mg' = ~0.5) in natural occurrences has been reported as inclusion minerals in poikiloblastic garnets within ultrahigh-temperature (UHT) metamorphic rocks from major collisional orogenic belts in southern India and southern Africa. The experimental data presented in our study suggest the possibility that the staurolite-bearing UHT metamorphic rocks had experienced high-pressure metamorphism and/or UHT extreme metamorphism during the prograde metamorphic stage.

Hibonite-(Fe), (Fe,Mg)Al 12 O 19 , is the Fe 2+ -dominant analog of hibonite CaAl 12 O 19 , discovered in a highly altered Ca-,Al-rich refractory inclusion from the Allende meteorite. It occurs as scattered micrometer-sized single crystals within an aggregate of hercynite (Fe,Mg)Al 2 O 4 , adjacent to nepheline, ilmenite, ferroan spinel, perovskite, and hibonite. The mean chemical composition determined by electron microprobe analysis of hibonite-(Fe) is Al 2 O 3 90.05 wt%, FeO 3.60, SiO 2 2.09, MgO 1.61, Na 2 O 0.55, CaO 0.28, TiO 2 0.04, V 2 O 3 0.02, sum 98.25, corresponding to an empirical formula of (Fe 2+ 0.34 Mg 0.27 Na 0.12 Al 0.11 Ca 0.03 ) Σ0.87 (Al 11.77 Si 0.23 ) Σ12.00 O 19 . Hibonite-(Fe) is hexagonal, P6 3 /mmc; a = 5.613 Å, c = 22.285 Å, V = 608.0 Å 3 and Z = 2. Its electron backscatter diffraction pattern is a good match to that of the hibonite structure. Hibonite-(Fe) is apparently a secondary alteration product formed by iron-alkali-halogen metasomatism, whereas hibonite is a primary refractory phase.

Thin crystals of rutile, brookite, anatase, and cassiterite were irradiated in situ in the transmission electron microscope using 1.0 MeV Kr ions at 50-300 K. Synthetic rutile and natural cassiterite, with 0.1-0.2 wt% impurities, remain crystalline up to a fluence of 5 × 10 15 ions cm -2 without evidence for amorphization at 50 K. Natural brookite and anatase, with 0.3-0.5 wt% impurities, become amorphous at fluences of 8.1 × 10 14 and 2.3 × 10 14 ions cm -2 , respectively. We have also studied two natural rutile samples containing ~1.7 and 1.2 wt% impurities. These samples became amorphous at 9.2 × 10 14 and 8.6 × 10 14 ions cm -2 at 50 K, respectively. Further analyses of the fluence-temperature data for natural brookite, rutile, and anatase give critical amorphization temperatures of 168 ± 11, 209 ± 8, and 242 ± 6 K, respectively. Results are briefly discussed with respect to several criteria for radiation resistance, including aspects of the structure, bonding, and energetics of defect formation and migration.

We report the occurrence of unusual lamellar (random and regular subparallel) intergrowths of corundum and symplectic intergrowth of spinel with orthopyroxene in an ultrahigh-temperature Al-Mg granulite from the Southern Marginal Zone of the Limpopo Belt, South Africa. Separate compositions are suggested for the homogenous phase that might have existed prior to the formation of the two types of lamellar intergrowths of corundum with orthopyroxene. In the case of random lamellar corundum + orthopyroxene intergrowth, the estimated precursor composition is similar to Fe-Mg garnet, while, although speculative, an ultrahigh-Al orthopyroxene precursor is suggested to account for the exsolution nature of the unique regular subparallel corundum lamellae in orthopyroxene. Considering that the stability field of Fe-Mg-rich garnet relative to orthopyroxene + corundum extends to lower pressures, the estimated garnet composition (X Mg = ~0.65) is most likely stable at low pressures of ~5 kbar. Such low pressures are further supported by the Fe-Mg-rich garnet compositional nature of the pre-spinel-intergrowth orthopyroxene. Given the rare preservation of the mineral assemblages typical of prograde metamorphism in granulite facies rocks, our discovery of corundum lamellar intergrowth with orthopyroxene from an ultrahigh-temperature Al-Mg granulite is unique.

Single-crystal diffraction data collected for CaFe 2 O 4 at high pressure reveal above 50 GPa an isosymmetric phase transition (i.e., no change in symmetry) marked by a volume decrease of 8.4%. X-ray emission spectroscopic data at ambient and high pressure confirm that the nature of the phase transition is related to the Fe 3+ high-spin/low-spin transition. The bulk modulus K 0 calculated with a Birch Murnaghan EoS (K′ = 4) is remarkably different [K 0 = 159(2) GPa for CaFe 2 O 4 “high spin” and K 0 = 235(10) GPa for CaFe 2 O 4 “low spin”]. Crystal structure refinements reveal a decrease of 12% of the Fe 3+ crystallographic site volume. The geometrical features of the low-spin Fe 3+ crystallographic site at high pressure (bond lengths, volume) indicate a relevant decrease of Fe 3+ -O bond lengths, and the results are in agreement with tabulated values for crystal radii of Fe 3+ in high- and low-spin state. The reduced crystal size of Fe 3+ in the low-spin state suggest that in lower mantle assemblages, Fe 3+ partitioning in crystallographic sites should be strongly affected by the iron spin state.

The ordering of octahedral Al 3+ , Fe 3+ , and Mg 2+ cations in dioctahedral 2:1 phyllosilicates was studied theoretically. Quantum mechanical calculations based on density functional theory (DFT) were performed for optimizing different cation distributions along the octahedral sheet. Three systems of two species (Al 3+ /Mg 2+ , Al 3+ /Fe 3+ , and Fe 3+ /Mg 2+ ) were studied to obtain the cation exchange potentials J n as first, second, third, and fourth nearest neighbors. Monte Carlo (MC) simulations based on the previously calculated cation exchange potentials J n of these binary systems showed ordering phase transition in the distribution of the octahedral cations, with different ordering patterns. Ordered phases are depending on composition and on third and fourth neighbor range interactions. The effect of hydrostatic pressure can affect the cation ordering of the octahedral sheet. The two-species model was extended to a three-species ordering MC simulation model. In our case, we do not find perfect long-range ordering in the three-species systems. Instead we find some domains with different ordering patterns.