Volume 96, Issue 4

Study of the evolution of the Earth’s crust advanced greatly in the last half-century through the dynamic collaboration of three equally contributing partners: field observations, together with microanalysis of mineral assemblages, experimental petrology, and stability calculations based on systematically measured and compiled thermodynamic properties of minerals. In addition to providing the initial inspiration for experimental and theoretical studies of petrogenesis, quantitative field observations have had an important feedback relationship with the indoor approaches. It has been demonstrated that physico-chemical properties of mineral systems can, in favorable circumstances, be inferred from purely geologic criteria, and, in some cases, field criteria have been used to challenge the validity of existing experimental or thermodynamic data, leading to reinvestigations and revisions. The “geo-experimental” method may be particularly useful to infer phase equilibrium relations among minerals where experimental reactions are prohibitively slow at geologically reasonable temperatures or where fluid fluxing of mineral reactions is necessarily limited, as in investigations concerning the low-H 2 O-activity deep-crustal rocks, the granulites. Geo-experimental phase equilibrium diagrams, constructed largely from quantitative petrographic observations, have been successful in deducing polymorphism, order-disorder, and solid solution properties of such mineral systems as the aluminum silicate minerals, feldspars, epidote minerals, scapolites, and cordierite. The last minerals are particularly important in that they may retain a record of volatile components of an intergranular mineralizing fluid. Knowledge of the important effects of mixed volatile fluids has been inhibited by lack of experimental data on devolatilization, ion exchange, redox, sulfidation, and fluid-complexing reactions of the volatile-bearing minerals biotite, amphibole, apatite, scapolite, cordierite, carbonates, and sulfides with the major minerals of rocks of the deeper interior. A concerted approach among the experimental, thermodynamic, and petrographic disciplines is necessary to make progress on the still poorly understood role of volatiles in shaping the crust. The relatively young science of fluid inclusion analysis will have an increasingly important role in the interdisciplinary effort to interpret the evolution of the continental crust and its underpinnings.

Carbon isotope exchange between calcite and graphite is a useful and reliable geothermometer for medium- to high-grade marbles. However, in rare instances, such as at Naxos, Greece, apparent disequilibrium carbon isotope fractionation between calcite and graphite has been previously reported. In this study, new results are presented on the morphological features, X-ray diffraction studies, Raman spectroscopic studies, and carbon isotope studies of graphite. Three morphologically distinct graphite types are identified. The first type is fine grained with distinct polygonal crystal aggregates having smooth pinacoid faces. The second type consists of large botryoidal aggregates with typical crystal overgrowth features, and the third type is “normal” large platy crystals associated with phlogopite. Graphite morphological features using an FE-SEM suggests that the botryoidal aggregates are composed of complexly intergrown and stacked graphite layers, comprised of cone-helix structures. Different types of graphite display distinct Lc(002) and DG values as well as a sharp first-order Raman peak at -1580 cm -1 with disordered bands. The presence of Raman disordered bands in fine-grained well-crystallized graphite is attributed to edge effects. Carbon isotope analysis of graphite reveals that marble with fine-grained well-crystallized graphite show a consistent isotope fractionation with the calcite, which perfectly match temperature estimates based on the mineral isograds. In contrast, coarse botryoidal graphite gives heterogeneous carbon isotope values (about a few per mill) and show anomalous carbon isotope fractionation between the calcite and graphite. The graphite associated with phlogopite also shows variations in the carbon isotope values. Combining morphologic, crystallographic, and carbon isotopic data we conclude that the variations in carbon isotopic composition were caused by the overgrowth of graphite on the preexisting grain and that morphological observations and structural characterizations of graphite crystals is a key for predicting isotopic equilibration during metamorphism.

Deformation of olivine in a volcanic context is poorly constrained, although deformed olivine is abundant in some volcanic rocks, and its presence is important for the definition of the magmatic history of volcanic edifices such as Kilauea Volcano, Hawaii. Deformed olivines at Kilauea originate in the lower crust; therefore, the classic approaches and interpretations applied to mantle-derived olivine are not applicable. Deformed olivine crystals from Kilauea lava samples were examined using an in situ XRD technique. Our results validate and refine optical observations of olivine deformation. We also confirm the presence of deformation and quantify it for olivine crystals of any size, even for small crystals (0.15 mm). There are significant correlations between deformation intensity (strainrelated mosaicity) and olivine composition and crystal size. Although this technique does not allow the simple estimation of the P-T conditions of deformation, crystal formation, or magmatic history, some constraints are provided herein. In particular we estimate the threshold degree of mosaicity, above which we consider that a crystal underwent deformation. In situ XRD is shown to be an easyto- use, fast, low-cost, non-destructive technique and is less ambiguous than optical microscopy. For crystals optically exhibiting subgrain formation, analysis of asterism by in situ XRD has been used to reconstruct the mosaic spread of the original grain, and thus its original strain condition prior to subgrain formation.

We used small-angle and ultra-small-angle neutron scattering (SANS/USANS) to characterize the evolution of nanoscale features in weathering Rose Hill shale within the Susquehanna/Shale Hills Observatory (SSHO). The SANS/USANS techniques, here referred to as neutron scattering (NS), characterize porosity comprised of features ranging from approximately 3 nm to several micrometers in dimension. NS was used to investigate shale chips sampled by gas-powered drilling (“saprock”) or by hand-augering (“regolith”) at ridgetop. At about 20 m depth, dissolution is inferred to have depleted the bedrock of ankerite and all the chips investigated with NS are from above the ankerite dissolution zone. NS documents that 5-6% of the total ankerite-free rock volume is comprised of isolated, intraparticle pores. At 5 m depth, an abrupt increase in porosity and surface area corresponds with onset of feldspar dissolution in the saprock and is attributed mainly to peri-glacial processes from 15 000 years ago. At tens of centimeters below the saprock-regolith interface, the porosity and surface area increase markedly as chlorite and illite begin to dissolve. These clay reactions contribute to the transformation of saprock to regolith. Throughout the regolith, intraparticle pores in chips connect to form larger interparticle pores and scattering changes from a mass fractal at depth to a surface fractal near the land surface. Pore geometry also changes from anisotropic at depth, perhaps related to pencil cleavage created in the rock by previous tectonic activity, to isotropic at the uppermost surface as clays weather. In the most weathered regolith, kaolinite and Fe-oxyhydroxides precipitate, blocking some connected pores. These precipitates, coupled with exposure of more quartz by clay weathering, contribute to the decreased mineral-pore interfacial area in the uppermost samples. These observations are consistent with conversion of bedrock to saprock to regolith at SSHO due to: (1) transport of reactants (e.g., water, O 2 ) into primary pores and fractures created by tectonic events and peri-glacial effects; (2) mineral-water reactions and particle loss that increase porosity and the access of water into the rock. From deep to shallow, mineral-water reactions may change from largely transport-limited where porosity was set largely by ancient tectonic activity to kinetic-limited where porosity is changing due to climate-driven processes.

The dry thermal transformation of 2-line ferrihydrite to hematite was investigated using combinations of thermogravimetric (TG) and differential scanning calorimetric (DSC) analysis, along with in situ DSC and pair distribution function (PDF) analysis of X-ray total scattering data and in situ temperature controlled infrared (IR) spectroscopy. TG data show a 25.6 ± 0.1% weight loss below 300 °C, ascribed to the removal of surface water since PDF analysis shows no change in the structure of ferrihydrite up to this temperature. The transformation to hematite occurs at around 415 ± 1 °C (peak temperature) at a heating rate of 10 °C/min, with no obvious weight change during or after the transformation. In situ PDF analysis indicates that the ferrihydrite bulk structure remained intact up to the direct transition to crystalline hematite, with no intermediate phases, crystalline or amorphous, formed. In situ IR data shows the extent of absorption attributable to OH stretching in ferrihydrite at 215 °C dropped to 10% of its room-temperature value. These results suggest ferrihydrite contains very little structural OH: the molar ratio of OH/Fe is 0.18 ± 0.01. A recently proposed akdalaite-like ferrihydrite model has an OH/Fe equal to 0.2, consistent with this result. The 3-phase model proposed by Drits et al. (1993) has an average formula close to FeOOH, with an OH/Fe equal to 1.0, far more than suggested by our experiments. Based on the constraints set by the estimated water content and the PDF signatures, we examined possible anion packing types and local structural motifs in ferrihydrite, and demonstrate that ABAC is the only feasible packing type and that a peak at 3.44(2) Å in PDF provides indirect evidence for the presence of tetrahedral Fe.

The defect-free akdalaite model (fhyd6) for six-line ferrihydrite (6Fh) derived from a pair distribution function (PDF) analysis of high-energy X-ray scattering (HEXS) data was revised (model ferrifh) by Michel et al (2010) using data from a sample produced by heating two-line ferrihydrite (2Fh) at 175 °C for 8 h in the presence of citrate. We show here that the scattering pattern for this sample is similar if not the same as that for hydromaghemite, which in turn is a mixture of maghemite (γ-Fe 2 O 3 ), hematite and 6Fh. As in the case of fhyd6, the PDF of ferrifh was regressed using the structure of the weakly hydrated phase akdalaite [Al 10 O 14 (OH) 2 ] after substituting Fe for Al as a starting model. We show that the ferrifh model is implausible for the following reasons. (1) It is derived from a sample, ferrifh, that appears to be hydromaghemite, not pure 6Fh. (2) It has 20% tetrahedral Fe, a coordination that had been eliminated previously using XANES, Mössbauer, and EELS spectroscopies. (3) 75% of the Fe octahedra have shared edge lengths considerably longer than the shortest unshared edges in violation of Pauling’s distortion rule. (4) Three tetrahedral Fe-O bonds are longer than three octahedral Fe-O bonds, inducing significant polyhedral distortions. And (5) the calculated composition [Fe 10 O 14 (OH) 2 ⋅1.2H 2 O] disagrees with literature data on weight loss from dehydration for 6Fh. We present an alternative interpretation of the histogram of Fe-Fe distances up to 3.7 Å obtained from the PDF of the fhyd6 ferrihydrite as a mixture of local structures of goethite/akaganeite (α/β- FeOOH) and feroxyhite/hematite (δ-FeOOH/α-Fe 2 O 3 ). Within this interpretation Fe only occupies octahedra that are bonded to each other by faces, edges, or double-corners. This polyhedral connectivity is confirmed experimentally by analysis of the EXAFS spectra of six-line ferrihydrites measured at room and liquid helium temperature. The Fe-Fe pairs from EXAFS data are described reasonably well by a mixture of approximately 70% feroxyhite (containing some nanohematite) and 30% akaganeite, without resorting to other phases. This set of evidence indicates that HEXS data are consistent with the Drits model for ferrihydrite (Drits et al. 1993a).

The positions of hydrogen (deuterium) cations within the interlayer of synthetic Na- and K-birnessite samples have been determined for the first time using Rietveld analysis and Fourier difference syntheses, from time-of-flight powder neutron diffraction data. This study revealed that two symmetry-related D(H) positions are located -1 Å above and below the midpoint between the split O interlayer sites in Na-birnessite. This result confirms our earlier interpretation that the split interlayer sites include O atoms from two symmetrically equivalent H 2 O molecules. These molecules are oriented 180° to each other, and they are pivoted about a single set of H(D) atoms positions. The interlayer H 2 O molecules in K-birnessite are oriented such that one of the H(D) atoms is pointed approximately toward the Mn octahedral layer O atoms and the other is directed approximately within the net of interlayer K/O sites. In K-birnessite, six K-Ooct distances are shorter than 3.35 Å, compared with only two Na-Ooct bond lengths in Na-birnessite, suggesting that K cations are more tightly bonded to the octahedral O atoms than are Na cations.

Pressless split-sphere apparatus (BARS) equipped with an 8-6 type multi-anvil system is used extensively for large (6 carats) gem quality diamond crystal growth and various mineralogical and geochemical studies at shallow mantle conditions (P ≤ 7.5 GPa). In this study, we extend the pressure range for this apparatus to the conditions of the Earth’s transition zone (P = 20 GPa) using a 6-8 multi-anvil system. A scaled-up Kawai-cell consisting of 47 mm WC cubic anvils with 10 mm truncation and 18 mm MgO pressure medium was employed for the experiments. A 300 mm spherical multi-anvil block compressed by a hydraulic system with 2500 atm capacity was used to compress the Kawai-cell. A pressure of 15.5 GPa was successfully generated at the oil pressure of 1875 atm. The comparison of the BARS apparatus calibrations with those obtained using a uniaxial split-sphere press USSA-5000 suggests that pressure up to 17 and 20 GPa in compressed cell/sample volumes of 1200/60 and 400/18 mm 3 , respectively, can be generated using a combination of the BARS apparatus and the scaled-up Kawai-cell. This cell volume is much larger than that in conventional multi-anvil systems, which does not exceed 80 mm3 at the same conditions

Majorite garnets with the composition Mj70Py30 have been deformed in the multi-anvil apparatus at 17 GPa and 2000 °C. The microstructure has been characterized by transmission electron microscopy. It is shown that under these conditions majorite garnet is ductile. Dislocations with <100> and ½<111> Burgers vectors are observed with a density 1-5 × 10 12 m −2 . The absence of clear glide planes and the occurrence of subgrain boundaries suggest the importance of diffusion and climb in the plasticity of majorite garnets in mantle transition zone conditions.

The density of carbonated peridotite magma was measured up to 3.8 GPa and 2100 K using an X-ray absorption method. A fit of the pressure-density-temperature data to the high-temperature Birch-Murnaghan equation of state yielded the isothermal bulk modulus, K T0 = 22.9 ± 1.4 GPa, its pressure derivative, K 0 ′ = 7.4 ± 1.4, and the temperature derivative of the bulk modulus (∂K T /∂T) P = -0.006 ± 0.002 GPa/K at 1800 K. The bulk modulus of carbonated peridotite magma is larger than that of hydrous peridotite magma. The partial molar volume of CO 2 in magma under high pressure and temperature conditions was calculated and fit using the Vinet equation of state. The isothermal bulk modulus was K T0 = 8.1 ± 1.7GPa, and its pressure derivative was K 0 ′ = 7.2 ± 2.0 at 2000 K. Our results show that the partial molar volume of CO 2 is less compressible than that of H 2 O, suggesting that, on an equal molar basis, CO 2 is more effective than H 2 O in reducing peridotite melt density at high pressure.

We found a new nickel sulfide that is isostructural with Fe 3 S. The synthesized nickel sulfide is a non-stoichiometric compound with a Ni deficiency and its composition is Ni 2.90±0.5 S. In situ synchrotron X-ray observations indicate that Ni 3-x S forms above 5.1 GPa and melts incongruently into Ni+liquid up to 10 GPa. The bulk modulus of Ni 3-x S at 300 K was determined to be 140 ± 2 GPa with a fixed K' = 4 by static compression with a liquid pressure medium. The eutectic point of the Ni-NiS system lies between Ni 3 S and Ni 3 S 2 up to 10 GPa and its composition changes from Ni 66.6 S 33.4 at 0.1 MPa to Ni 70.7 S 29.3 at 10 GPa. The eutectic melting temperature of the Ni-NiS system decreases to 5.1 GPa and 720 K as the pressure increases where Ni and α-Ni 3 S 2 are the eutectic solids. On the other hand, the eutectic melting temperature increases with a positive slope above 5.1 GPa where Ni 3 S+α-Ni 3 S 2 is stable under subsolidus conditions. Intermediate compounds appear at a lower pressure in the Ni-NiS system than that for the Fe-FeS system. Ni 3 S 2 is stable at atmospheric pressure and Ni 3 S forms at 5.1 GPa, whereas Fe 3 S 2 and Fe 3 S form at 14 and 20 GPa, respectively. The addition of Ni complicates the melting relationship in the Fe-FeS system at high pressure because of the wider stability field of Ni 3 S. The low-melting temperature of the Fe-Ni-S system plays an important role in the percolative core-formation of planetesimals during planetary accretion.

Far-infrared spectroscopy and X-ray diffraction Rietveld structure-refinement of synthetic kinoshitalite (Kn) solid solutions, BaMg 3 [(Al 2-x Ga x )(Si 2-y Ge y )]O 10 (OH,OD,F) 2 : (x = 0.0-2.0, y = 0.0-2.0), show that there is complete solid solution for all compositions in each (OH/OD)- and F-series: [4] [Al 2 (Si 2-y Ge y )]-, [4] [(Al 2-x Ga x )Si 2 ]-, [4] [Ga 2 (Si 2-y Ge y )]-, [4] [(Al 2-x Ga x )Ge 2 ]-Kn, and in OH/OD-for-F substituted [4] (Al 2 Si 2 )-, [4] (Ga 2 Si 2 )-, [4] (Al 2 Ge 2 )-, [4] (Ga 2 Ge 2 )-Kn end-member compositions. In the far-infrared region, 170-40 cm -1 , three kind of bands are observed; an in-plane tetrahedral torsional mode, an interlayer Ba-O inner stretching vibration and a Ba-O outer stretching vibration. With increasing tetrahedral [4] Al-for- [4] Ga and Si-for-Ge substitution, the frequencies and intensities of the tetrahedral in-plane torsional bands decrease in both the (OH/OD)- and F-bearing phases, but in the [4] (Al 2 Si 2 )-, [4] (Ga 2 Si 2 )-, [4] (Al 2 Ge 2 )-, [4] (Ga 2 Ge 2 )-Kn end-member compositions, the frequencies are unaffected by (OH/OD)-for-F substitution. The frequencies of both the Ba-O inner and Ba-O outer stretching bands increase with increasing [4] Al-for- [4] Ga and Si-for-Ge substitution, but the frequencies of the Ba-O inner stretching bands decrease with increasing (OH/OD)-for-F substitution in the [4] (Al 2 Si 2 )-, [4] (Ga 2 Si 2 )-, [4] (Al 2 Ge 2 )-, [4] (Ga 2 Ge 2 )-Kn end-member compositions. The frequency difference between the Ba-O inner and Ba-O outer stretching bands is linearly related to the tetrahedral rotation angles (α), and these differences are about 10 cm -1 larger in the (OH/OD)-bearing phases than in the corresponding F-bearing phases. The ranges of absorption frequencies and their corresponding deformation modes are as follows: (1) inplane tetrahedral torsional mode, 105-150 cm -1 ; (2) Ba-O inner stretching vibration, 105-140 cm -1 ; and (3) Ba-O outer stretching vibration, 75-90 cm -1 .

Compressional and shear wave velocities of a polycrystalline specimen of MgSiO 3 orthoenstatite have been measured by ultrasonic interferometry to 1373 K at 300 MPa in an internally heated gas-medium apparatus. The elastic wave velocities and bulk and shear moduli vary linearly with temperature to 1073 K. Below 1073 K, the temperature derivatives of the elastic moduli [(∂KS/∂T) P = -28.3(7) MPa/K and (∂G/∂T) P = -14.5(1) MPa/K, respectively] determined in this study are consistent with averages of single-crystal elastic constants measured using Brillouin spectroscopy by Jackson et al. (2007). The measured temperature dependence of elastic moduli, along with pressure dependence of elastic moduli, thermal expansion and calorimetric data have been assimilated into a finite-strain equation of state of the type proposed by Stixrude and Lithgow-Bertelloni (2005). This analysis suggests significant revisions to the optimal values of the zero-pressure Grüneisen parameter γ 0 and its zero-pressure logarithmic volume derivative q 0 . The unusually high absolute values of (∂K/∂P) T and (∂K/∂T) P are related through the extrinsic part of the temperature derivative. Above 1073 K, a pronounced softening of the elastic wave velocities is observed, which is plausibly associated with a phase transformation for which there is microstructural evidence: The recovered specimen was found to have transformed to the low-pressure clinoenstatite polymorph.

The illite layer content of mixed-layer illite/smectite (I/S) in a 2.5 m thick, zoned, metabentonite bed from Montana decreases regularly from the edges to the center of the bed. Traditional X-ray diffraction (XRD) pattern modeling using Markovian statistics indicated that this zonation results from a mixing in different proportions of smectite-rich R0 I/S and illite-rich R1 I/S, with each phase having a relatively constant illite layer content. However, a new method for modeling XRD patterns of I/S indicates that R0 and R1 I/S in these samples are not separate phases (in the mineralogical sense of the word), but that the samples are composed of illite crystals that have continuous distributions of crystal thicknesses, and of 1 nm thick smectite crystals. The shapes of these distributions indicate that the crystals were formed by simultaneous nucleation and growth. XRD patterns for R0 and R1 I/S arise by interparticle diffraction from a random stacking of the crystals, with swelling interlayers formed at interfaces between crystals from water or glycol that is sorbed on crystal surfaces. It is the thickness distributions of smectite and illite crystals (also termed fundamental particles, or Nadeau particles), rather than XRD patterns for mixed-layer I/S, that are the more reliable indicators of geologic history, because such distributions are composed of well-defined crystals that are not affected by differences in surface sorption and particle arrangements, and because their thickness distribution shapes conform to the predictions of crystal growth theory, which describes their genesis.

Six synthetic single crystals of spinel phases with different compositions along the ZnAl 2 O 4 -ZnCr 2 O 4 solid solution were structurally and chemically characterized by X-ray diffraction and electron microprobe techniques. As predicted, unit-cell parameters and octahedral bond lengths (M-O) increase with increasing replacement of Al 3+ by Cr 3+ . Despite the constant occupancy of the T site by Zn, also the tetrahedral bond length T Zn-O shows significant variations along this binary. These variations are positively correlated with variations in M-O bond lengths. The present data in conjunction with data from literature provide a basis for quantitative analyses of the variation in T Zn-O in normal spinel structures. A negative correlation between T Zn-O and the ionic potential at M (MIP) suggests that increasing M IP is related to a stronger electrostatic cation-cation repulsion across the shared octahedral edge M(O-O) shared of the structure. An observed negative correlation between M IP and M(O-O) shared suggests that a decrease of M(O-O) shared provides a more efficient shielding effect to reduce the octahedral cation interactions. In normal ZnB 2 O 4 spinels (where B = Al 3+ , Cr 3+ , Ga 3+ , V 3+ , Fe 3+ , and Mn 3+ ) cations with a smaller size provides a higher charge density. Increasing charge density at the M site causes shortening of M(O-O)shared, which in turn results in shorter T Zn-O bond length. In general, variations in T Zn-O are required by the structure to better provide an oxygen shielding effect to the octahedral cationcation repulsion.

Some errors and metastable phase relations are found in a recently reported oxygen fugacity-silica activity [logf O₂ -loga(SiO 2 )] diagram of the fayalite-kirschsteinite-hedenbergite-magnetite-wollastonite (Fa-Kst-Hd-Mt-Wo) system at 400 °C and 1 kbar (Markl et al. 2001). Systematic thermodynamic analysis reveals that the composite exsolution lamellae of Mt and Hd in the olivine from the Ilimaussaq Intrusion, Greenland, should form when the reaction 3Fa+3Kst+O 2 = 3Hd+2Mt is overstepped on rapid cooling. This process is also accompanied by favorable f O₂ and a(SiO 2 ) conditions. It is also found that the reaction mechanisms from Ca-Fe-rich olivine (Fa+Kst) to Mt+Hd are very different above and below the equilibrium temperature of the Mt-absent reaction Hd+Kst = Fa+2Wo. The mechanisms above the equilibrium temperature are obviously simpler than those below the equilibrium temperature. Despite this difference, the two types of mechanisms have many features in common: (1) Mt and Hd can form in Ca-Fe-rich olivine on continuous cooling; (2) If a(SiO 2 ) is not buffered, an increase in f O₂ alone is enough to transform Ca-Fe-rich olivine into Mt and Hd; (3) At a fixed f O₂ or a(SiO 2 ), the formation of Mt and/or Hd on cooling requires appropriate a(SiO 2 ) or f O₂ ; and (4) The increase in a(SiO 2 ) is favorable to the formation of Hd, but it is not essential for the intergrown exsolution of Mt and Hd. These common features are closely related to the fact that the process from Fa+Kst to Mt+Hd is an entropy decreasing oxidation reaction.

Oriented single crystals of stibioellisite (Tl 3 SbS 3 ), parapierrotite (TlSb 5 S 8 ), weissbergite (TlSbS 2 ), and lorandite (TlAsS 2 ) were investigated by polarized Raman spectroscopy. Whereas stibioellsite shows isolated SbS 3 groups, the rest of the minerals show interconnected pyramidal (As,Sb)S 3 groups. Raman bands of the investigated minerals occur between 400 and 10 cm -1 . The internal vibrations for stibioellsite occur between 350-100 cm -1 . Those of the interconnected pyramidal groups occur between 350 and 10 cm -1 in parapierrotite, 350 and 90 cm -1 in weissbergite, and 420 and 130 cm -1 in lorandite. Approximate similarities in the spectral features are evident when comparing the spectra of minerals containing XS 3 pyramids with the spectra of the minerals in the present study. A clear distinction between Raman spectra of separated and interconnected SbS 3 groups is not observed.

Optical absorption spectroscopy and X-ray structural refinements of seven different spinel single crystals on the (Mg 1-x Mn x )Al 2 O 4 solid solution (x = 0.02-1.00) evidences exceptionally strong relaxation (ε = 0.83) of IV Mn 2+ -O bonds. Our single-crystal structure refinements demonstrate that the ideal IV Mn 2+ -O bond distance in fully ordered galaxite (MnAl 2 O 4 ) should be 2.050 Å, which is 0.014 Å longer than previously suggested, and that structural parameters are mainly affected by the variations occurring at the TO 4 tetrahedron. The very strong structural relaxation observed around the T site may be explained by the fact that the TO 4 polyhedra of the spinel structure share only corners with neighboring MO 6 octahedra and are fully isolated from neighboring TO 4 tetrahedra. This provides structural flexibility around the T site and allows for considerable local T-O bond distance variations.

The high-pressure behavior of a natural Mg-rich anthophyllite (space group Pnma) from Talcville, New York, U.S.A., with empirical formula A Na 0.04 B (Mg 1.30 Mn 0.57 Ca 0.09 Na 0.04 ) C (Mg 4.96 Fe 0.02 Al 0.02 ) T (Si 7.99 Al 0.01 )O 22 W (OH) 2 has been studied to 7 GPa. Bulk and axial moduli determined by fitting a thirdorder Birch-Murnaghan equation-of-state to lattice parameters are: K T0 = 66(2) GPa, K′ = 11(1), K T0 (a) = 41(3) GPa, K′(a) = 11(2), KT0(b) = 97(5) GPa, K′(b) = 6(2), K T0 (c) = 83(7) GPa, and K′(c) = 12(4). Structure refinements were also obtained at 0.0001, 1.98(2), 4.57(2), and 6.13(4) GPa. A comparison with the recent study of Fe-rich proto-amphibole, space group Pnmn, by Zanazzi et al. (2010) allows the effects of symmetry and composition upon high-P behavior of amphibole to be evaluated. The effects of composition have been identified and the central role of the ribbon of M1,2,3 octahedra in the compression of amphibole is discussed. The only significant differences between the elastic properties of anthophyllite and proto-amphibole are the greater stiffness of anthophyllite along [100] and the much higher-pressure derivatives K′(b) and K′(c) of anthophyllite. It is proposed that a high Mg content of the M1,2,3 ribbon stiffens the amphibole structure considerably on compression, compared with an Fe-rich ribbon. This result indicates the importance of determining K′, as the zero-pressure moduli can be very similar but change significantly with P.

The Fe oxidation state, coordination geometry, and <Fe-O> distances have been determined by Fe K-edge XANES and EXAFS for a set of silicate glasses of phonolite composition produced at different oxygen fugacity conditions with the aim of determining the effect of iron oxidation state and local structural environment on the viscosity of the corresponding melts. Comparison of the pre-edge peak data with those of Fe model compounds with known oxidation state and coordination number allowed for determination of the Fe oxidation state and coordination number for all the glasses analyzed. The Fe 3+ /(Fe 3+ +Fe 2+ ) ratio varies from 0.44 to 0.93 (±0.05) in the glasses studied. The determined values are in excellent agreement (within 0.03 difference) with those independently measured by the titration method. Moreover, pre-edge peak data clearly indicate that Fe 3+ is in fourfold coordination, whereas Fe 2+ exists both in fourfold and fivefold coordination for this phonolitic composition, although the presence of minor amounts of sixfold-coordinated Fe cannot be ruled out by XANES data alone. EXAFS data of the most oxidized sample indicate that Fe 3+ is in tetrahedral coordination with <Fe-O> = 1.85 Å (±0.01). This value compares well with literature data for [4] Fe 3+ (e.g., in tetra-ferriphlogopite or rodolicoite). Calculated NBO/T ratios decrease with Fe oxidation (from 0.23 to 0.19). For phonolitic glasses of this study, going from reducing to oxidizing conditions results in a higher fraction of network-forming Fe, thus increasing the polymerization of the tetrahedral network and producing shorter (and stronger) <Fe-O> bond distances. Both the polymerization increase and the structural variations in the Fe local environment can qualitatively explain the strong increase in melt viscosity observed at higher oxygen fugacity

Field, textural, paragenetic, and chemical data for micas from pegmatites of the Fregeneda-Almendra pegmatitic field (Central-Iberian Zone) are used to characterize and evaluate their petrogenesis. These pegmatites show a zonal distribution from barren to evolved, with an increase in degree of evolution with increasing distance from the Mêda-Penedono-Lumbrales leucogranite. Five types of evolved pegmatites have been recognized: (1) petalite-rich, (2) spodumene-rich, (3) Li-mica + spodumenerich, (4) Li-mica-rich, and (5) cassiterite-rich pegmatites, plus six types of barren and intermediate pegmatites. Representative micas from the different pegmatite types and from the leucogranite were analyzed for major and trace elements. All micas belong to the muscovite-lepidolite series. Lithium is incorporated into Li-micas via the Li 3 Al -1 □ -2 and Si 2 LiAl -3 substitutions, where □ represents vacancies. The Al 4 Si-3□ -1 and Al 2 □ 1 R 2+ -3 substitutions, where R 2+ = (Fe + Mg + Mn), account for the compositional variability of micas from the Li-mica-free pegmatites. The Li, Rb, Cs, Be, Ta, and Nb contents of micas increase in the order: leucogranites and barren pegmatites < intermediate pegmatites < spodumene-bearing and petalite-bearing dikes < Li-mica-bearing pegmatites. The Ba content decreases in the same order, and Sn and Zn are relatively abundant in the intermediate pegmatites. These variations are consistent with rare-element enrichment via fractionation processes combined with partitioning of rare elements from the pegmatite melt into the minerals and volatile phases. However, some pegmatite types occurring in this area, such as the cassiterite-rich dikes, do not seem to form part of the same evolutionary trend.

To investigate the behavior of the As 4 S 4 molecule within a crystal-chemical environment differing from realgar, α-As 4 S 4 , and its high-temperature polymorph, β-As 4 S 4 , the effects of the light exposure on the structure of the HgI 2 ·As 4 S 4 adduct have been studied. The structure of this compound consists of a packing of nearly linear HgI 2 molecules and As4S4 cage-molecules. A crystal [V = 1295.9(4) Å 3 ] was exposed to filtered polychromatic light (550 nm long-wavelength pass filter). A marked increase of the unit-cell volume as a function of the exposure time was observed up to V = 1338.9 Å 3 at 3060 min of exposure. Structure refinements indicated that the increase of the unit-cell volume is to ascribe to the formation of an increasing fraction (up to 59%) of pararealgar replacing the realgar-type molecule. After this point, further light-exposure did not cause any further increase of the lattice parameters. On the contrary, a slow, continuous decrease of the unit-cell volume down to 1292.9 Å 3 occurred by keeping the crystal in the dark (39 days). The “reverse” evolution of the unit-cell parameters in the dark almost overlap that observed during the light-induced process and the structural model obtained after the “dark stage” was found to be identical to that of the unaltered crystal, although the diffraction quality was lower and powder-like diffraction rings were observed together with single-crystal reflections. Apart from few peaks belonging to the original unaltered HgI 2 ·As 4 S 4 adduct, most of the collected peaks can be assigned to β-As4S4. This feature could indicate decomposition into micro-crystalline β-As 4 S 4 and HgI 2 ; no diffraction effect ascribable to any HgI 2 phase, however, was observed. Micro-Raman spectra were collected on crystals exposed to the above-mentioned wavelength light for increasing times (up to 3000 min). The peak at 274(±1) cm -1 whose intensity increases as a function of the exposure time confirms the transition from a realgar- to a pararealgar-type molecule in the HgI 2 ·As 4 S 4 adduct.

We report here for the first time low-temperature (123-295 K) electron paramagnetic resonance (EPR) spectroscopic data on naturally occurring calumetite [Cu(OH,Cl 2 )∙2H 2 O] from the Khetri copper complex, Jhunjhunu district, Rajasthan, India. The sample was characterized by X-ray powder diffraction (XRD), differential thermal analysis (DTA), and thermogravimetric (TG) analysis techniques. DTA scans show two endothermic reactions at about 378 and 598 K, which are attributed to dehydration of the sample. TG data indicate weight loss of up to 24 wt% in the temperature region between 600 and 700 K. The room-temperature EPR spectrum exhibits parallel and perpendicular components centered at g ∥ = 2.26 and g ⊥ = 2.10, respectively. The evaluated spin-Hamiltonian parameters indicate that these resonance signals are characteristic of Cu 2+ ions in distorted octahedral symmetry. The population difference (N) between the Zeeman levels has been evaluated and is found to increase with decreasing temperature.

Chromatite (CaCrO 4 , tetragonal) is mainly known from Cr 6+ -contaminated soils associated with chromium ore processing residue. This extremely rare mineral was found at the Nabi Musa locality (Judean Desert, Israel), in a peculiar rock complex of the Mottled Zone. We have explored the possible mechanisms responsible for leaching Cr 6+ from natural rocks, by means of field observations, batch experiments, thermodynamic modeling, and mineralogical analyses of the Nabi Musa rocks (XRPD, EMPA, SEM, FTIR, and optical microscopy). A remarkable feature of the Mottled Zone rocks is a broad occurrence of high- and ultrahigh-temperature combustion metamorphic rocks, with Cr 3+ accumulated mainly in opaque minerals such as Fe-spinel, brownmillerite, and perovskite. Another feature of the Mottled Zone sequence is abundant Cr 3+ (bentorite and volkonskoite) and Cr 6+ mineralization (Cr 6+ -bearing ettringite and baryte-hashemite solid solution) in low-temperature hydrothermal veins. Field, mineralogical, and thermodynamic modeling data suggest that Cr was leached from Cr 3+ -bearing opaque minerals during late hydrothermal alteration of combustion metamorphic rocks by unusual hyperalkaline waters (pH up to 12). The Cr 3+ was then oxidized to Cr 6+ , and subsequently partially immobilized in Cr 6+ -bearing ettringite. As a consequence of the highway construction across Nabi Musa hill in 2006, the buried veins filled by Cr 6+ -substituted ettringite were exhumed and exposed to supergene alteration. The ensuing decomposition of Cr 6+ -bearing ettringite was followed by Cr 6+ release into pore waters in rainy seasons, and then by precipitation of chromatite on the evaporation barrier under the hard desert insolation in dry seasons. The chromatite formation has been due to both unique rock and water chemistry of the Mottled Zone sequence and to the arid climate of the Judean Desert.

A new biologically related, struvite-type phosphate mineral, hazenite, ideally KNaMg 2 (PO 4 ) 2 ⋅14H 2 O, has been found in and/or on completely dried-out or decomposed cyanobacteria on porous calciumcarbonate (mainly calcite and aragonite) substrates in Mono Lake, California. The mineral occurs as radiating clusters of prismatic crystals and is colorless, transparent with white streak and vitreous luster. It is brittle, with the Mohs hardness of 2-2.5; cleavage is good on {001} and no twinning was observed. The measured and calculated densities are 1.91(3) and 1.88(2) g/cm 3 , respectively. Hazenite is biaxial (+), with n α =1.494(1), n β = 1.498(1), n γ = 1.503(1), 2V meas = 41(2)°, 2V calc = 42°, X = b, Y = c, Z = a, and does not fluoresce under long- or short-wave ultraviolet rays. The dispersion is strong with r < v. It is soluble in water. The electron microprobe analysis yielded an empirical formula of K 0.97 (Na 0.96 Ca 0.02 )Mg 2.07 [(P 0.98 S 0.02 )O 4 ] 2 ⋅13.90H 2 O. Hazenite is orthorhombic with space group Pmnb and unit-cell parameters a = 6.9349(4) Å, b = 25.174(2) Å, c = 11.2195(8) Å, and V = 1958.7(3) Å 3 . There are many structural similarities between hazenite and struvite, as also revealed by their Raman spectra. The hazenite structure contains six symmetrically independent non-hydrogen cation sites, two for Mg 2+ (Mg1 and Mg2), two for P 5+ (P1 and P2), one for Na + , and one for K + . It can be viewed as three types of layers stacking along the b-axis, in a repeating sequence of ABCBABCB…, where layer A consists of Mg1(H 2 O) 6 octahedra and NaO 6 trigonal prisms, layer B of P1O 4 and P2O 4 tetrahedra, and layer C of Mg2(H 2 O) 6 octahedra and very irregular KO 6 polyhedra. These layers are linked together by hydrogen bonds, plus the K-O bonds between layers B and C (K-O5-P2). Interestingly, the combination of layers B and C in hazenite exhibits a configuration analogous to the struvite-(K) structure. Hazenite is believed to form in high pH environments through the involvement of cyanobacterial activities. To our knowledge, hazenite is the first struvite-type compound that contains two structurally distinct monovalent cations (K and Na), pointing to an exclusive role of biological activity in the mineralization process

The fractional latent heat of a crystallizing system is simply the latent heat of fusion divided by the total enthalpy. When plotted against temperature, this function displays robust pulses with the successive saturation of each incoming crystal phase. Each pulse endures for tens of degrees, corresponding to tens to hundreds of thousands of years in large magma bodies. From an original 1963 development by P.J. Wyllie using a synthetic system, I show that the liquidus slopes (degrees per gram of solid produced) decrease discontinuously at the arrival of a new phase, and their inverse, the crystal productivity, undergoes sharp upward pulses at the same time. The overall liquidus slope increases continuously, interrupted by small downward jumps, with the evolution of the multicomponent melt. The crystal productivity pulses feed the fractional latent heat pulses, which dominate the crystallization history of a melt. These elementary relationships govern the near-solidus growth of dihedral angles in cumulates as they relate to the liquidus events. The latent heat ordinarily approximates to about 80% of the total enthalpy budget, but jumps to 100% (by definition) when solidification occurs by isothermal adcumulus growth and approaches 100% when mafic phases such as augite and Fe-Ti oxides are over-produced (relative to their equilibrium saturation) in layered intrusions. The feedback of latent heat to a self-regulating cooling history of large magma bodies is deduced here in principle. The overall results help to clarify the crystallization history of mafic magmas. In particular, they support the solidification of floor cumulates by interchange with parent magma and without the help of compaction.