Showing papers in "American Mineralogist in 2002"

Infrared and Raman study of interlayer anions CO32–, NO3–, SO42– and ClO4– in Mg/Al-hydrotalcite

Abstract: The difference in the local environment of CO32−, NO3−, SO42−, and ClO4− in Mg/Al-hydrotalcite compared to the free anions was studied by infrared and Raman spectroscopy. In comparison to free CO32− a shift toward lower wavenumbers was observed. A band around 3000–3200 cm−1 has been attributed to the bridging mode H2O-CO32−. The IR spectrum of CO3− hydrotalcite clearly shows the split ν3 band around 1365 and 1400 cm−1 together with weak ν2 and ν4 modes around 870 and 667 cm−1. The ν1 mode is activated and observed as a weak band around 1012 cm−1. The Raman spectrum shows a strong ν1 band at 1053 cm−1 plus weak ν3 and ν4 modes around 1403 and 695 cm−1. The symmetry of the carbonate anions is lowered from D 3 h to C 2 s resulting in activation of the IR inactive ν1 mode around 1050–1060 cm−1. In addition, the ν3 shows a splitting of 30–60 cm−1. Although NO3-hydrotalcite has incorporated some CO32− the IR shows a strong ν3 mode at 1360 cm−1 with a weak band at 827 cm−1, and the ν4 band is observed at 667 cm−1, although it is largely obscured by the hydrotalcite lattice modes. The Raman spectrum shows a strong ν1 mode at 1044 cm−1 with a weaker ν4 band at 712 cm−1. The ν3 mode at 1355 cm−1 is obscured by a broad band due to the presence of CO32−. The symmetry of NO3− did not change when incorporated in hydrotalcite. The IR spectrum of SO4-hydrotalcite shows a strong ν3 at 1126, ν4 at 614 and a weak ν1 mode at 981 cm−1. The Raman spectrum is characterized by a strong ν1 mode at 982 cm−1 plus medium ν2 and ν4 bands at 453 and 611 cm−1; ν3 cannot be identified as a separate band, although a broad band can be seen around 1134 cm−1. The site symmetry of SO42− is lowered from T d to C 2 v . The distortion of ClO4− in the interlayer of hydrotalcite is reflected in the IR spectrum with both ν3 and ν4 bands split around 1096 and 1145 cm−1 and 626 and 635 cm−1, respectively. A weak ν1 band is observed at 935 cm−1. The Raman spectrum shows a strong ν1 mode at 936 cm−1 plus ν2 and ν4 bands at 461 and 626 cm−1, respectively. A ν3 mode cannot be clearly recognized, but a broad band is visible around 1110 cm−1. These data indicative a lowering of symmetry from T d to C s .

Fluid-induced nucleation of (Y + REE)-phosphate minerals within apatite: Nature and experiment. Part I. Chlorapatite

Abstract: This study investigates chlorapatite [ideally Ca10(PO4)6Cl2)] from two spatially separated locations in the Odegardens Verk apatite mine, Bamble Sector, SE Norway, which was partially metasomatized under amphibolite-facies conditions. Relative to the original chlorapatite, metasomatized areas in sample DL136 are enriched in OH and F, depleted in Na, Si, and (Y + REE) and contain numerous inclusions (1–15 μm) of monazite and xenotime. In contrast, metasomatized areas in sample TN174 are enriched in F and Si, depleted in Na, show (Y + REE) abundances similar to the original chlorapatite, and host only a few monazite and xenotime grains. In order to define the role of fluids in the formation of (Y + REE)-phosphate minerals associated with apatite, a series of experiments covering wide ranges of temperature (900–300 °C) and pressure (1000–500 MPa) for a series of fluid compositions (pure H2O, a 50/50 molar mix of H2O and CaF2, and CO2 with 1–2 wt% H2O) were conducted on unaltered chlorapatite from the Odegardens Verk. In the H2O experiments, features due to metasomatic alteration [depletion in Na, Si, and (Y + REE) as well as numerous monazite and xenotime inclusions], as observed in the natural sample DL136, were fully reproduced. Monazite and xenotime grains were also observed growing on the surface of the apatite in dissolution embayments in metasomatized areas. In the 50/50 CaF2-H2O experiments, metasomatized regions are strongly enriched in F and Si, depleted in Na, show unchanged (Y + REE) abundances, and contain only a very few, small monazite and xenotime grains similar to what was observed in sample TN174. Natural and experimental observation imply the presence of two coupled substitutions: Na+ + (Y + REE)3+ = 2 Ca2+ and Si4+ + (Y + REE)3+ = P5+ + Ca2+. In the case of DL136 and H2O–fluid experiments, Na+ and Si4+ went into solution, whereas the released (Y + REE) contributed to the growth of monazite and xenotime within the metasomatized apatite structure. For sample TN174 and the 50/50 CaF2 + H2O–fluid experiments, Na in apatite became depleted but Si was enriched thereby stabilizing the (Y + REE) in the now recrystallized fluor-hydroxylapatite structure, which retarded the growth of monazite or xenotime inclusions. The basic conclusion of this study is that nucleation of monazite and xenotime in the metasomatized chlorapatite is principally a function of the composition of the infiltrating fluid and, to a much lesser extent, temperature and pressure.

The nature of disorder in montmorillonite by simulation of X-ray powder patterns

Abstract: The planar disorder of Ca-montmorillonite (Fuller’s earth) has been investigated using structural simulations of X-ray powder patterns. A standard sample was fully characterized using chemical, microscopic, and diffraction methods. Earlier models of disorder taken from the literature and newly formulated combined models were used to generate simulated powder patterns to be compared with the experimental spectrum. A new model of disorder with random shifts of − a /3 and ± b /3, with a total density of defects of 75%, gives the best fit to the observed data. Thus, the sample cannot be classified as a turbostratic structure (fully disordered) and consequently turbostratic disorder does not invariably apply to all smectite samples. These findings open a debate on the nature and application of turbostratic disorder: is it possible for smectite samples to have intermediate degrees of disorder between a fully disordered stacking (turbostratic) and a highly faulted but well-defined stacking or is the result obtained for the Ca-montmorillonite just an exception? This model of disorder is useful for the quantitative phase analysis by X-ray powder diffraction based on the Rietveld method, which can now benefit from a more reliable initial structure model for Ca-montmorillonite and which will improve the accuracy of the weight-fraction estimates.

A combined rapid-quench and H2-membrane setup for internally heated pressure vessels: Description and application for water solubility in basaltic melts

Abstract: This study presents improvements of internally heated pressure vessels to realize high-pressure experiments at controlled f O 2 in low-viscosity systems such as basaltic ones. The new design is a combination of two experimental techniques: a hydrogen sensor membrane made of platinum to measure f H 2 , and therefore f O 2 , and a rapid-quench system to avoid crystallization of low-viscosity melts during quench. The experimental setup has been tested successfully at temperatures up to 1250 °C and pressures up to 500 MPa. Basaltic melts containing up to 9.38 wt% water can be quenched as bubble-free and crystal-free glasses. The improvements allow synthesis of hydrated glass or partly crystallized samples with a large volume (for further studies) and to perform routine phase-equilibrium studies in basaltic systems at geologically relevant conditions. We used the new technique to determine the effect of f O 2 on water solubility in a melt with MORB composition. The results show that there is a small but significant decrease of water solubility with decreasing f O 2 from MnO-Mn 3 O 4 to QFM buffer conditions in the pressure range 50–200 MPa. Kinetic problems in crystallization experiments in basaltic systems and the duration necessary to attain equilibrium Fe 2+ /Fe 3+ ratio in the charge are discussed.

Raman spectroscopy of Fe2O3 to 62 GPa

Abstract: Raman spectra of Fe2O3 were measured to 62 GPa in a diamond anvil cell. All group theoretically predicted Raman-active phonon modes were detected to 54 GPa. In addition, some high-pressure spectra show an IR-active Eu mode (~660 cm ‐1 ), possibly induced by surface defects or stress. This mode is related by a factor of two to a mode at 1320 cm ‐1 . The assignment of the 1320 cm ‐1 mode has been controversial (two-magnon scattering or two-phonon scattering), and our observation supports the phonon assignment. All Raman-active phonons show nonlinear pressure-induced shifts. The mode Gruneisen parameters and their logarithmic volume dependences for two low-frequency phonons, A1g and Eg, become negative and infinite, respectively, near 50 GPa as a result of the instability of the corundum structure at this pressure. Using the observed Raman-active phonons together with acoustic phonons previously measured by ultrasonics, and Kieffer’s model, we calculate the phonon contribution to the thermodynamic parameters of hematite. Comparison with experimentally measured values allows an estimation of an upper bound to the magnon contribution to the heat capacity at ambient pressure. This increases continuously above the Morin temperature and reaches a maximum at the Neel temperature (~37%). The Raman spectra change dramatically at pressures greater than 54 GPa as indicated by the appearance of new peaks and a significant increase in background. Although direct structural analysis is not possible due to the low signal-to-background ratio and the lack of polarization information, we were able to examine the consistency of our Raman observation with the corundum-to-perovskite phase transformation using the results for an analog system: MgSiO3 ilmenite (ordered corundum type) and perovskite. This analysis shows that observed new features in Fe2O3 Raman spectra may not be consistent with the GdFeO3 perovskite structure.

Scales of disequilibrium and rates of equilibration during metamorphism

Abstract: Mounting evidence suggests that partial disequilibrium—meaning disequilibrium for some elements, but not for others—may be a common but rarely detected phenomenon during metamorphic mineral growth, even in ordinary prograde reactions that progress to completion. Detailed examination of compositional variations in garnet crystals over a range of metamorphic grade suggests distinctly different scales of equilibration for common elements, with strong temperature dependence. Under lower greenschist-facies conditions, Fe and Mg may equilibrate at hand-sample scale, whereas Mn and Ca may not equilibrate even at millimeter-scale. Although Mn may achieve hand-sample-scale equilibration under upper greenschist-facies conditions, Ca and many trivalent cations (e.g., REEs) may not do so until temperatures exceed those of the middle amphibolite facies. Even in the lower granulite facies, some elements (e.g., Y, Yb) show indications of disequilibrium at sub-centimeter-scales during garnet growth. Analysis and numerical modeling of undisputed disequilibrium textures demonstrate that the most common impediment to equilibration during metamorphism is the sluggishness of intergranular diffusion, the same mechanism known to govern porphyroblast crystallization in many metamorphic environments. Despite this importance to petrology, few quantitative determinations exist of inter-granular diffusion rates under metamorphic conditions. Using numerical simulations of coupled intergranular and intracrystalline diffusion processes in coronal textures around partially resorbed garnet crystals from the Llano Uplift, U.S.A., a very precise and relatively accurate estimate is obtained for the rate of intergranular diffusion of Al in fluid-undersaturated systems. This result and an earlier estimate for fluid-saturated systems provide bracketing values for Al diffusivity during metamorphism.

Isotopic and elemental partitioning of boron between hydrous fluid and silicate melt

Abstract: The fractionation of B and its isotopes between aqueous fluid and silicate melt has been studied from 550 to 1100 °C and 100–500 MPa. Fluid-melt partition coefficients are 1 for rhyolite melt. This shows that B is not always strongly extracted from melts into hydrous fluids. Boron isotopic fractionation is large compared with the carbon and oxygen stable isotopic systems (especially at high T ) and is most simply explained by differences in coordination (trigonal vs. tetrahedral) among coexisting phases. Combined with earlier measurements on illite-water (300–350 °C), B isotopic fractionation defines a temperature-dependent trend from 300 to 1100 °C. Because of the large magnitude and apparent low sensitivity to bulk composition, B isotopic fractionation can be readily applied to studies of diagenesis, hydrothermal alteration of planetary bodies, subduction-zone processing and arc magma generation, and magma chamber evolution.

Determination of molar absorptivities for infrared absorption bands of H2O in andesitic glasses

Abstract: We have determined infrared molar absorptivities for water absorption bands in Fe-bearing and Fe-free andesitic glasses. Water dissolves in andesitic glasses as both hydroxyl groups and molecular water as observed in other silicate glasses. Concentrations of molecular water and hydroxyl species are a strong function of total water content. IR molar absorptivities for Fe-bearing andesite are e 3 5 7 0 = 62.32′0.42 L/mol.cm, e 4 5 0 0 = 0.79 ′ 0.07 L/mol.cm, e 5 2 0 0 = 1.07 ′ 0.07 L/mol.cm, and e 1 6 3 0 = 42.34 ′ 2.77 L/mol.cm. Molar absorptivities for Fe-free andesite are 69.21 ′ 0.52 L/mol.cm for e 3 5 7 0 , 0.89 ′ 0.07 L/mol.cm for e 4 5 0 0 , 1.46 ′ 0.07 L/mol.cm for e 5 2 0 0 , and 52.05 ′ 2.85 L/mol.cm for e 1 6 3 0 . Molar absorptivities show significant compositional dependencies that can be predicted based on tetrahedral cation (Si + 4 , Al + 3 )/total cation fraction.

Sulfur saturation limits in silicate melts and their implications for core formation scenarios for terrestrial planets

Abstract: This study explores the controls of temperature, pressure, and silicate melt composition on S solubility in silicate liquids. The solubility of S in FeO-containing silicate melts in equilibrium with metal sulfide increases significantly with increasing temperature but decreases with increasing pressure. The silicate melt structure also exercises a control on S solubility. Increasing the degree of polymerization of the silicate melt structure lowers the S solubility in the silicate liquid. The new set of experimental data is used to expand the model of Mavrogenes and O’Neill (1999) for S solubility in silicate liquids by incorporating the influence of the silicate melt structure. The expected S solubility in the ascending magma is calculated using the expanded model. Because the negative pressure dependence of S solubility is more influential than the positive temperature dependence, decompression and adiabatic ascent of a formerly S-saturated silicate magma will lead to S undersaturation. A primitive magma that is S-saturated in its source region will, therefore, become S-undersaturated as it ascends to shallower depth. In order to precipitate magmatic sulfides, the magma must first cool and undergo fractional crystallization to reach S saturation. The S content in a metallic liquid that is in equilibrium with a magma ocean that contains ~200 ppm S (i.e., Earth’s bulk mantle S content) ranges from 5.5 to 12 wt% S. This range of S values encompasses the amount of S (9 to 12 wt%) that would be present in the outer core if S is the light element. Thus, the Earth’s proto-mantle could be in equilibrium (in terms of the preserved S abundance) with a core-forming metallic phase.

Experimental and modeled solubilities of chlorine in aluminosilicate melts, consequences of magma evolution, and implications for exsolution of hydrous chloride melt at Mt. Somma-Vesuvius

Abstract: Solubility experiments were conducted with forty-one aluminosilicate rock compositions to determine how extensively Cl dissolves in hydrous chloride melt- ± vapor-saturated silicate melts containing low to moderate water contents at 2000 bars. Chlorine solubility in most silicate melts is dominated by the abundances of Mg ≈ Ca > Fe > Na > K > network-forming Al > Li ≈ Rb ≈ Cs, but Ti, F, and P also have strong influences. The relationship of composition to Cl solubility is more complex in peraluminous and peralkaline felsic melts, because network-modifying Al, Na, and K have a greater influence than their network-forming counterparts. Also, the effects of Ca, Mg, and Al in mafic melts characterized by high (Ca + Mg + Al)/(Na + K + Li) are much greater than their effects in silica-enriched melts. Association coefficients that express the influence of each ion on Cl solubility were determined, and the solubility data and coefficients were employed to develop a model that predicts Cl solubility at 2000 bars for water-undersaturated melts ranging from rhyolite to basalt. The coefficients were also used to investigate the predominant chloride complexes in melt, and the bulk of the solubility data are consistent with the interaction of Cl with alkaline-earth metals that provide charge balance for network-forming Al. The Cl solubility model is applied to Mt. Somma-Vesuvius magmas as they evolved from phonotephrite to phonolitic compositions, via fractional crystallization, to investigate the role of Cl in magmatic degassing. The results clearly demonstrate that Cl solubility was dramatically reduced by subtle changes in melt composition. Decreasing abundances of Ca, Mg, and Fe in the residual melt induced a dramatic reduction in Cl solubility that occurred simultaneously with gradual increases in the abundance of volatiles in melt due to crystallization of volatile-free minerals. The increasing abundance of volatiles and concurrent reduction in Cl solubility may have forced the exsolution of a hydrous chloride melt directly from the Cl-enriched mafic magmas. It is likely that the exsolution of hydrous chloride melt may occur in other Cl-enriched magmas, because Cl solubility depends so strongly on melt composition.

Closure in crystal size distributions (CSD), verification of CSD calculations, and the significance of CSD fans

Abstract: Crystal size distribution (CSD) measurements are susceptible to the closure problem, just like chemical compositions. In its simplest form this means that the total crystal content of a rock cannot exceed 100%. Where chemical or thermal effects limit the total quantity of a single phase, closure can occur at lower volumetric phase proportions. This means that parts of the CSD diagram (ln(population density) vs. size) are not accessible. If the volumetric phase proportion is constant, then straight CSDs will appear to rotate around a point at small sizes giving a fan of CSDs. These fans are significant and do show changes in crystal sizes that can be interpreted in terms of magmatic pro- cesses. However, the correlation between the slopes (or characteristic lengths) and intercepts of individual CSDs in a family is not significant, but just a consequence of the constant phase propor- tion effect. Many other graphs, such as characteristic length vs. volumetric phase proportion, can give more information on magmatic processes. LETTERS

Structure of synthetic Na-birnessite: Evidence for a triclinic one-layer unit cell

Abstract: The structure of a synthetic analogue of Na-birnessite (NaBi) was studied by powder X-ray diffraction (XRD). It is shown that NaBi has a one-layer triclinic structure with sub-cell parameters: a P = 2.9513(4) A, b P = 2.9547(4) A, c P = 7.334(1) A, αP = 78.72(2) A, βP = 101.79(1) A, γP = 122.33(1) A, and space group P1 . This sub-cell is equivalent to the base-centered sub-cell with parameters: a = 5.174 A, b = 2.848 A, c = 7.334 A, α = 90.53 A, β = 103.20 A, and γ = 90.07 A. A structure model has been refined using the Rietveld technique. NaBi consists of vacancy-free Mn-bearing octahedral layers whose negative charge arises mostly from the substitution of Mn3+ for Mn4+. The departure from the hexagonal symmetry of layers results from Jahn-Teller distortion of Mn3+ octahedra, which are elongated along the a axis, segregated in Mn3+-rich rows parallel to the b axis, and separated from each other along the a axis by two Mn4+-rows. Structural sites of interlayer Na cations and H2O have been determined as well as their occupancies. The sub-cells of the two NaBi modifications described by Drits et al. (1997) as types I and II likely contain four sites for interlayer species, two of which are occupied by Na and the other two by H2O molecules. In the two NaBi varieties, these pairs of sites are split along the c axis and related by a center of symmetry. This splitting is consistent with the modulated structure of both NaBi types, which arises from the periodic displacement of interlayer species along the b axis with a periodicity λ = 6 b (Drits et al. 1997).

Reply to comment on “Closure in crystal size distributions (CSD), verification of CSD calculations and the significance of CSD fans”

Abstract: Geochemists have long been aware of closure in chemical analysis (e.g., Chayes 1960). The purpose of my 2002 paper was to point out that closure also occurs in crystal-size distribution analysis (CSD; Higgins 2002). In both geochemistry and CSD analysis closure can lead to meaningless correlations, but carefully designed studies can easily avoid this situation and yield significant results. In my paper, I developed some very simple equations that are applicable to all CSDs, whatever their shape (Eqs. 1, 2, and 6). This is the context for the phrase “applies to all CSDs” that is quoted by Pan. I later developed equations that can only be applied to straight CSDs that extend to all crystal sizes, zero to infinity (Eqs. 3 and 4). These cannot be applied to all CSDs. The phrase “any process that changes the slope of the CSD must also change the intercept” quoted by …

Temperature-induced Al -zoning in hornblendes of the Fish Canyon magma, Colorado

Abstract: An extensive electron microprobe survey of amphibole compositions in the Fish Canyon magma (2146 analyses), more than 80% of which are from high-resolution (<10 μm steps) core-to-rim traverses across large euhedral phenocrysts, provides: (1) temporal constraints on the immediately pre-eruptive P - T - f H2O evolution of the magma, and (2) a means of evaluating recent calibrations of the Al-in-hornblende barometer (Anderson and Smith 1995; hereafter AS1995) and thermometers (Blundy and Holland 1990; thermometers A and B of Holland and Blundy 1994; hereafter BH1990, HB1994TA, and HB1994TB). Hornblende phenocrysts are variable for most major elements (e.g., 5–9 wt% Al2O3 and 44–50 wt% SiO2). This compositional range is controlled by two major temperature-sensitive coupled substitutions. Approximately 50% of the total Al variation (~0.8 atoms per formula unit = apfu) is due to the edenite exchange [TSi + A□ = TAl + A(Na + K)] and another 25–30% is the consequence of a Ti-Tschermak exchange (TSi + M1–M3 Mn = TAl + M1–M3Ti). In contrast, the pressure-sensitive Al-Tschermak substitution (TSi + M1–M3 Mg = TAl + M1–M3Al) did not play a significant role, as M1–M3Al does not correlate with TAl and is always <0.2 apfu. In order to constrain the ranges of absolute P and T over which these hornblendes crystallized and to assess the sensitivity of the recent thermo-barometric algorithms of BH1990, HB1994TA (requiring silica saturation), HB1994TB (not requiring silica saturation) and AS1995, we have calculated pressures and temperatures for two selected populations of analyses wherein Al2O3 contents are within analytical error (5.95 to 6.05 wt% Al2O3, N = 78 and 7.7 to 7.8 wt% Al2O3, N = 40). The barometric formulation of AS1995 gives a mean pressure of 2.24 ± 0.05 for the high-Al population at 760 °C, which is indistinguishable from the 2.4 ± 0.5 kbar estimate of Johnson and Rutherford (1989a). A high sensitivity to temperature at low P is suggested by the geologically implausibly shallow depths calculated for the low-Al population (<1 kbar at 760 °C). The three thermometric formulations give reasonable results between 706 and 814 °C, but the HB1994TA calibration gives a mean temperature higher by ~50 °C and is more sensitive to small analytical differences (~100 °C spread for each population). HB1994TB is considered the most reliable calibration of the Al-in-hornblende thermometer as it most precisely reproduces T estimates determined by independent methods. Nine out of 14 traverses across large phenocrysts from the Fish Canyon magma display rimward increases in TAl, A(Na + K), and M1–M3Ti, compensated by decreases in TSi, and M1–M3Mn. Using the HB1994TB algorithm, the low-Al population, typical of near-core compositions, gives a mean temperature of ~715 °C, which is ~35–45 °C above the water-saturated granite solidus at 2–2.5 kbar. The high-Al population, representing the average rim composition, gives a value around 760 °C, which is indistinguishable from independent T determinations using coexisting Fe-Ti oxides and Qtz-Mag oxygen isotope thermometry. These profiles suggest that Fish Canyon hornblendes crystallized during near-isobaric reheating over a temperature range of ~40 °C, which is consistent with our model of rejuvenation and remobilization of a pre-existing near-solidus crystal mush of batholithic dimensions via shallow intrusion of more mafic magma (Bachmann et al. 2002). Crystallization of hornblende from a high-SiO2, low-MgO melt during reheating requires an open system, in which both heat and mass, in particular volatiles, are transferred from the underlying mafic magma.

Ultrahigh-pressure metamorphism in western Tianshan, China: Part I. Evidence from inclusions of coesite pseudomorphs in garnet and from quartz exsolution lamellae in omphacite in eclogites

Abstract: Inclusions of coesite pseudomorphs in garnets have been found in type I eclogites interlayered with mafic blueschists and carbonate eclogites, and quartz exsolution lamellae in omphacites have been recognized in type II eclogites with pillow structure in western Tianshan, China. Based on mineralogy and petrology, the metamorphic evolution of western Tianshan eclogites can be divided into three stages: (1) pre-peak, ultrahigh-pressure (UHP), eclogite-facies stage (356–443 °C, 8–10 kb); (2) UHP eclogite-facies stage (496–598 °C, 25.7–26.7 ± 1 kbar); and (3) retrograde epidote blueschist-facies stage (500–530 °C, 10–12 kbar). Consequently, the eclogites from the western Tianshan region of China have undergone unambiguous UHP metamorphism.

Structure of heavy-metal sorbed birnessite: Part 1. Results from X-ray diffraction

Abstract: Selected-area electron diffraction (SAED) and energy dispersive analysis were used to study the structure of synthetic heavy-metal sorbed birnessites (MeBi). Samples were prepared by equilibrating a suspension of Na-rich buserite (NaBu) at pH4 in the presence of various heavy metal cations (Me), including Pb, Cd, Zn, and Cu. Five main types of SAED patterns were observed. Types I and II were observed only for ZnBi micro-crystals, and they both consist of two super-cell reflection networks related by a mirror plane parallel to the a *c* plane. In direct space, these twinned networks correspond to the hexagonal supercells with AH = BH = 7b/ 3, and AH = BH = 7b, for ZnBi type I and II, respectively. In the two varieties, the supercells result from an ordered distribution of vacant layer octahedra capped by interlayer Zn in ZnBi layers. This distribution is described by a hexagonal cell with AH = 7b. In ZnBi micro-crystals of type I, interstratified twinned right- and left-handed fragments are similar to chalcophanite (ZnMn3O7-3H2O - Wadsley 1955; Post and Appleman 1988), and distributions of vacant layer octahedra from adjacent layers are regularly shifted with respect to each other by 1/3 of the long diagonal of the hexagonal layer unit cell. In ZnBi micro-crystals of type II, distributions of vacant layer octahedra are not regularly shifted from one layer to the adjacent one. SAED patterns of types III and IV occur for PbBi, ZnBi, and CdBi micro-crystals and contain super-cell reflections distributed parallel to [100] * with a periodicity which is not commensurate with that of the MeBi sub-structure (a */2.15 and a*/5.25, respectively). The super-cell reflections result from the ordered distribution within MeBi layers of vacant layer sites capped by Me as pairs along the a axis. Within each pair, vacant sites are separated by 2a for type III, and by 5a for type IV. In one-layer monoclinic structures, the apparent incommensurability arises from the +a/3 shift between adjacent layers having a similar one-dimensional periodic distribution of interlayer Me located above and below vacant octahedra sharing three corners with Mnlayer octahedra (TC sites). Tetrahedral coordination of these Me cations in TC sites, as in ZnBi, leads to the formation of strong H-bonds between adjacent layers. A similar incommensurate effect occurs in one layer hexagonal MeBi if octahedrally coordinated Me cations periodically distributed along the a axis are located above and/or below empty tridentate cavities sharing three edges with Mnlayer octahedra ( VITE sites, PbBi). SAED patterns of type V contain only sub-cell reflections and were observed mostly for PbBi and CuBi micro-crystals. Three different conditions can lead to the absence of supercell reflections: (1) a low amount of sorbed Me (PbBi); (2) the presence of Me having a similar scattering power as that of Mn on a single side of vacant layer sites (CuBi); or (3) a random distribution of interlayer species.

Titanium in biotite from metapelitic rocks: Temperature effects, crystal-chemical controls, and petrologic applications

Abstract: An extensive natural biotite data set from western Maine constrains the temperature and crystal-chemical controls on the saturation Ti levels in biotites from metapelites. The geologically and petrologically well-characterized metamorphic terrain associated with the M3 metamorphism of the Acadian Orogeny of western Maine is ideal for this approach in that metamorphism occurred at roughly isobaric conditions of 3.3 kbar, and chemical equilibrium was closely approached. The data set from these metapelites exhibits systematic variations in Ti contents over a continuum of metamorphic grades (garnet through sillimanite-K-feldspar zones), mineral assemblages, and bulk compositional ranges. Samples were selected so that competing substitutions are restricted to those in metapelites with quartz, aluminous phases (chlorite, staurolite, or sillimanite), Ti phases (ilmenite or rutile), and graphite. Due to crystal-chemical factors, in any given metamorphic zone, an inverse linear relationship exists between Ti and Mg contents. Decreasing octahedral Ti and increasing tetrahedral Si in Mg-rich biotite helps alleviate size disparity between octahedral and tetrahedral sheets. For a biotite with a given Mg content, Ti most dramatically increases above staurolite zone conditions. Our constrained data set allows us to calculate a Ti saturation surface for natural biotite as a function of temperature and Mg content at 3.3 kbar. The Ti saturation surface can be used to establish several important metamorphic features in similar metamorphic settings. These include a general approach to equilibrium, local and/or subtle departures from equilibrium due to minor alteration to chlorite, and relative and absolute geothermometry based on Ti in biotite inclusions in refractory minerals and in matrix biotite.

Experimental determination of calcite solubility in H2O-NaCl solutions at deep crust/upper mantle pressures and temperatures: Implications for metasomatic processes in shear zones

Abstract: The solubility of calcite in NaCl-H 2 O solutions was measured at 600–900 °C, 10 kbar, at NaCl concentrations ranging from dilute to near halite saturation, and at 6–14 kbar, 700 °C, in 30 mol% NaCl solutions. Solubility was determined from the weight loss of cleavage rhombs of a pure natural calcite after experiments of 1/2 to 6 days in a piston-cylinder apparatus with NaCl-graphite furnaces. CaCO 3 molality ( m CaCO 3 ) increases greatly with NaCl mole fraction ( X NaCl): at 800 °C and 10 kbar, m CaCO 3 increases from ~0.1 in pure H 2 O to near 4.0 at halite saturation ( X NaCl ~ 0.6). There is also a large temperature effect at 10 kbar, with m CaCO 3 increasing from 0.25 at 600 °C to 3.0 at 900 °C at X NaCl = 0.3. There is only a 20% increase with increasing pressure between 6 and 14 kbar at 700 °C and X NaCl = 0.3. Melting to a carbonate-rich liquid was inferred at 900 °C, 10 kbar, from X NaCl of 0 to 0.2. The composition, temperature, and pressure dependence of m CaCO 3 are described by: \[\mathit{m}_{CaCO_{3}}\ =\ [{-}0.051\ +\ 1.65\ {\times}\ 10^{{-}4}\ \mathit{T}\ +\ \mathit{X}^{2}_{NaCl}exp({-}3.071\ +\ 4.749\ {\times}\ 10^{{-}6}\mathit{T}^{2})]\ (0.76\ +\ 0.024\mathit{P})\] with T in Kelvins and P in kbar. The predicted increase of calcite solubility with salinity and temperature is so great that critical mixing of NaCl-rich hydrous carbonate liquid and CaCO 3 -rich saline solution is probable at 10 kbar near 1000 °C and X NaCl ~ 0.4. The experimental results suggest a genetic mechanism for the enigmatic carbonated shear zones, such as the Attur Valley of southern India, where crustal rocks have been replaced by up to 20% by calcite and ankerite with mantle-like stable-isotope signatures. The high CaCO 3 carrying capacity of concentrated alkali-chloride solutions, together with the drastic decrease in solubility between 1000 and 700 °C, plausibly account for large-scale emplacement of mantle-derived carbonate from concentrated chloride-carbonate solutions (or hydrosaline magmas) formed as immiscible fluids in the late stages of alkalic magmatism. Such solutions may also mobilize sulfate and phosphate minerals, which would have important consequences for redistribution of incompatible and heat-producing elements in the crust.

Deciphering Ni sequestration in soil ferromanganese nodules by combining X-ray fluorescence, absorption, and diffraction at micrometer scales of resolution

Abstract: X-ray microprobes are among the most important new analytical techniques to emerge from third generation synchrotron facilities. Here we show how X-ray fluorescence, diffraction, and absorption can be used in parallel to determine the structural form of trace elements in heterogeneous matrices at the micrometer-scale of resolution. Scanning X-ray microfluorescence (microSXRF) and microdiffraction (microSXRD) first are used to identify the host solid phase by mapping the distributions of elements and solid species, respectively. Micro-extended X-ray absorption fine structure (microEXAFS) spectroscopy is then used to determine the mechanism of trace element binding by the host phase at the molecular scale. To illustrate the complementary application of these three techniques, we studied how nickel is sequestered in soil ferromanganese nodules, an overwhelmingly complex natural matrix consisting of submicrometer to nanometer sized particles with varying structures and chemical composition s. We show that nickel substitutes for Mn3+ in the manganese layer of the MnO2-Al(OH)3 mixed-layer oxide lithiophorite. The affinity of Ni for lithiophorite was characteristic of micromodules sampled from soils across the U.S.A. and Europe. Since many natural and synthetic materials are heterogeneous at nanometer to micrometer scales, the synergistic use of microSXRF, microSXRD and microEXAFS is expected to have broad applications tomore » earth and materials science.« less

Coesite exsolution from supersilicic titanite in UHP marble from the Kokchetav Massif, northern Kazakhstan

Abstract: Coesite exsolved from supersilicic titanite was discovered in an impure calcite marble at Kumdy-kol in the Kokchetav UHP (ultrahigh-pressure) metamorphic terrane, northern Kazakhstan. This impure marble consists mainly of calcite, K-feldspar, diopside, and symplectites of diopside + zoisite, with minor amount of titanite, phengite, and garnet. No diamond was found in the marble. Coesite and quartz, which have needle or platy shapes measuring about 20–60 μm in length, occur as major exsolved phases in the cores and mantles of titanite crystals with minor calcite and apatite. The strongest Raman band for the coesite needles and plates was confirmed at about 524 cm−1 with a weak band at about 271 cm−1. To estimate the initial composition of the titanite before coesite exsolution, exsolved phases were reintegrated by measuring their area fractions on digital images. The highest excess Si in titanite was thus determined to be 0.145 atoms per formula unit (apfu). This composition requires a pressure higher than 6 GPa on the basis of phase relations in the system CaTiSiO5–CaSi2O5. This pressure is consistent with other evidence of high pressure in the same marble, such as 1.4–1.8 wt% K2O and over 1000 ppm H2O in diopside. Supersilicic titanite and coesite exsolution also indicate that SiO2 exsolution occurred in the coesite stability field during exhumation of the UHP metamorphic unit.

Diamond formation through carbonate-silicate interaction

Abstract: Crystallization of diamond and graphite from the carbon component of magnesite, upon its decarbonation in reactions with coesite and enstatite at pressures of 6-7 GPa and temperatures of 1350-1800 °C has been accomplishedexperimentally. In a series of experiments, diamond was obtained in association with enstatite, coesite, and magnesite, as well as with forsterite, enstatite, and magnesite. Octahedral diamond crystals with sizes up to 450 pm were studied by FTIR spectroscopy and were found to contain nitrogen and hydrogen, which are known as the most abundant impurities in natural type Ia diamonds. We found that growth of diamond on the cubic faces of seed crystals proceeds with formation of a cellular surface structure, which is similar to natural fibrous diamonds. The isotopic composition of synthesized diamonds (δ 1 3 C = -1.27‰) was determined to be close to that of the initial magnesite (δ 1 3 C = -0.2‰).

Structure change of Ca1−xSrxTiO3 perovskite with composition and pressure

Abstract: Structure refinements of solid solutions of (Ca 1 - x Sr x )TiO 3 (x = 0.0, 0.25, 0.5, 0.6, 0.65, and 1.0) were undertaken using single crystals at ambient conditions. Their lattice constants, c/a axial ratios, and cell volumes indicate continuous changes from orthorhombic to cubic through a tetragonal phase. The orthorhombic structure is continuous between x = 0.0 and x = 0.6, and a phase at x = 0.65 shows a tetragonal structure with space group I4/mcm. With increasing Sr substitution, the symmetry changes to cubic with Pm3m space group. A-O and B-O distances in ABO 3 perovskite were determined as a function of the composition of the A cation (Ca and Sr). Tilting and rotation angles of the TiO 6 octahedral linkage with x of (Ca 1 - x Sr x )TiO 3 were also evaluated. Single-crystal structure refinements of Ca 0 . 3 5 Sr 0 . 6 5 TiO 3 perovskite at 3.5, 4.1, and 7.0 GPa at 300 K were carried out using a diamond anvil cell. The tetragonal phase transforms to an orthorhombic structure with space group Pbnm at 3.5 GPa. The polymorphic transition of V I I I A 2 + V I B 4 + O 3 perovskites under compression is discussed.

Ultra-high pressure metamorphism in western Tianshan, China : Part II. Evidence from magnesite in eclogite

Abstract: Carbonates of ultrahigh-pressure metamorphic (UHPM) origin were discovered in eclogites from western Tianshan, China. In the eclogites, relict magnesite ( X Mg = 0.79) occurs as rounded to subidiomorphic inclusions (0.01–0.1 mm) within matrix dolomite, and also as rounded inclusions with thin reaction rims of dolomite in glaucophane. Based on the textural evidence and calculated phase relationship, these eclogites record an early UHPM assemblage overprinted by a blueschist assemblage, which can be explained by the reaction: \begin{eqnarray*}&&{30MgCO_{3}_{mag}^{}}\ +\ {10NaAlSi_{2}O_{6}_{jd}^{}}\ +\ {9Ca_{3}Al_{3}Si_{3}O_{12}_{gr}^{}}\ +\ {11SiO_{2}_{coe}^{}}\ +\ {8H_{2}O_{w}^{}}\ =\\&&{15CaMg(CO_{3})_{2}_{dol}^{}}\ +\ {5\ Na_{2}Al_{2}Mg_{3}Si_{8}O_{22}(OH)_{2}_{gl}^{}}\ +\ {6Ca_{2}Al_{3}Si_{3}O_{12}(OH)._{cz}^{}}\end{eqnarray*} Using THERMOCALC (v2.75) and the updated internally consistent mineral thermodynamic database of Holland and Powell (1998), we have calculated the P - T grid involving glaucophane (gl), grossular (gr), jadeite (jd), magnesite (mag), dolomite (dol), clinozoisite (cz), coesite (coe), CO 2 , and H 2 O in a 7-component system of NCMASCH (Na 2 O-CaO-MgO-Al 2 O 3 -SiO 2 -CO 2 -H 2 O). The calculated P - T slope of the above reaction is nearly parallel to the temperature axis and suggests that it can be used as an excellent geobarometer. Using the analyzed compositions and mineral-thermodynamic database of Holland and Powell (1998), the above retrograde metamorphic reaction in Tianshan eclogites occurred at 27–28 kbar and 525–607 °C calculated from Grt-Cpx geothermometers.

Structural refinements of magnesite at very high pressure

Abstract: Unit-cell parameters of magnesite were measured between ambient pressure and 80 GPa using angle dispersive powder X-ray diffraction. The isothermal bulk modulus determined from a third order Birch Murnaghan equation of state is K T = 108(3) GPa with K T ′ = 5.0(2), and V 0 = 279.2(2) A 3 , in agreement with previously reported values. Combining this result with previous measurements, we show that magnesite with R 3 c structure is stable compared to the assemblage periclase + carbon dioxide at pressures and temperatures corresponding to the coremantle boundary. Crystal structure refinements have also been carried out up to 80 GPa. The main structural change is a strong compression of the MgO 6 octahedra with increasing pressure, largely reflected in the anisotropic compression of the c axis. This compression, however, tends to level off at around 50–60 GPa. On the other hand, the CO 3 groups do not remain invariant since they undergo first a slight expansion and then a compression above the same threshold pressure of 60 GPa above which Mg-O bonds cannot compress further. Thus, in this structure-type, the energy gain due to a drastic volume reduction of the MgO 6 octahedron compensates in a given pressure range for the energy cost of the small expansion of the CO 3 carbonate unit.

On geological interpretations of crystal size distributions: Constant vs. proportionate growth

Abstract: Geological interpretations of crystal size distributions (CSDs) depend on understanding the crystal growth laws that generated the distributions. Most descriptions of crystal growth, including a population-balance modeling equation that is widely used in petrology, assume that crystal growth rates at any particular time are identical for all crystals, and, therefore, independent of crystal size. This type of growth under constant conditions can be modeled by adding a constant length to the diameter of each crystal for each time step. This growth equation is unlikely to be correct for most mineral systems because it neither generates nor maintains the shapes of lognormal CSDs, which are among the most common types of CSDs observed in rocks. In an alternative approach, size-dependent (proportionate) growth is modeled approximately by multiplying the size of each crystal by a factor, an operation that maintains CSD shape and variance, and which is in accord with calcite growth experiments. The latter growth law can be obtained during supply controlled growth using a modified version of the Law of Proportionate Effect (LPE), an equation that simulates the reaction path followed by a CSD shape as mean size increases.

Experimental evidence of three coexisting immiscible fluids in synthetic granitic pegmatite

Abstract: We present an experimental study of synthetic peraluminous granite doped with H2O, B, P, and F, which confirms that aluminosilicate melt, hydrous fluid, and hydrosaline melt (high-temperature brine) can stably coexist at 450–900 °C and 0.1–0.2 GPa in bulk compositions similar to those of natural granitic pegmatites. Hydrosaline melt is not quenchable, unstable at room conditions, and requires special techniques for synthesis and preservation. Raman spectroscopy and electron microprobe analyses of hydrosaline melt synthesized in our experiments show that it is composed of H3BO3, Na3AlF6, AlPO4, H2O, and aluminosilicate components. Aluminosilicate melt saturated in both hydrosaline liquid and hydrous fluid at 850 °C and 0.2 GPa contains 3.6 wt% F, 4.2 wt% P2O5, and 4 wt% B2O3. Natural hydrosaline melts have previously been found as inclusions trapped in rock-forming minerals. They are not restricted to granites and can be effective agents for enhanced crystal growth, metasomatism, and ore formation. In addition, hydrosaline melts may account for many characteristic features of rare-element and miarolitic pegmatites, such as giant size and perfect shapes of crystals in pegmatite cores, diverse mineralogy, and strong enrichment in rare elements.

Hydroxyl-rich topaz in high-pressure and ultrahigh-pressure kyanite quartzites, with retrograde woodhouseite, from the Sulu terrane, eastern China

Abstract: Hydroxyl-rich topaz was recognized in thick kyanite quartzites from both high-pressure (HP) and ultrahigh-pressure (UHP) belts of the Sulu terrane, China. These quartzites contain variable proportions of quartz, kyanite, and topaz, with minor phengite, pyrite, and rutile. Some topaz grains from the UHP belt contain abundant inclusions of oriented kyanite, whereas those from the HP belt are partially replaced by woodhouseite [CaAl 3 (PO 4 )(SO 4 )(OH) 6 ]. Most topaz crystals contain 9.5 to 13.5 wt% F [0.92–1.30 atoms per formula unit (apfu)], indicating 35–55% substitution of F by OH. Such naturally occurring, hydroxyl-rich topaz has not been previously reported. Some topaz grains from the HP belt show distinct zoning: (1) decreasing F content from narrow cores (13.3–16.5 wt%) to thick rims (9.5 wt%) or (2) oscillatory zoning (9.44–12.77 wt%). Unit-cell parameters of topaz show a positive linear correlation between the OH content and a and b as well as volume. Based on our petrologic data, the experimentally determined curve of Ky + H 2 O = topaz-OH at very high pressures, and calculated OH/(OH + F) isopleths of topaz at low pressures, the topaz with X OH ~ 0.35 from the UHP belt may have formed at P-T conditions within the coesite stability field. The P-T condition of HP topaz is less well constrained; its high X OH (0.40–0.55) may have been caused by lower metamorphic temperatures and higher initial X OH in comparison with UHP topaz. Hydroxyl-rich topaz together with other hydrous minerals in UHP rocks may be important carriers of H 2 O to mantle depths of 100–200 km during continental subduction.

First-principles calculation of the infrared spectrum of hematite

Abstract: The theoretical infrared spectrum of lizardite [Mg3Si2O5(OH)4] was computed using first-principles quantum mechanical calculations. Density functional perturbation theory allowed us to derive the low-frequency dielectric tensor of lizardite as a function of the light frequency. The infrared spectrum was then calculated using a model that takes into account the platy shape of particles. A very good agreement was obtained between theory and experiment. This agreement allows us to make an unambiguous assignment of the major absorption bands observed in the IR spectrum of lizardite, including the stretching bands of OH groups.

Analytical techniques for volatiles: A case study using intermediate (andesitic) glasses

Abstract: Small-scale analyses of volatiles in minerals and glasses provide information on how volatiles influence high-temperature geologic processes and low-temperature alteration processes. Four techniques for determining the C-O-H volatile contents of andesitic glasses are compared: manometry, secondary ion mass spectrometry, micro-Fourier transform infrared spectroscopy, and a technique where the H2O content is calculated using the difference between electron microprobe analysis totals and 100% sum. We present a method to determine the H content of a wide range of glass and mineral compositions using secondary ion mass spectrometry and a model for calibration factors. The extinction coefficients for H-O volatile contents in intermediate composition synthetic glasses are determined, and it is demonstrated that C-O speciation changes as total H2O content increases, with molecular CO2 decreasing, CO32− increasing, and carbonate peak splitting increasing. For glasses with low H2O content and oxy-substituted minerals, the methods of choice for volatile analysis are secondary ion mass spectrometry or micro-Fourier transform infrared spectroscopy.

Trace-element partitioning between vacancy-rich eclogitic clinopyroxene and silicate melt

Abstract: We investigated experimentally trace-element partitioning between clinopyroxene and anhydrous silicate melt at conditions relevant to near-solidus melting of quartz eclogite in the upper mantle. Partitioning experiments were carried out at 3.0 GPa and 1335–1365 °C on a synthetic mixture similar to a low-degree melt of a quartz eclogite. Clinopyroxene is Al2O3-rich (~17 wt%), cation deficient (~3.92 cations per six O atoms), and coexists with andesitic melt (~57 wt% SiO2), quartz, and traces of rutile. The clinopyroxene cation deficiency indicates a large proportion of vacant M2 sites in form of the Ca-Eskola (CaEs) component (Ca0.5□0.5AlSi2O6). Clinopyroxene-melt partition coefficients ( D -values) were determined for Sr, Y, Zr, Nb, REE, Th, and U by SIMS, and for Sc, V, Cr, Mn, Co, Ni, K, and major elements by electron microprobe analysis. Compared to previous studies of clinopyroxene with >10 wt% Al2O3, partition coefficients in vacancy-rich aluminous clinopyroxene for elements substituting into the M2 site are elevated for monovalent cations ( D Na ~ 1.0, D K = 0.027), and reduced for trivalent cations, particularly for LREE ( D La = 0.029). D Th and D U range from 0.0017 to 0.0037, much lower than previously assumed for clinopyroxene pertaining to partial melting of eclogite in the upper mantle, and D U/ D Th is apparently >1. Observed partitioning behavior of elements substituting into M2 is attributable to the low average charge (1.53–1.57) of the site. This low average charge increases the number of configurations available to charge balance 1+ cations and vacancies, and decreases the number of configurations available to charge balance cations with high valences. Stablization of low-valence species suggests that these vacancy-rich clinopyroxenes may be substantial hosts of noble gases in the mantle. Partition coefficients for cations substituting into the M1 site show relatively little effect from the high vacancy concentrations of M2, but partitioning systematics may be obscured by strong crystal field stabilization of some cations in the M1 site, notably Ni2+ and Cr3+. Application of the measured D -values of this study to partial-melting calculations results in small increases in the Sm/Yb of liquids produced by modest extents of melting of quartz eclogite. Significant enrichments in (230Th)/(238U) activity ratios (>1.21) are expected for decompression melting of quartz eclogite at upwelling rates below 5 cm/year, but enrichments would be small (<1.11) above 10 cm/year.