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Interfacial tension between immiscible liquids is an important thermodynamic parameter of silicate melt unmixing and a property that determines the kinetics of phase separation. In this study, we present experimental measurements of interfacial tension between immiscible Fe-rich and silica-rich melts in the system [K.sub.2]O-FeO-[Fe.sub.2][O.sub.3]-[A1.sub.2][O.sub.3]-Si[O.sub.2]. We have also measured densities and surface tensions of the individual immiscible liquid phases. The measurements were carried out in air at 1500-1550[degrees]C by the maximum detachment force method employing vertical cylinder geometry and using a gravimetric balance system. We have chosen the most oxidized and contrasting liquid compositions containing 73 and 17 wt% Si[O.sub.2] and 14 and 80 wt% FeOt, respectively, that have been shown to coexist in air at and above 1465[degrees]C. Interracial tension between the synthetic immiscible liquids decreases with increasing temperature from 16.4 [+ or -] 3.1 mN/m at 1500[degrees]C to 7.8 [+ or -] 1.1 mN/m at 1550[degrees]C. Interfacial tension between natural, less compositionally contrasting ferrobasaltic and rhyolitic melts should be even lower by a factor of 2 or 3. Very low interracial tension implies easy nucleation of immiscible liquid droplets and very slow coarsening of resulting silicate emulsions. Keywords: Immiscibility, interface, tensiometry, nucleation DOI: 10.2138/am.2010.3456

Chemical zoning according to the substitution Al + (OH,F) = Ti + O is a common phenomenon in natural titanite and can be used to reconstruct its growth conditions. We synthesized titanite, which has a zoning pattern similar to that found in natural titanite that grew at the expense of rutile, visible in element distribution maps as a patchy irregular pattern. Run conditions in the system CaO-[Al.sub.2][O.sub.3]- Ti[O.sub.2]-Si[O.sub.2]-NaCl-[H.sub.2]O(-Ca[F.sub.2]) were 600[degrees]C, 0.4 GPa, 1 to 107 days duration. Natural rutile crystals were placed in a perforated Pt-tube within an outer Au-capsule containing wollastonite, [gamma]-[Al.sub.2][O.sub.3], optionally powdered Ca[F.sub.2], and a NaCl brine, to simulate conditions, where ruffle is transformed into titanite by a hydrous Ca-Si-Al(-F) fluid, driven by a chemical potential gradient between inner and outer capsule. Spontaneous nucleation of titanite is restricted to a small number of crystals on the rutile surface in the inner capsule. Growth proceeds from sparse isolated titanite crystals after 1 day to assemblages of several 100 [micro]m large crystals in a reaction rim in the long-time runs. Titanite is strongly zoned in Al-Ti and shows up to ~0.5 Al per formula unit (pfu) in experiments containing F; without F, substitution is limited to ~0.25 Al pfu. The range of the Ti-Al exchange is already large in short run times and the same heterogeneity is observed in long-time runs. The Al-Ti distribution (with and without F) in compositional domains (up to several tens of micrometers) is patchy and irregular. No relation of the zoning with time (core-rim) or preferred growth directions was observed. In the case of fluid-mediated growth, mineral growth zoning represents a disequilibrium at supersaturated conditions. The concentration of Ti in the fluid is likely the limiting factor for titanite growth considering the low solubility compared to that of Al, Ca, and Si. Titanium is not transported in significant amounts into the outer capsule, indicating low mobility, in contrast to Al, Ca, and Si, which are transported into the inner capsule. Assuming buffered Al concentration from dissolution of the Al-source, we speculate that the Ti/Al of the fluid at the precipitation site was controlled by the local Ti concentration, which varies over the distance from the rutile surface to the growing titanite crystal and produces the irregular patchy zoning. Keywords: Dissolution-precipitation reaction, Al-Ti substitution, mineral zoning, experiments DOI: 10.2138/am.2010.3518

The system [Al.sub.2][O.sub.3]-[P.sub.2][O.sub.5]-[H.sub.2]O contains many phosphate minerals that occur in various geologic environments. The natural occurrence of variscite (AlP[O.sub.4]*2[H.sub.2]O, orthorhombic), including its monoclinic polymorph metavariscite, is largely restricted to soils and aluminous rocks like Al-rich igneous rocks and shales, which interacted with P-rich hydrothermal solutions or groundwater. Variscite dehydrates to berlinite (AlP[O.sub.4]) and a hydrous P-Al-rich fluid. This dehydration reaction AlP[O.sub.4]*2[H.sub.2]O = AlP[O.sub.4] + 2[H.sub.2]O is, however, metastable, at low concentrations of P in the fluid, because berlinite breaks down at lower temperature to augelite [[Al.sub.2]P[O.sub.4][(OH).sub.3]] + [H.sub.3]P[O.sub.4] + [H.sub.2]O and trolleite [[Al.sub.4][(P[O.sub.4]).sub.3][(OH).sub.3]] + [H.sub.3]P[O.sub.4] + [H.sub.2]O. The variscite/metavariscite = berlinite equilibrium has been investigated by synthesis experiments from mixtures of [gamma]-[Al.sub.2][O.sub.3] and excess phosphoric acid at pressures between 1 and 5 kbar and temperatures of 100-200 [degrees]C using standard cold-seal vessels. The hydration-dehydration equilibrium is mainly controlled by temperature and only weakly by pressure. At 4 and 5 kbar, variscite/metavariscite were found at temperatures of ~150 [degrees]C, at lower pressure at 115-125 [degrees]C in accordance with their natural mode of occurrence. In this T range, however, variscite/metavariscite is not the sole phase but is always accompanied by variable but generally minor amounts of wavellite and trolleite [Al.sub.4][(P[O.sub.4]).sub.3][(OH).sub.3]. Secondary wavellite [Al.sub.3][(P[O.sub.4]).sub.2][(OH).sub.3]*5[H.sub.2]O, hydrated Al-phosphate AlP[O.sub.4]*x[H.sub.2]O (1.1 [less than or equal to] x [less than or equal to] 1.3) and Al-metaphosphate hexahydrate Al[([H.sub.2]P[O.sub.4]).sub.3] formed during quenching and/or drying of the runs. Berlinite is the reaction product at temperatures 200 [degrees]C/4-5 kbar and 150 [degrees]C/1-3 kbar and may be associated with augelite and trolleite. Keywords: Al-orthophosphates, berlinite, hydrothermal experiments, metavariscite, stability, variscite

A new type of sector zoning, with an hourglass shape, has been identified in metamorphic tourmalines that formed under a wide variety of physical and chemical conditions. The two sectors in the c-direction are not equivalent due to asymmetry in the crystal structure of tourmaline along the c-axis. The [c.sup.+] sector is characterized by low concentrations of Ti, Ca, Mg, and Na, although A1 is high, and has a pale (commonly blue or pale-green) color. Conversely, the [c.sup.-] sector is low in Mg and A1, and high in Ca, Fe, and Ti (the latter two causing the dark-brown color of this sector). The a-sector has intermediate characteristics and probably approximates a sector-free tourmaline. Thin sectioning of these sector-zoned tourmalines perpendicular to the c-axis can produce three types of apparent radial zoning patterns: blue-green cores, dark-brown cores, or no distinct cores. These apparent cores will further vary in relative diameter depending on the sectioning level. Furthermore, 'core' boundaries can be straight or ragged depending on whether the relative growth speeds for the different faces was constant or variable. These textures have been used to argue for a prograde or detrital origin of tourmaline cores. However, sector zoning is a more appealing explanation for most of these textures, and can further explain the textural resemblance among metamorphic tourmalines from highly variable bulk-rock composition, metamorphic history, and mineral paragenesis. The sector zoning that is described here develops by preferential uptake of elements on the r growth plane, resulting from a combined effect of differences in surface charge and morphology of this plane in the [c.sup.+] and [c.sup.-] directions. This leads to the preferential incorporation of more positively charged elements in the [c.sup.-] direction, and a preference for a vacant X-site in the [c.sup.+] direction. Because the compositional differences among the sectors are pronounced in both major and trace elements and in the same order of magnitude as growth zoning variability, the presence of sector zoning must be established and taken into account when making inferences from tourmaline chemistry. Keywords: Hourglass, sector zoning, tourmaline, metamorphic rocks, element fractionation

The [Fe.sup.3+]-Al substitution in synthetic zoisite was studied in the system CFASH at 2.0 GPa and 750[degrees]C (compositional range: 0.0-0.14 [X.sub.ps]). The samples were characterized by powder X-ray diffraction, FTIR, and electron microprobe. Discontinuities in refined lattice parameters at N0.05 [X.sub.ps] are attributed to two distinct and hitherto unknown modifications, zoisite I (0.05 [X.sub.ps]. The following lattice parameters were derived: (zoisite I) a ([Angstrom]) = -3.72 x [10.sup.-2] x [X.sub.ps] + 16.1913 b ([Angstrom]) = 6.43 x [10.sup.-2] x [X.sub.ps] + 5.5488 c ([Angstrom]) = 3.43 x [10.sup.-2] x [X.sub.ps] + 10.0320 V ([[Angstrom].sup.3]) = 11.4 x [X.sub.ps] + 901.3 (zoisite II) a ([Angstrom]) = -8.26 x [10.sup.-2] x [X.sub.ps] + 16.2061 b ([Angstrom]) = 8.14 x [10.sup.-2] x [X.sub.ps] + 5.5486 c ([Angstrom]) = 1.18 x [10.sup.-1] x [X.sub.ps] + 10.0263 V ([[Angstrom].sup.3]) = 19.3 x [X.sub.ps] + 901.6 In both modifications, substitution of [Fe.sup.3+] expands the M3 octahedron, resulting in opposed rotations of the corner-linked T1 and T2 tetrahedra of the [Si.sub.2][O.sub.7] group. The extent of rotation is limited and controls the maximum [Fe.sup.3+] content in zoisite I and II. With increasing [Fe.sup.3+] content, zoisite I transforms to zoisite II and zoisite II to clinozoisite. The transformation from zoisite I to II can be classified as a substitutionally induced isosymmetric displacive phase transition. Four significant IR bands were observed at ~3250, ~3195, ~3155, and ~2170 [cm.sup.-1]. The first three bands are attributed to the configurations [Al.sub.2][sup.M1,2]-O10 ... O2-[Al.sub.2][sup.M1.2]([Al,[Fe.sup.3+])[sup.M3], [Al.sub.2][sup.M1,2]-O10H ... O4-[Al.sub.2][sup.M1,2][Fe.sup.3+][sup.M3], and [Al.sub.2][sup.M1,2]-O10-H ... O4-[Al.sub.2][sup.M1,2]Al[sup.M3]. O10-H ... O2 is bifurcated between the two symmetrically arranged O2 and O2' atoms. The band at ~2170 [cm.sup.-1] is interpreted as the first overtone of the bending vibration of O10-H ... O2. In analogy with the results from powder XRD the IR bands show discontinuities at ~0.05 [X.sub.ps], confirming the two modifications of zoisite.

Carbonates are excellent carriers for divalent cations such as Ca, Mg, and Sr, and knowledge about their stability and solid solutions is important to understanding the global strontium cycle. To shed light on the topology of the two-phase field between calcite-type and aragonite-type (Ca,Sr)CO3 solid solutions as a function of temperature and pressure, and to learn more about the distribution of Sr and Ca between carbonates and fluid, we studied the system CaCO3-SrCO3-H2O at 600 °C in the pressure range 0.5-5 kbar. Conventional and rapid-quench hydrothermal syntheses were performed using a range of different starting materials. All bulk compositions were within the assumed/postulated two-phase field of calcite-type and aragonite-type (Ca,Sr)CO3 solid solutions. The run products were analyzed by scanning electron microscopy, electron microprobe analysis, and powder X-ray diffraction with Rietveld refinement. The results show that the heterotype solid solution is more extensive than previously assumed, with calcite incorporating up to 20 mol% SrCO3, which is twice as much as previously predicted. The compositional range of the aragonite-type solid solution was identical to that found in the literature. Using the data from this study, an updated version of the phase diagram P-X (Sr) at 600 °C for the CaCO3-SrCO3 system was calculated. The phase diagram does not support a phase transition within the trigonal (Ca,Sr)CO3 solid solution associated with rotational disorder of the CO3-groups. This order-disorder phase transition was previously postulated to explain some observed compositional trends in this system. Our new data are in line with other more recent studies. The distribution of Sr and Ca between the fluid and solid phases D = XsolidSr/XfluidSr is near to 1.0 for calcite-type and on average around 2.0 for aragonite-type solid solutions. This contrasts with silicate-fluid systems in which Sr typically shows a strong preference for the fluid phase compared with Ca. [ABSTRACT FROM AUTHOR]

Enthalpies of drop solution in molten lead borate have been measured for a series of four zoisite and 12 clinozoisite samples, spanning both epidote solid solutions. Mixing within the zoisite series between 0.0 and 0.2 [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] appears to be ideal. The enthalpy of drop solution varies between 493.0 [+ or -] 3.2 and 496.0 [+ or -] 3.6 kJ/mol with a slight increase with increasing Fe content, based on a linear fit to the data. The clinozoisite solid solution shows more complex behavior. The enthalpy of drop-solution for samples between 0.28 and 0.65 [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] varies between 491.7 [+ or -] 4.2 and 500.7 [+ or -] 3.0 kJ/mol and shows significant scatter. For samples between 0.65 and 0.95 [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] the drop solution enthalpies increase abruptly, going from 494.3 [+ or -] 3.4 to 521.1 [+ or -] 5.4 kJ/mol. End-member drop solution values for Fe-free zoisite were estimated by the linear fit and for clinozoisite by assuming a fictive Fe-free end-member, taking into account the estimated energy of the monoclinic-orthorhombic transition, and by extrapolation of the Fe-rich data to [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. Standard enthalpies of formation from the elements for Fe-free zoisite, Al-clinozoisite, and Fe-clinozoisite have been calculated based on the present data and data from the literature. The values are [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] kJ/mol; [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] kJ/mol; [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] kJ/mol. The overall trend in the drop solution data reflects an apparent positive enthalpy of mixing for the clinozoisite series, consistent with the presence of a miscibility gap in the series between about 0.5 and 0.8 [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] An attempt to fit the highly asymmetric enthalpy of mixing data with a two-parameter asymmetric Margules mixing model, however, failed to yield realistic values for the interaction parameters and the solvus closure temperature. The significant scatter in the data, between [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] = 0.0 and 0.65, may be due to short-range ordering, suggested by Fehr and Heuss-Assbichler (1997), or could be due to different states of metastable disorder in the samples. The steep increase in enthalpy going from [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] the may be due to the effect of possible increasing [Fe.sup.3+]-[Al.sup.3+] disorder among the M1 and M3 octahedral sites. The break in slope of the enthalpy data occurs at or near the composition where the onset of [Fe.sup.3+] disorder had been observed previously in clinozoisite.

Olenitic tourmaline with high amounts of tetrahedral B (up to 2.53 [4]B pfu) has been synthesized in a piston-cylinder press at 4.0 GPa, 700 °C, and a run duration of 9 days. Crystals are large enough (up to 30 × 150 μm) to allow for reliable and spatially resolved quantification of B by electron microprobe analysis (EMPA), single-crystal X‑ray diffraction, and polarized single-crystal Raman spectroscopy. Tourmalines with radial acicular habit are zoned in [4]B-concentration [core: 2.53(25) [4]B pfu; rim: 1.43(15) [4]B pfu], whereas columnar crystals are chemically homogeneous [1.18(15) [4]B pfu]. An amount of 1.4(1) [4]B pfu was found in the columnar tourmaline by single-crystal structure refinement (SREF) (R = 1.94%). The EMPA identify [T]Si–1 [V,W]O–1 [T]B1 [V,W](OH)1 as the main and [X]...–1 [T]Si–1 [X]Na1 [T]B1 as minor exchange vectors for [4]B-incorporation, which is supported by the SREF. Due to the restricted and well-defined variations in chemistry, Raman bands in the OH-stretching region (3000–3800 cm–1) are unambiguously assigned to a specific cation arrangement. We found the sum of the relative integrated intensity (Irel) of two low-frequency bands at 3284–3301 cm–1 (ν1) and 3367–3390 cm–1 (ν2) to positively correlate with the [4]B concentrations: [4]B [pfu] = 0.03(1) × [Irel (ν1) + Irel (ν2)]. Hence, those bands correspond to configurations with mixed Si/B occupancy at the T site. Our semi-quantitative correlation also holds for well-characterized natural [4]B-bearing tourmaline from the Koralpe, Austria. This work shows the potential for Raman spectroscopy as a non-destructive method for the chemical classification of (precious) natural tourmaline, and as a tool to rapidly characterize chemical zonation of tourmalines in thin section. [ABSTRACT FROM AUTHOR]

The occurrence of a trioctahedral analog of illite, the dioctahedral interlayer-deficient K-mica, has long been debated. Due to the inherent difficulties of determining structure and chemical composition of the extremely fine-grained material, earlier descriptions based on separated material are equivocal. Here we describe low-temperature (diagenetic) formation of fluorophlogopite, which is interlayer-deficient and therefore analogous to illite, using high-resolution in situ methods (transmission electron microscopy, TEM, with preparation by focused ion beam milling, combined with wavelength-dispersive analysis by field-emission gun electron microprobe). The average composition is K0.5Mg2.8V0.01Fe0.005 [Si3.15Al0.85O10 (OH) 0.65F1.35], including minor amounts of NH4 for charge compensation as determined by electron energy loss spectroscopy. The K-deficient Mg-mica occurs in layer packages of ~10 layers, and no indications for interlayering with other sheet silicate layers such as chlorite or vermiculite could be identified with TEM. X-ray powder diffraction patterns of separated material confirm the absence of smectite components. The mineral was identified in phosphorites from the lowermost Cambrian Tal Group, Mussoori Syncline, Lesser Himalayas, India. The rocks are alternating phosphatic mudstones and phosphatic dolostones, at times interbedded with phosphate-poor carbonate layers, which are rich in organic matter. Sedimentary fluorophlogopite occurs in both rock types and in two textural associations; one in vesicles filled with amorphic organic matter, the other as reaction rims around illite, which contains up to 5 wt% V2O3 in its rims. Textural arguments favor an early diagenetic formation of both, V-bearing illite and fluorophlogopite, closely associated with organic matter and linked to dolomitization. The high-F content stabilizes phlogopite to low temperatures. Our findings confirm that the stability field of fluorophlogopite extends from magmatic to metamorphic and sedimentary conditions. [ABSTRACT FROM AUTHOR]

The Ca-Sr fractionation between zoisite and, respectively, lawsonite and an aqueous fluid has been determined by synthesis experiments in the presence of a 1 M (Ca,Sr)Cl2 aqueous fluid at 2.0 GPa/550, 600, and 700 °C and 4.0 GPa/800 °C for zoisite and 2.0 GPa/400 °C and 4.0 GPa/600 °C for lawsonite. Solid run products were characterized by EMP, SEM, and XRD with Rietveld refinement and fluids were analyzed by ICP-OES. Zoisite exhibits notable intracrystalline Ca-Sr fractionation between the A1 and A2 sites and calculated intracrystalline exchange coefficients KD(Sr-Ca)A1-A2 = 1.5 to 26 show strong preference of Sr over Ca for the slightly larger A2 site. Calculated individual site-dependent zoisite/aqueous fluid (af, in superscripts)-exchange coefficients for the studied 1 M (Ca,Sr)Cl2 aqueous fluids are K(Sr-Ca)zo A1-af = 3.38 to 41.08 for the A1 site and K(Sr-Ca)zo A1-af = 0.45 to 6.51 for the A2 site. Assuming γCaaf = γSraf and a symmetric mixing model, the thermodynamic evaluation of the site-dependent exchange reactions Ca2+(af) + SrA1(M2+)A2 Al3[Si3O11(O/OH)] = Sr2+(af) + CaA1 (M2+)A2Al3[Si3O11(O/OH)] and Ca2+(af) + (M2+)A1 SrA2 Al3[Si3O11(O/OH)] = Sr2+(af) + (M2+)A1 CaA2 Al3[Si3O11(O/OH)] yields Δµ0 = --29 kJ/mol and WSr-Cazo A1 = 5.5 kJ/mol for the A1 site and Δµ0 = -1.1 kJ/mol and WSr-Cazo A2 = 0 kJ/mol for the A2 site at P and T of the experiments. The data indicates ideal Ca-Sr substitution on the A2 site. Lawsonite formed in both the orthorhombic Cmcm and the monoclinic P21/m form. Calculated lawsonite-aqueous fluid-exchange coefficients indicate overall preference of Ca over Sr in the solid and are KD(Sr-Ca)law Cm -cm-af = 1.12 to 11.32 for orthorhombic and KDD(Sr-Ca)law Cm -cm-af = 1.67 to 4.34 for monoclinic lawsonite. Thermodynamic evaluation of the exchange reaction Ca2+(af) + SrAl2Si2O7(OH)2·H2O = Sr2+(af) + CaAl2Si2O7(OH)2·H2O assuming γCaaf = γSraf and a symmetric mixing model yields similar values of Δµ0 = --9 kJ/mol and WSr-Calaw Cmcm-af = 10 kJ/mol for orthorhombic and Δµ0 = -10 kJ/mol and WWSr-Calaw P21/m = 11 kJ/mol for monoclinic lawsonite. Calculated Nernst distribution coefficients for the studied 1 M (Ca,Sr)Cl2 aqueous fluids are DSrzo-af = 2.8 ± 0.7 for zoisite at 2 GPa/600 °C and DSrlaw Cmcm-af = 0.6 ± 0.2 for orthorhombic lawsonite at 4 GPa/600 °C and show Sr to be compatible in zoisite but incompatible in lawsonite. This opposite mineral-aqueous fluid-fractionation behavior of Sr with respect to zoisite and lawsonite on the one hand and the ideal Ca-Sr substitution on the zoisite A2 site in combination with the strong intracrystalline Ca-Sr fractionation in zoisite on the other hand, make Sr a potential tracer for fluid-rock interactions in zoisite- and lawsonite-bearing rocks. For low Sr-concentrations, xSrzo directly reflects xSraf and allows us to calculate Sr-concentrations in a metamorphic aqueous fluid. During high-pressure aqueous fluid-rock interactions in subduction zone settings the opposite mineral-aqueous fluid-fractionation behavior of Sr results in different aqueous fluid characteristics for lawsonite- vs. zoisite-bearing rocks. Ultimately, subduction zone magmas may trace these different aqueous fluid characteristics and allow distinguishing between cold, lawsonite-bearing vs. warm, zoisite-bearing thermal regimes of the underlying subduction zone. [ABSTRACT FROM AUTHOR]

We performed hydration experiments of pure and Nb-, Cr-, and V-doped synthetic dry (H2O < 3 ppm) single rutile crystals. They were equilibrated with pure H2O in hydrothermal experiments at constant conditions of 600 °C, 400 MPa, and fO2 near the Ni-NiO buffer, run time between ~25 min and 14 days. Slabs cut parallel to (110) of the reacted single crystals (1 to 2 mm³) were analyzed for H+ by FTIR. Hydration occurs almost spontaneously and the H2O-equivalent is uniformly distributed in the samples, but depends extremely on trace element contents. In pure rutile, the average H2O-content is 314 ± 50 ppm, the saturation level at these conditions. Rutile doped with 500 ppm Nb has a lower average H2O content of ~235 ppm, rutile with 2000 ppm Cr has ~900 ppm H2O, and rutile with 2000 ppm V does not incorporate H2O. During stepwise heating at atmospheric pressure of a reacted Nb-doped rutile, H+ is quickly released between 450 and 550 °C. UV-VIS spectra of unreacted colorless and reacted deep blue pure rutile show the rutile-characteristic sharp absorption edge in the UV spectra. The reacted rutile has a broad absorption band at 6500 cm-1 wavenumber attributed to intervalence charge transfer transition Ti3++Ti4+ → Ti4++Ti3+. The reduction of Ti4+ to Ti3+ is charge balanced by the incorporation of H+. The Nb-doped rutile changed its color from light greenish-blue (untreated) to deep blue. In the untreated Nb rutile, the UV-VIS absorption band at 6500 cm-1 indicates that Nb5+ is charge balanced by Ti3+. In the reacted Nb-rutile the absorption band is more intense, but compared with the pure rutile, H+ incorporation is lower by the equivalent of Ti3+ reduced in the untreated rutile. Reacted Cr-rutile almost retains its brownish-orange color, but the spectrum shows a prominent Ti3+/Ti4+ IVCT band at ~6400 cm-1 with moderate intensity considering the high-H2O contents of ~900 ppm. The high-H+ contents are best explained by the reduction of Cr4+ to Cr2+. The UV-VIS spectra of the dark-blue to opaque V-doped rutile show a very strong absorption toward low energies, which is likely caused by reduction of Ti4+ to Ti3+ for charge balance of V5+. This forms a deep narrow window of transmittance in the range 25 000--20 000 cm-1, which causes the dark-blue color. To explore the possible use of H-in-rutile as a geohygrometer, geothermobarometer, and oxybarometer, we measured the H+ content in a natural rutile crystal from a retrograded eclogite with a zoned trace element (Fe, Nb, and Zr) content. The crystal reveals a slight correlation between the variable H2O (~200 to 900 ppm) and its trace element concentrations. The observations indicate that the preservation of H+ contents in this natural rutile is a complicated interplay of diffusive reequlibration of fast H+, slower Fe and very slow other trace elements. An interpretation of the H2O contents of the natural crystal in terms of fO2 or aH2O is not possible. [ABSTRACT FROM AUTHOR]

Fluid-assisted mass transport reactions by dissolution-precipitation, where a precursor mineral reacts with a fluid, play an important role in metamorphism and metasomatism. We investigated titanite growth on rutile in time series experiments between one and 107 days at constant P-T conditions of 600 °C and 400 MPa in the system TiO2-CaO-SiO2-Na2O-HCl-H2O. A two-capsule assemblage allows for transport of Ca and Si from dissolving wollastonite to dissolving rutile, the Ti-source, in a NaCl-bearing aqueous fluid, according to the general reaction CaSiO3 + TiO2 = CaTiSiO5. Complete overgrowth of rutile by titanite occurred after just one day of experiment. Fine-grained lozenge-shaped titanite crystals of short-time runs (up to 14 days) reorganize to larger predominantly prismatic crystals after >14 days. After investigation by scanning electron microscopy, the titanite overgrowth was removed from the rutile by hydrofluoric acid, to provide a three-dimensional view of the dissolution-reaction front on the rutile surface. The morphology of the rutile surface is dominated by humps or ridges beneath the central region of a titanite crystal and valleys at the grain boundaries between adjacent titanite crystals. The dissolution pattern on the rutile surface mimics the titanite overgrowth and changes with changing grain size and shape of the titanite with longer run times. The preferred dissolution of rutile in the valleys is clearly linked to the position of the titanite grain boundaries, which served as pathways for fluid-assisted element transport. Rutile-titanite and titanite-titanite boundaries show a significant porosity in transmission electron microscopy images of foils prepared by focused ion beam milling. The large-scale dissolution pattern on the rutile surface is independent of the crystallographic orientation of the rutile and entirely dominated by the arrangement of titanite crystals in the overgrowth. Dissolution features on a scale smaller than ~1 µm are dominated by stepwise dissolution and etch-pits following the crystallographic orientation of the rutile. Similar observations were made in experiments with an additional Al-source, although these experiments result in a different overgrowth pattern; i.e., an exposed rutile surface is always present and solitary titanite crystals are accompanied by partial overgrowths. Quantitative characterization of the surface morphology by white-light interference microscopy demonstrates that, with increasing grain size of titanite, dissolution of rutile is strongly enhanced at the titanite grain boundaries. Natural examples of titanite overgrowths on rutile show the same relations between element pathways, arrangement of titanite crystals and 3D dissolution pattern on rutile as in the experimental systems. We conclude that the transport of Ti away from the rutile and of Ca + Si into the reaction rim occurred in a grain boundary fluid, the composition of which must have been strongly different from the composition of the bulk fluid in the experiment, as well as in the natural system. The reaction progress depends on the availability of a fluid, and relicts of rutile in titanite indicate restricted availability of fluid in the natural system (e.g., a fluid pulse that was consumed by the reactions). The reaction examined here can serve as a proxy for other reactions of the conversion of oxide minerals (e.g., spinel or corundum) into silicates. [ABSTRACT FROM AUTHOR]

Presents the first systematic study of synthetic iron-free and iron-bearing zoisite using X-ray powder diffraction and infrared spectroscopy. Electrone microprobe analysis; Fourier-transform infrared analysis; Lattice parameters and atom coordinates of all of the samples.

Interfacial tension between immiscible liquids is an important thermodynamic parameter of silicate melt unmixing and a property that determines the kinetics of phase separation. In this study, we present experimental measurements of interfacial tension between immiscible Fe-rich and silica-rich melts in the system K2O-FeO-Fe2O3-Al2O3-SiO2. We have also measured densities and surface tensions of the individual immiscible liquid phases. The measurements were carried out in air at 1500-1550 °C by the maximum detachment force method employing vertical cylinder geometry and using a gravimetric balance system. We have chosen the most oxidized and contrasting liquid compositions containing 73 and 17 wt% SiO2 and 14 and 80 wt% FeOt, respectively, that have been shown to coexist in air at and above 1465 °C. Interfacial tension between the synthetic immiscible liquids decreases with increasing temperature from 16.4 ± 3.1 mN/m at 1500 °C to 7.8 ± 1.1 mN/m at 1550 °C. Interfacial tension between natural, less compositionally contrasting ferrobasaltic and rhyolitic melts should be even lower by a factor of 2 or 3. Very low interfacial tension implies easy nucleation of immiscible liquid droplets and very slow coarsening of resulting silicate emulsions. [ABSTRACT FROM AUTHOR]

Chemical zoning according to the substitution Al + (OH,F) = Ti + O is a common phenomenon in natural titanite and can be used to reconstruct its growth conditions. We synthesized titanite, which has a zoning pattern similar to that found in natural titanite that grew at the expense of rutile, visible in element distribution maps as a patchy irregular pattern. Run conditions in the system CaO-Al2O3-TiO2-SiO2-NaCl-H2O(-CaF2) were 600 °C, 0.4 GPa, 1 to 107 days duration. Natural rutile crystals were placed in a perforated Pt-tube within an outer Au-capsule containing wollastonite, γ-Al2O3, optionally powdered CaF2, and a NaCl brine, to simulate conditions, where rutile is transformed into titanite by a hydrous Ca-Si-Al(-F) fluid, driven by a chemical potential gradient between inner and outer capsule. Spontaneous nucleation of titanite is restricted to a small number of crystals on the rutile surface in the inner capsule. Growth proceeds from sparse isolated titanite crystals after 1 day to assemblages of several 100 µm large crystals in a reaction rim in the long-time runs. Titanite is strongly zoned in Al-Ti and shows up to ~0.5 Al per formula unit (pfu) in experiments containing F; without F, substitution is limited to ~0.25 Al pfu. The range of the Ti-Al exchange is already large in short run times and the same heterogeneity is observed in long-time runs. The Al-Ti distribution (with and without F) in compositional domains (up to several tens of micrometers) is patchy and irregular. No relation of the zoning with time (core-rim) or preferred growth directions was observed. In the case of fluid-mediated growth, mineral growth zoning represents a disequilibrium at supersaturated conditions. The concentration of Ti in the fluid is likely the limiting factor for titanite growth considering the low solubility compared to that of Al, Ca, and Si. Titanium is not transported in significant amounts into the outer capsule, indicating low mobility, in contrast to Al, Ca, and Si, which are transported into the inner capsule. Assuming buffered Al concentration from dissolution of the Al-source, we speculate that the Ti/Al of the fluid at the precipitation site was controlled by the local Ti concentration, which varies over the distance from the rutile surface to the growing titanite crystal and produces the irregular patchy zoning. [ABSTRACT FROM AUTHOR]