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We have determined mineral-melt partition coefficients (D values) for 20 trace elements in garnet-pyroxenite run products, generated in 3 to 7 GPa, 1,425-1,750[degree]C experiments on a high-Fe mantle melt (97SB68) from the Parana-Etendeka continental-flood-basalt (CFB) province. D values for both garnet (~[Py.sub.63][Al.sub.25][Gr.sub.12]) and clinopyroxene (~[Ca.sub.0.2][Mg.sub.0.6][Fe.sub.0.2][Si.sub.2][O.sub.6]) show a large variation with temperature but are less dependent on pressure. At 3 GPa, [D.sup.cpx/liq] values for pyroxenes in garnet-pyroxenite run products are generally lower than those reported from Ca-rich pyroxenes generated in melting experiments on eclogites and basalts ([~Ca.sub.0.3-0.5][Mg.sub.0.3-0.6][Fe.sub.0.07-0.2][Si.sub.2][O.sub.6]) but higher than those for Ca-poor pyroxenes from peridotites ([~Ca.sub.0.2][Mg.sub.0.7][Fe.sub.0.1][Si.sub.2][O.sub.6]). [D.sup.grt/liq] values for light and heavy rare-earth elements are [less than or equal to] 0.07 and >0.8, respectively, and are similar to those for peridotitic garnets that have comparable grossular but higher pyrope contents ([PY.sub70-88][Al.sub.17-20][Gr.sub.8-14]). 97SB68[D.sup.grt/liq.sub.LREE] values are higher and [D.sup.grt/liq.sub.LREE] values lower than those for eclogitic garnets which generally have higher grossular contents but lower pyrope contents ([PY.sub.20-70] [Al.sub.10-50] [Gr.sub.10-55]). D values agree with those predicted by lattice strain modelling and suggest that equilibrium was closely approached for all of our experimental runs. Correlations of D values with lattice-strain parameters and major-element contents suggest that the wollastonite component and pyrope:grossular ratio exert major controls on 97SB68 clinopyroxene and garnet partitioning, respectively. These are controlled by the prevailing pressure and temperature conditions for a given bulk-composition. The composition of coexisting melt was found to have a relatively minor effect on 97SB68 D values. The variations in D values displayed by different mantle lithologies are subtle and our study confirms previous investigations which have suggested that the modal proportions of garnet and clinopyroxene are by far the most influential factor in determining incompatible trace-element concentrations in mantle melts. The trace-element partition coefficients we have determined may be used to place high-pressure constraints on garnet-pyroxenite melting models. Keywords Ferropicrites * Garnet pyroxenite * Partition coefficients * Trace elements

Hydrous defects in nominally anhydrous minerals influence the physical properties and reduce the melting point of mantle rocks. Consequently, it is believed that extraction of [H.sub.2]O beneath mid-ocean ridges by dehydration melting enhances the creep strength of the upper mantle and produces a chemical lithosphere. However, recent studies show that trace water induces melting at shallower depths beneath ridges than previously believed. Here we explore the hypothesis that incipient melting, dehydration, and strengthening of the subridge mantle is promoted by trace quantities of carbon. Experiments at 3 GPa on carbonated peridotite with 1 and 2.5 wt% C[O.sub.2], produce partial melts that change from carbonatites with 25 wt% Si[O.sub.2] between 1325 and 1350 [degrees]C. Enhanced melting of carbonated peridotite relative to volatile free peridotite is parameterized as a simple function of the concentration of C[O.sub.2] in the melt. Application of this model to mantle with 100 ppm bulk C[O.sub.2] suggests that the subridge mantle undergoes silicate melting ~130-140 [degrees]C below the C[O.sub.2]-free solidus, corresponding to a depth of ~110 km beneath ridges, and that 0.2 wt% partial melting will be attained at a depth of ~75 km. The combined effects of [H.sub.2]O and C[O.sub.2] further enhance deep silicate melting, as [H.sub.2]O partitions from nominally anhydrous silicates to carbonated silicate melts. Thus, the deepest silicate melting beneath ridges will be induced by the combination of [H.sub.2]O and C[O.sub.2], rather than simply by dehydration melting of nominally anhydrous peridotite, and this combination is likely responsible for dehydration strengthening of the oceanic lithosphere and for geochemical signatures of deep melting in mid-oceanic ridge basalt. Keywords: mid-oceanic ridge, partial melting, lithosphere, mantle volatiles, C[O.sub.2], dehydration.

[Li.sub.0.52][Mg.sub.0.96][Sc.sub.0.52][Si.sub.2][O.sub.6] orthopyroxene was synthesized by the flux method and its structure studied with single-crystal X-ray diffraction. The crystal is orthorhombic with space group Pbca and unit-cell parameters a = 18.259(5), b = 8.883(2), c = 5.271(1) [[Angstrom].sup.3], and V= 854.9(3) [[Angstrom].sup.3]. The structure refinement shows that the M1 and M2 sites are occupied by (0.48 Mg + 0.52 Sc) and (0.48 Mg + 0.52 Li), respectively. While the 03-03-03 kinking angle (165.40[degrees]) of the silicate tetrahedral A chain appears to be normal when compared with reported data, the kinking angle (151.92[degrees]) of the B chain is the largest of all orthopyroxenes examined at ambient conditions. This is the first orthopyroxene structure that contains more than 50% trivalent and monovalent cations in the M1 and M2 sites, respectively, and displays a kinking angle of the tetrahedral B chain that is greater than 150[degrees]. Our study demonstrates the stability of the new pyroxene structure type predicted by Pannhorst (1979) at room temperature. Keywords: [Li.sub.0.52][Mg.sub.0.96][Sc.sub.0.52][Si.sub.2][O.sub.6], orthopyroxene, crystal structure, X-ray diffraction

Zincalstibite, a new mineral occurring within the cavities of marble of the Apuan Alps, Tuscany, Italy, has chemical formula [Zn.sub.2]AlSb[(OH).sub.12], space group P[bar.3], a = 5.321(1), c = 9.786(2)[Angstrom]. It is associated with sub-millimeter tufts of white crystals of minletite and sub-millimetric stalactite aggregates of opal and an amorphous copper-silicate phase (possibly crisocolla). The crystals are trigonal prismatic, with forms (110), (001),elongated [001], generally less than 10 x 10 x 40 + 50 [micro]m, with few larger crystals. They are colorless, transparent, with vitreous luster, white streak and {001} cleavage. The stronger reflections are [(hkl), d ([Angstrom]), [I.sub.rel]: (002). 4.904, 100: (100). 4.620, 35; (101), 4.179.57: (103, 110). 2.669,31;(112.11[bar.2]), 2.343,88;(114, 17[bar.4]), 1.805, 57. Zincalstibite is structurally related to cualstibite, as evidenced by the structural determinations and refinements we present ]\/r both the minerals. They are built up by layers of isolated Sb[(OH).sub.6], octahedra alternating along c with trioctahedral layers, which contain Zn and Al cations and Cu and Al cations in zincalstibite and cualstibite, respectively. In cualstibite the ordering of Al and Cu within these trioctahedral layers results in a supercell, with [a.sub.cual] = 9.150(2) [Angstrom] [approximately equal to] [square root of 3] [a.sub.zinc] The name of zincalstibite is related to its chemical composition and points to its relationships with cualstibite. Both the mineral and its name were approved by the IMA Commission for New Minerals and Mineral Names (IMA 1998-033). Keywords: Zincalstibite, cualstibite, new mineral, crystal structure, XRD data

Melanophlogite, a clathrasil, possesses a framework of corner-linked silica tetrahedra forming framework cavities that can enclose small guest molecules. Synchrotron X-ray diffraction experiments of the guest-free melanophlogite have been conducted at pressures up to 12 GPa and temperatures up to 1473 K. Upon compression at room temperature, melanophlogite gradually lost its crystallinity and became completely X-ray amorphous at ~8 GPa. The amorphization process was similar to those of denser silica polymorphs, but it reached completion at a much lower pressure (e.g., quartz becomes X-ray amorphous at ~30 GPa). The decreased amorphization pressure of melanophlogite may be attributed to its lower framework density and the ease of bending of its Si-O-Si linkages, thereby accelerating the collapse of the structure under high pressure. Determination of cell volumes of melanophlogite prior to its amorphization yielded a room-temperature bulk modulus of 26.3 [+ or -] 1.7 GPa, which is consistent with the relatively large compressibilities reported for the structurally similar zeolites. When heated at -8 GPa, the amorphous phase started to crystallize at 873 K into coesite, the stable silica phase at these pressure and temperature conditions. Thus the occurrence of pressure-induced amorphization in melanophlogite appears to result from the kinetic hindrance to its transformation to the thermodynamically stable coesite. Keywords: Melanophlogite, clathrasil, pressure-induced amorphization, compressibility, synchrotron X-ray diffraction

The thermodynamic properties of crystalline and liquid KA1Si308 are used to calculate the fusion curve of sanidine to 6.5 GPa. New values for the enthalpy and entropy of fusion of sanidine at one bar and 1200 [degrees]C ([DELTA][H.sub.Tf] = 63.0 kJ/mol, [DELTA][S.sub.Tf] = 42.8 J/mol-K) are recommended on the basis of improved heat-capacity equations for KAl[Si.sub.3][O.sub.8] crystal, glass, and liquid. On the basis of phase-equilibrium experiments on the congruent melting reaction between 2 and 6.5 GPa, the pressure dependence of the liquid compressibility ([K'.sub.0] = d[K.sub.0]/dP, where [K.sub.0] = 1/[[beta].sub.0]) is constrained to be 12.2 [+ or -] 1.0 in a third-order Birch-Mumaghan equation of state (EOS). The metastable, one-bar melting temperature ([T.sub.f]) is additionally constrained to be 1203 [+ or -] 26 [degrees]C. Determination of the liquid [K'.sub.0] allows the density and compressibility of KA1Si308 liquid to be calculated to 6.5 GPa (2.709 [+ or -] 0.014 g/[cm.sup.3] at 1600 [degrees]C). The uncertainty in [K'.sub.0] of [+ or -]1.0 leads to an error in melt density at 6.5 GPa of [+ or -]0.52%. With a [K'.sub.0] = 12.2, the relatively high compressibility of KAl[Si.sub.3][O.sub.8] liquid at 1600 [degrees]C ([K.sub.0] = 15.8 GPa) drops rapidly with increasing pressure. The dominant mechanism of compression for KAl[Si.sub.3][O.sub.8] liquid between 0 and 6.5 GPa most likely involves topological changes and increases in network connectivity with pressure. It is probable that highly compressible liquids, such as hydrous, silica-rich liquids formed by partial melting of a subducted slab, may have K6 values that exceed 12 (at pressures [greater than or equal to]6.5 GPa). Keywords: Sanidine, fusion curve, enthalpy of fusion, topology, equation of state

The single-crystal elastic properties of [Ca.sub.0.07][Mg.sub.1.93][Si.sub.2][O.sub.6] orthopyroxene (space group Pbca) have been investigated by Brillouin spectroscopy at ambient conditions. The aggregate bulk and shear moduli, ([K.sub.0,S] = 102.5 GPa (1.5) and p. = 74.2 GPa (1.1), respectively, are ~5% and ~3% lower than commonly accepted values for MgSi[O.sub.3] end-member ([K.sub.0,S] = 107.6, [mu] = 76.8 GPa). These results indicate that the incorporation of small amount of Ca in the orthoenstatite structure does not greatly affect its elastic properties. As a consequence, the increase in bulk modulus reported in natural orthopyroxenes relative to the Mg-end-member is not related to the substitution of Ca in the M2 octahedral sites, but more probably to the substitution of A1 in tetrahedral sites. Keywords: Brillouin spectroscopy, elastic properties, enstatite, orthopyroxene

Scheuchzerite, Na[(Mn,Mg).sub.9][V[Si.sub.9][O.sub.28](OH)][(OH).sub.3], is a new mineral from the metamorphosed synsedimentary exhalative Mn deposit of Fianel, Val Ferrera, Central Alps, Switzerland. It is dedicated to the Swiss naturalist Johann Jakob Scheuchzer (1672-1733). Scheuchzerite is associated with saneroite and tiragalloite in veins resulting from the remobilization of ore components during retrograde Tertiary Alpine metamorphism. Scheuchzerite forms yellow-orange, transparent acicular crystals up to 0.5 mm in length with yellow-orange streak and vitreous luster, Mohs' hardness ~2.5, [d.sub.cale] 3.47 (electron microprobe) to 3.52 g/[cm.sup.3] (structure refinement); [d.sub.meas] 3.50(2) g/[cm.sup.3], good cleavage parallel to fiber elongation. Scheuchzerite is biaxial positive, [n.sub.min] = 1.74 and [n.sub.max] = 1.75; [n.sub.mean] (Gladstone-Dale) 1.74; weakly pleochroic, X = brown yellow, Y = pale yellow. The empirical chemical formula is [Na.sub.0.97][([Mn.sub.7.79][Mg.sub.0.95][Z n.sub.0.16][Ni.sub.0.04][Ca.sub.0.03][Al.sub.0.01]).sub.[SIGMA]=8.98] [([V.sub.0.95][As.sub.0.02][Si.sub.9.08]).sub.[[SIGMA]=10.05] [O.sub.32.05][H.sub.4].9.08) Scheuchzerite is triclinic, P[bar.1], a = 9.831(5) [Angstrom], b = 10.107(5) [Angstrom], c = 13.855(7) [Angstrom], [alpha] = 86.222(10)[degrees], [beta] = 73.383(9)[degrees], [gamma] = 71.987(9)[degrees]; V= 1254.2(10) [[Angstrom].sub.3]; Z = 2. The crystal structure was solved with direct methods on the basis of 1616 unique reflections with I > 4[sigma]F and refined to [R.sub.1] = 9.4%. The crystal structure consists of tetrahedral layers separated by layers containing chains of edge-sharing [Mn[(O,OH).sub.6]] octahedra as well as [Na[O.sub.8]] polydedra. The tetrahedral layers consist of [[Si.sub.9][O.sub.25](OH)] loop-branched chains of corner-sharing silicate tetrahedra extending along [011]. The loops contain 6 tetrahedra and are separated by 3 tetrahedra in a broken 4-loop arrangement. A hydrogen atom is probably shared by two O atoms (symmetrical hydrogen bond), replacing the missing silicon atom. A vanadate [(V[O.sub.4]).sup.3-] tetrahedron branches off the 6-tetrahedra loop, and hence the overall formula of the tetrahedral chains is [V[Si.sub.9][O.sub.28](OH)]. In the notation of Liebau (1985), scheuchzerite is a single chain silicate (monopolysilicate) {olB, [1.sup.1.sub.[infinity]}[V[Si.sub.9][O.sub.28](OH)]. The topology of the scheuchzerite structure is reminiscent of that of the double-chain silicates of the amphibole group, but scheuchzerite contains a new type of silica chain. Keywords: Scheuchzerite, new mineral, Fianel mine, Val Ferrera, Central Alps, Switzerland, crystal structure, single chain silicate

The activity of alkali metal oxides can be controlled in one-atmosphere wire-loop experiments at high temperature by suspending a crucible containing alkali silicate melt beneath the samples. The method has been applied to measuring the activity coefficient of Na[O.sub.0.5] in a series of CMAS-Na[O.sub.0.5] melts relative to that in the anorthite-diopside eutectic composition at 1400 [degrees]C, using a reservoir of Na[O.sub.0.5]-Si[O.sub.2] melt. The results show that this relative activity coefficient decreases strongly with Si[O.sub.2], increases with CaO and MgO, but is insensitive to Al[O.sub.0.5]. This latter behavior is inconsistent with 'quasi-crystalline' models of melt thermodynamics that hypothesize Na-Al species.

The Ti content of biotite can serve as a geothermometer for graphitic, peraluminous metapelites that contain ilmenite or rutile and have equilibrated at roughly 4-6 kbar. The relationship between Ti-content, temperature, and Mg/(Mg + Fe) value was calibrated empirically using an extensive natural biotite data set (529 samples) from western Maine and south-central Massachusetts in combination with the petrogenetic grid of Spear et al. (1999}. The calculated Ti-saturation surface is curved such that for a given Mg/(Mg + Fe) value, Ti concentration increases as a function of temperature in a nonlinear fashion, and for a given temperature Ti concentrations decrease with an increase in Mg/(Mg + Fe). The fit to the Ti-saturation surface can be reformulated as the geothermometric expression: T = [{[ln(Ti) - a - c[(X.sub.Mg).sup.3]/b}.sup.0.333], in which T is temperature in degrees Celsius, Ti is the number of atoms per formula unit (apfu) normalized on the basis of 22 O atoms, [X.sub.Mg] is Mg/(Mg + Fe), a = -2.3594, b = 4.6482 x [10.sup.-9] and c = 1.7283. The calibration range for this expression is [X.sub.Mg] = 0.275-1.000, Ti = 0.04-0.60 apfu, and T = 480-800 [degrees]C. Precision of the Ti-in-biotite geothermometer is estimated to be [- or +]24 [degrees]C at the lower temperature range and improves to [+ or -]12 [degrees]C at higher temperatures. Application of the Ti-in-biotite geothermometer to ilmenite- or rutile-bearing, graphitic, peraluminous metapelites equilibrated at 3-6 kbar is generally consistent with independent temperature determinations, but with some deviations that represent local reequilibration. Consequently, the Ti systematics in biotite can also serve as the basis of a very sensitive indicator of chemical equilibrium, or lack thereof. Application of the geothermometer to metapelites not containing the requisite mineral assemblages can lead to minor-to-significant errors in estimated temperatures. Biotite Ti-substitution mechanisms are controlled by several factors. Based on the biotite calibration data set, magnesian biotites {[X.sub.Mg] > 0.65) incorporate Ti in accordance with the exchange vector Ti[Al.sub.2][R.sub.-1][Si.sub.-2], where R is the sum of the divalent cations Mg + Fe + Mn. This substitution mechanism is primarily a response to misfit of the octahedral and tetrahedral layers in magnesian biotites. Intermediate biotites ([X.sub.Mg]

Although many formulations of plagioclase + liquid equilibria have been calibrated in the last decade, few models specifically address the issue of temperature (T) prediction. Moreover, for those that do, T error is not addressed, greatly limiting their use as geothermometers. Several recent models of plagioclase-liquid equilibria are thus tested for their ability to recover T from their calibration data, and predict T from experiments not used for calibration. The models of Sugawara (2001) and Ghiorso et al. (1995, 2002) outperform earlier calibrations. These models perform reasonably well at T > 1100[degrees]C, though recovery and prediction of T is less precise for hydrous compositions. In addition, these models cannot be integrated with geo-hygrometers, or other mineral-melt thermometers and barometers; the following expression predicts T with up to 40% greater precision: (1) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] Because these thermometers are pressure (P) sensitive, a temperature-sensitive barometer was also developed: (2) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] In these models, T is in Kelvins and P is in kbar. A[n.sub.pl] and A[b.sub.pl] are the fractions of anorthite and albite in plagioclase, calculated as cation fractions: An = CaO/(CaO + Na[O.sub.0.5] + K[O.sub.0.5]) and Ab = Na[O.sub.0.5] / (CaO + Na[O.0.5] + K[O.sub.0.5]. Terms such as [Al.sup.liq] refer to the anhydrous cation fraction of Al in the liquid: [H.sub.2]O in Equation 1 is in units of wt%. Errors on these models are comparable to those for clinopyroxene thermobarometers: In Equation 1, R = 0.99 and the standard error of estimate (SEE) is 23 K: for Equation 2, R = 0.94 and the SEE is 1.8 kbar. The models successfully recover mean pressures for experimental data that are not used for calibration, and are furthermore able to recover near-1-atm P estimates for volcanic rocks from Kilauea, Hawaii. which are thought to have crystallized at or very near Earth's surface.

Mineralogical-textural analysis of a multi-layered plagioclase (Pl) + orthopyroxene/biotite (Opx/Bt) vermicular microstructure around garnet from the Bolangir anorthosite massif is presented. X-ray images demonstrate the garnet was zoned prior to decomposition, with Mg decreasing toward its rim, and Mn, Ca, and [X.sub.Fe] [= Fe/(Fe + Mg)] increasing. On decomposition, the Ca zonations in garnet were truncated by the consuming garnet margin. By implication, the retreat velocity of the garnet margin was faster than the diffusive velocity for Ca within the garnet. By contrast, the pre-decomposition Fe, Mn, and Mg variations in garnet were considerably modified adjacent to Opx + Pl, but were unaffected adjacent to Bt + Pl intergrowths. In the corona, [X.sup.Opx.sub.Fe] and [X.sup.Bt.sub.Fe] decrease toward garnet, whereas the An content in associated Pl increases from [An.sub.50] to [An.sub.80-83] Thermo-barometry involving nearest neighbor Opx, Grt, Pl, and Qtz indicate that the corona formed at 6.0 [+ or -] 1.0 kbar, 750 [+ or -] 50 [degrees]C. At the isothermal-isobaric condition of decomposition, the compositional variation across the Bt + Pl corona is explained best by the inward retreat of the garnet margin at a velocity faster than the fastest diffusing element (Fe, Mn). In zones where garnet decomposed to Opx + Pl, the velocity of the consuming garnet margin was such that the intra-garnet Fe, Mn redistribution was aborted in transit. The compositions and volume proportions of vermicular orthopyroxene and plagioclase in the multi-layered corona are best approximated by the NCFMAS reaction [Grt.sub.ss] + [Pl.sub.1] ([+ or -] Qtz) [right arrow] Opx + [Pl.sub.2]. The early formed/matrix plagioclase ([Pl.sub.1]) and inward-retreating garnet ([Grt.sub.ss]) margin provided the necessary components for successive Opx + [Pl.sub.2] layers to grow at Grt-[Pl.sub.1] interfaces. Tie-line topology in ACFN space indicates [Pl.sub.2] to be An-rich relative to [Pl.sub.1]. The mineralogical relations indicate the CFMAS reaction, [Grt.sub.ss] + Qtz [right arrow] Opx + An, to be an inadequate analogue of the actual decomposition. Consequently, to assume that the texture indicates near-isothermal decompression across the low-dP/dT CFMAS reaction is possibly an oversimplification.

Dellaventuraite is a new amphibole species from the Kajlidongri manganese mine, Jhabua District, Madhya Pradesh, India. It occurs with leakeite, kornite, albite, braunite, and bixbyite associated with cross-cutting epigenetic veins that have reacted with regionally metamorphosed rocks containing Mn-rich minerals (braunite, bixbyite, jacobsite, spessartine) to produce Mn-rich amphiboles, Mn-rich pyroxenes, Mn-rich mica, piemontite, and manganophyllite. Dellaventuraite occurs as anhedral grains, the color of which varies from pink to red, depending on Mn content. It is brittle, H = 5, [D.sub.calc] = 3.184 g/[cm.sup.3], has a pale pink streak, vitreous luster, and does not fluoresce in ultraviolet light; it has perfect cleavage on {110} and conchoidal fracture. In transmitted plane-polarized light, dellaventuraite is strongly pleochroic, X = pale mauve-brown. Y ~ Z = dark red-brown: Y ^ a = 20[degrees] (in [beta] obtuse), Z = b, with absorption X < Y ~ Z. It is biaxial positive, [[eta].sub.[alpha]] = 1.688 [+ or -] 0.003, [[eta].sub.[beta]] = 1.692 [+ or -] 0.005, [[eta].sub.[gamma]] = 1.721 [+ or -] 0.003, 2[V.sub.(obs)] = 49 [+ or -] 3[degrees], 2[V.sub.(calc)] = 41[degrees]. Dellaventuraite is monoclinic, space group C2/m, a = 9.808(1), b = 17.840(2), c = 5.2848(5) [angstrom], [gamma] = 104.653(1)[degrees], V = 894.6(2) [[angstrom].sup.3], Z = 2. The strongest ten X-ray diffraction lines in the powder pattern are [d(l,hkl)]: 2.697(10,151), 2.542(9,-202), 3.127(8,310), 3.378(7,131), 2.154(7,261), 1.434 (7.-661), 4.450(6,021), 8.459(5,110), 2.727(5,-331), 2.328(5,-351). Analysis by a combination of electron microprobe, SIMS and crystal-structure refinement gives Si[O.sub.2] = 54.22, [Al.sub.2][O.sub.3] = 0.81, Ti[O.sub.2] = 5.45, [Fe.sub.2][O.sub.3] = 6.44, [Mn.sub.2][O.sub.3] = 7.57. ZnO = 0.12, NiO = 0.16, MgO = 8.26, [Li.sub.2]O = 1.53, CaO = 1.85, [Na.sub.2]O = 8.12, [K.sub.2]O = 2.12, [H.sub.2]O = 0.80, Cr, V, F, Cl not detected, sum 97.41 wt%. The formula unit, calculated on the basis of 24(O,OH,F) is ([K.sub.0.40][Na.sub.0.61])([Na.sub.1.71][Ca.sub.0.29]) ([Mg.sub.1.81][Zn.sub.0.01][Ni.sub.0.02][Li.sub.0.90] [Fe.sup.3+.sub.0.71][Mn.sup.3+.sub.0.85][Ti.sup.4+.sub.0.60] [Al.sub.0.10])([Si.sub.7.96][Al.sub.0.04])[O.su b.22][[(OH).sub.0.80] [O.sub.1.20]; the ideal end-member composition Na[Na.sub.2](Mg[Mn.sup.3+][sub.2]Li[Ti.sup.4+])[Si.sub.8] [O.sub.22][O.sub.2]. The crystal structure of dellaventuraite was refined to an R index of 3.8% using MoK[alpha] X-ray intensity data. The M1 site is occupied by [Ti.sup.4+], [Mn.sup.3+], and Mg in approximately equal amounts, the M2 site is occupied primarily by Mg and [Fe.sup.3+], and M3 is occupied by Li with minor Mg and [Mn.sup.2+]. Local bond-valence considerations suggest that [O.sup.2-] at O3 is linked to [Ti.sup.4+]Mg or [Mn.sup.3+][Mn.sup.3+] at the adjacent M1 sites, and that OH at O3 is linked to MgMg at the adjacent M1 sites.

We have investigated the physical and structural properties of phase D (Mg[Si.sub.2][O.sub.6][H.sub.2]) at high pressure by means of a first-principles calculation method. Phase D is important as a dense hydrous magnesium silicate with high stability under pressure, and as one of the most likely candidates for a water reservoir in the Earth's lower mantle. The calculated compression behavior of phase D is in very good agreement with experimental results. We found a distinct but continuous change from asymmetric to symmetric hydrogen bonding in phase D at 40 GPa. This pressure-induced hydrogen bond symmetrization has a significant effect on the compression behavior of phase D. The bulk modulus increases by about 20% with this structural change. This behavior of pressure-reduced hydrogen bond symmetrization is very similar to that previously reported by us for 6-A100H. The transition is reversible and second-order, and thus the high-pressure state is probably unquenchable.

Important aspects of melt inclusion formation and potential compositions effects have been addressed through a series of experiments using anorthite/fosterite saturated anhydrous mafic liquids. Experimental charges were cooled from 1,300 to 1,230 and 1,210[degrees]C at rates of 1-10[degrees]/min followed by 0-24 h isothermal periods. Hopper and skeletal crystal morphologies with variable degrees of completeness developed during the cooling period. Planar overgrowth of these textures during isothermal periods led to the formation of inclusions, the majority of which formed after 6 h of isothermal run time. We suggest that the change in morphologies is related to a decrease in growth rates and changes in dominant growth mechanisms. In general, inclusion compositions were uniform and similar to the host glass, indicating that with the isothermal times required for most inclusions to form, a boundary layer was not entrapped that could be detected within the limits of our analyses.

With recent findings of substantial amounts of B in rock-forming minerals, it has become evident a need exists for microprobe techniques allowing quantitative B determinations in minerals. A microprobe method based on the nuclear reaction [sup.11]B(p,[alpha])2[alpha] has the potential to become a powerful tool for analyses of minerals with B contents spanning a wide concentration range. In this paper, we present nuclear microprobe analyses of B in a set of silicate minerals containing stoichiometric concentrations of B in the range 1.9-8.8 wt%. Our results strongly indicate that nuclear reaction analyses of silicates utilizing the [sup.11]B(p,[alpha])2[alpha] nuclear reaction are virtually matrix-insensitive, and saturation effects are negligible within the studied range of B concentrations. This finding suggests that it is adequate to use a single B standard for calibration purposes for analyses covering wide ranges of B concentrations in silicates. In addition, we demonstrate how B-concentration maps of high contrast and resolution may be produced by scanning the proton beam over the samples.

Interactions between a 5 mu(m) borosilicate glass sphere and a silica slide is studied in the presence of a nonadsorbing polyelectrolyte, sodium (polystyrene sulfonate) (NaPSS) by using total internal reflection microscopy (TIRM). Further, the effect of the polymer concentration, within the dilute solution regime, on the observed interactions is also investigated.

The infrared (IR) spectra of small clusters of atoms ([MM'[(OH).sub.2]] and [MM'[(OH).sub.2] [([H.sub.2]0).sup.6]], where M, M' = [Al.sup.3+], [Mg.sup.2+], [Fe.sup.2+], [Fe.sup.3+]) mimicking the environment of the OH group in dioctahedral phyllosilicates have been modeled using ab initio quantum mechanical calculations. These modeling results are relevant to establishing the connections between IR spectra of phyllosilicates and their composition, and to investigate the utility of quantum mechanical models for calculating IR frequencies of minerals. This study focused on the OH stretch and in-plane bend fundamentals, because they give rise to a combination band near 4545 [cm.sup.-1] (2.2 [micro]m) that can be observed in imaging spectrometer or hyperspectral remote sensing data. A comparison among results obtained using both ab initio [Hartree-Fock (HF) and Density Functional Theory (DFT)], and semi-empirical [PM3(tm)] methods, showed that the DFT model approaches IR frequency experimental values most closely. IR spectra of phyllosilicates were modeled using the DFT method. The modeled frequencies were scaled using a mode-dependent linear transformation, and experimental frequencies were reproduced satisfactorily. The modeling results show that most of the variability observed in the OH in-plane bend fundamental of dioctahedral phyllosilicates can be explained by the effects of neighboring octahedral cations alone. Discrepancies between modeling and experimental results in the case of the OH stretch point to the existence of factors other than the nature of the neighboring octahedral cations, such as tetrahedral substitution, affecting this fundamental mode.

Detailed X-ray single-crystal structure refinements and complete (SIMS + EMPA) microprobe chemical analyses of a series of non-metamict samples of hellandite recently found in Latium (Italy), together with a critical re-evaluation of the existing data on hellandite, allow a better understanding of the crystal-chemistry of the hellandite group. Relative to the crystal structure determined by Mellini and Merlino (1977) for a Y-rich sample from Predazzo (Italy), a new tetrahedrally coordinated site has been detected; this may be fully, or in part, occupied by Li and Be. These cations occur at the center of the tetrahedral cavity where the H atom, which is bonded to the O5 O atom, protrudes; thus the H content in hellandite is constrained to values [less than or equal to] 2 - (Li + Be + F). A new general formula for hellandite-group minerals is proposed: [X.sub.4][Y.sub.2]Z[T.sub.2][[B.sub.4][Si.sub.4][O.sub.22]][W.sub.2], where X = Na, Ca, Y, LRE[E.sup.3+] at the eightfold-coordinated M3 and M4 sites; Y = Ca, Y, HRE[E.sup.3+], [Th.sup.4+ at the eightfold-coordinated M2 site; Z = Al, [Mn.sup.3+], [Fe.sup.3+], [Ti.sup.4+] at the octahedral M1 site; T = [square] (vacancy), Li, Be at the new tetrahedrally coordinated site; and W = OH, F, [O.sup.2-] at the 05 site. Eight root end-member compositions were identified; four of these correspond to known compositions: hellandite-(REE), tadzhikite-(REE), and two new minerals, mottanaite-(Ce) and ciprianiite, which are described in a companion paper (Della Ventura et al. 2002). The root-name tadzhikite must be used for samples with [Ti.sup.4+] > 0.5 apfu, with no reference to the OH content. Hellandite is an REE mineral. Rare-earth elements occur at two distinct sites (M2 and M4), with a strong preference for M2 (particularly HREE). Thus, one or two Levinson modifiers can be added to the root name to correctly describe the species. A sequence of incorporation based on crystalchemical arguments is provided and allows evaluation of the site populations for nomenclature purposes. This redefinition of the hellandite group and the new nomenclature rules have been approved by the IMA Commission on New Minerals and Mineral names (code 00-F).

Two new minerals of the hellandite group were found within alkali-syenitic ejecta enclosed in pyroclastic formations of the Roman Comagmatic Province (Latium, Italy). Mottanaite-(Ce) [ideally [sup.X][Ca.sup.Y.sub.4][(CeCa).sup.Z][Al.sup.T][Be.sub.2] [([Si.sub.4][B.sub.4][O.sub.22]).sup.W][O.sub.2]] and ciprianiite [ideally [sup.X][Ca.sup.Y.sub.4][[(Th,U)(REE)].sup.Z][Al.sup.T][[square].sub.2] ([Si.sub.4][B.sub.4][O.sub.22]) [sup.W][(OH, F).sub.2]] occur as transparent, brown-colored, tabular euhedral crystals in miarolitic cavities and voids of the ejecta, which consist mainly of sanidine and plagioclase (An ranging from 20 to 80%), with minor amounts of feldspathoid, clinopyroxene and/or clinoamphibole, magnetite, titanite, and zircon. Locally, accessory minerals include britholite-(Ce), baddeleyite, phosphate to silico-phosphate phases close in composition to the brabantite-cheralite series, thorite, fluorite, danburite, and vonsenite. The genesis of the new hellandite end-members can be related to late-stage post magmatic hydrothermal fluids enriched in Zr, Ti, REEs, and actinide elements. Both mottanaite-(Ce) and ciprianiite have a vitreous luster and are non-fluorescent. Cleavage is absent in mottanaite-(Ce), fair to good in ciprianiite, {100}. Twinning is frequently observed in ciprianiite. Due to the strong intra-crystalline chemical zoning and twinning, physical properties could be measured only for mottanaite-(Ce). [D.sub.meas] is 3.61(4) g/[cm.sup.3], [D.sub.calc] is 3.88 g/[cm.sup.3]. Mottanaite-(Ce) is biaxial negative, with [alpha] = 1.680(5), [beta] = 1.694(2), [gamma] = 1.708(5); 2[V.sub.meas] ~90 [degrees]. Both minerals are monoclinic, space group P2/a, Z = 2. Unit-cell parameters for the crystals studied are: a = 19.032(9) [Angstrom], b = 4.746(3) [Angstrom], c = 10.248(5) [Angstrom], [beta] = 110.97(5) [degrees], V = 864.3(8) [[Angstrom].sup.3] for mottanaite-(Ce), and a = 19.059(5) [Angstrom], b = 4.729(1) [Angstrom], c = 10.291(4) [Angstrom], [beta] = 111.33(2) [degrees], V = 864.0(5) [[Angstrom].sup.3] for ciprianiite. Single-crystal structure refinement confirmed the presence of a further distorted tetrahedral site which was first detected in a hellandite-(Ce) sample from Latium (Oberti et al. 1999). This site is occupied by Be ([+ or -] Li) in stoichiometric mottanaite-(REE), whereas it mainly hosts hydrogen (bonded to the 05 oxygen atom) in ciprianiite and hellandite-(REE); solid solution between the end-members is possible, as shown by the studied samples. The chemical composition of the refined crystals was obtained by combining EMPA (for medium-Z elements) and SIMS analyses (for low- and high-Z elements); their results are in excellent agreement with the chemical information obtained from the structure refinements. The crystal-chemical formulae of the crystals studied, recalculated on the basis of 24 anions, are [sup.M3,4][Ca.sup.M2.sub.4] [[RE[E.sub.1.45][Ca.sub.0.37][(Th,U).sup.4+.sub.0.17] [Y.sub.0.01]].sub.[SIGMA]=2][sup.M1][([Al.sub.0.50] [Fe.sup.3+.sub.038][Mg.sub.0.03] [Ti.sup.4+.sub.0.07]).sub.[SIGMA]=0.99][sup.T] ([Be.sub.1.18][Li.sub.0.02][[square].sub.0.37]) [B.sub.3.99][Si.sub.3.98][O.sub.22][sup.O5][([O.sup.2-.sub.1.04] [F.sub.0.53]O[H.sub.0.43]).sub.[SIGMA]=2] for mottanaite-(Ce); and [sup.M3,4][Ca.sub.4][sup.M2][[RE[E.sup.3+.sub.0.72] [(Th,U).sup.4+.sub.0.66][Ca.sub.0.60][Y.sub.0.02]].sub.[SIGMA]=2] [sup.M1][([Al.sub.0.48][Fe.sup.3+.sub.0.38][Ti.sup.4+.sub.0.10][Mg.sub.0.05] [Mn.sup.3+.sub.0.02]).sub.[SIGMA]=1.03][sup.T] ([Be.sub.0.82][[square].sub.0.60][Li.sub.0.04]) [B.sub.4.00][Si.sub.4.00][O.sub.22][sup.O5][([O.sup.2-.sub.0.97]O [H.sub.0.54][F.sub.0.49]).sub.[SIGMA]=2] for ciprianiite.

Experiments to define the critical curve for a series of silicate melts in equilibrium with a hydrous fluid were carried out in a hydrothermal diamond anvil cell. Silicate compositions studied were albite with several wt% excess [Na.sub.2]O, [B.sub.2][O.sub.3] and [F.sub.2][O.sub.-1]. Complete miscibility between melt and water was observed at lower pressure and temperature conditions compared to pure albite for all compositions. For albite + excess [Na.sub.2]O, the critical curve had been lowered by 143 and 247[degrees]C at 10 kbar for 5 and 10 wt% excess Na. For albite + 5 and 10 wt% F, the difference at 10 kbar was 147 and 246[degrees]C respectively, and for albite +5 and 10 wt% B differences of 168 and 262[degrees]C were found. These results are likely to be additive, with the presence of more than one of the components depressing the critical curve to even lower temperatures and pressures. The results suggest that in complex pegmatitic systems, complete miscibility between melt and fluid may be important in the final stages of crystallisation. The unusual properties of fluid phases under conditions close to the critical curve in a silicate melt-water system may be essential for the enrichment of trace elements in pegmatites as well as for the formation of typical pegmatite texture.

To understand compositional variation in igneous biotite, full analyses of a suite of biotites of variable composition from volcanic and xenolith parageneses have been completed. Major and minor elements were determined by electron microprobe analysis, water was determined by manometry and SIMS analysis, and [Fe.sup.3+]/[Fe.sup.2+] was determined by microXANES and Mossbauer spectroscopy. Our new data, together with previous biotite analyses (total of 52 analyses), reveal correlations between [O.sup.2-] (2-F-Cl-OH) and the sum of the octahedral cations Al + Ti + [Fe.sup.3+] + Cr. This correlation allows estimation of either OH-or [Fe.sup.3+]/[Fe.sup.2+] as long as one or the other has been determined. The hydroxyl site in most mantle micas contains at least 1.0 [O.sup.2-] atoms per formula unit (apfu), indicating that the oxy-component cannot be ignored. The large oxy-component in melt inclusion micas from the martian meteorite Chassigny does not necessarily indicate oxidized or hydrous magmatic conditions because dehydrogenation may have occurred and/or because the oxy-component may be stable at low oxygen fugacity. The large variation in Ti, Al, and [Fe.sup.3+] in natural igneous micas is most likely dependent upon bulk compositional differences in each specific system such as variation of [a.sub.Ti[O.sub.2]] and [a.sub.[Al.sub.2][O.sub.3]] in silicate melts.

Fluorine-, boron- and phosphorus-rich pegmatites of the Variscan Ehrenfriedersdorf complex crystallized over a temperature range from about 700 to 500 [degrees] C at a pressure of about 1 kbar. Pegmatite quartz crystals continuously trapped two different types of melt inclusions during cooling and growth: a silicate-rich [H.sub.2]O-poor melt and a silicate-poor [H.sub.2]O-rich melt. Both melts were simultaneously trapped on the solvus boundaries of the silicate (+ fluorine + boron + phosphorus) -- water system. The partially crystallized melt inclusions were rehomogenized at 1 kbar between 500 and 712 [degrees] C in steps of 50 [degrees] C by conventional rapid-quench hydrothermal experiments. Glasses of completely rehomogenized inclusions were analyzed for [H.sub.2]O by Raman spectroscopy, and for major and some trace elements by EMP (electron microprobe). Both types of melt inclusions define a solvus boundary in an [X.sub.[H.sub.2]O]-T pseudobinary system. At 500 [degrees] C, the silicate-rich melt contains about 2.5 wt% [H.sub.2]O, and the conjugate water-rich melt about 47 wt% [H.sub.2]O. The solvus closes rapidly with increasing temperature. At 650 [degrees] C, the water contents are about 10 and 32 wt%, respectively. Complete miscibility is attained at the critical point: 712 [degrees] C and 21.5 wt% [H.sub.2]O. Many pegmatites show high concentrations of F, B, and P, this is particularly true for those pegmatites associated with highly evolved peraluminous granites. The presence of these elements dramatically reduces the critical pressure for fluid--melt systems. At shallow intrusion levels, at T [greater than or equal to] 720 [degrees] C, water is infinitely soluble in a F-, B-, and Prich melt. Simple cooling induces a separation into two coexisting melts, accompanied with strong element fractionation. On the water-rich side of the solvus, very volatile-rich melts are produced that have vastly different physical properties as compared to 'normal' silicate melts. The density, viscosity, diffusivity, and mobility of such hyper-aqueous melts under these conditions are more comparable to an aqueous fluid.

Experiments in the systems diopside-albite (Di-Ab) and diopside-albite-dolomite (Di-Ab-Dmt), doped with a wide range of trace elements, have been used to characterise the difference between clinopyroxene-silicate melt and clinopyroxene-carbonate melt partitioning. Experiments in Di-Ab-Dmt yielded clinopyroxene and olivine in equilibrium with C[O.sub.2]-saturated dolomitic carbonate melt at 3 GPa, 1375 [degrees] C. The experiments in Di-Ab were designed to bracket those conditions (3 GPa, 1640 [degrees] C and 0.8 GPa, 1375 [degrees] C), and so minimise the contribution of differential temperature and pressure to partitioning. Partition coefficients, determined by SIMS analysis of run products, differ markedly for some elements between Di-Ab and Di-Ab-Dmt systems. Notably, in the carbonate system clinopyroxene-melt partition coefficients for Si, Al, Ga, heavy REE, Ti and Zr are higher by factors of 5 to 200 than in the silicate system. Conversely, partition coefficients for Nb, light REE, alkali metals and alkaline earths show much less fractionation (

Experiments ranging from 2 to 3 GPa and 800 to 1300 [degrees] C and at 0.15 GPa and 770 [degrees] C were performed to investigate the stability and mutual solubility of the [K.sub.2]Zr[Si.sub.3][O.sub.9] (wadeite) and [K.sub.2]Ti[Si.sub.3][O.sub.9] cyclosilicates under upper mantle conditions. The [K.sub.2]Zr[Si.sub.3][O.sub.9]-[K.sub.2]Ti[Si.sub.3][O.sub.9] join exhibits complete miscibility in the P-T interval investigated. With increasing degree of melting the solid solution becomes progressively enriched in Zr, indicating that [K.sub.2]Zr[Si.sub.3][O.sub.9] is the more refractory end member. At 2 GPa, in the more complex [K.sub.2]Zr[Si.sub.3][O.sub.9]-[K.sub.2]Ti[Si.sub.3][O.sub.9]-[K.sub.2] [Mg.sub.6][Al.sub.2][Si.sub.6][O.sub.20][(OH).sub.4] system, the presence of phlogopite clearly limits the extent of solid solution of the cyclosilicate to more Zr-rich compositions [Zr/ (Zr + Ti) > 0.85], comparable to wadeite found in nature, with Ti[O.sub.2] partitioning strongly into the coexisting mica and/or liquid. However, at 1200 [degrees] C, with increasing pressure from 2 to 3 GPa, the partitioning behaviour of Ti[O.sub.2] changes in favour of the cyclosilicate, with Zr/(Zr + Ti) of the [K.sub.2](Zr, Ti)[Si.sub.3][O.sub.9] phase decreasing from ~0.9 to ~0.6. The variation in the Ti content of the coexisting phlogopite is related to its degree of melting to forsterite and liquid, following the major substitution [sup.VI]Ti + ?? = 2 [sup.VI]Mg.

By assuming an ideal two-component mixture of (Mg,Fe)SiO(sub 3) perovskite (MgPv) and (Mg,Fe)O magnesiowustite (Mw), and by using a thermoelastic model for mantle minerals developed previously, we can reproduce the PREM values of density and velocities (nu)(sub p) and (nu)(sub s) of compressional and shear waves of the lower mantle within +/- 0.12%, +/- 0.28%, and +/- 0.56% except for the transition layers at the both boundaries. The molar fractions and atomic fractions of iron for MgPv and Mw were adjusted to reproduce the PREM values of (RHO), (nu)(sub p), and (nu)(sub s) above the point of z = 871 km (which is slightly inside the lower mantle) under constant-entropy condition. This depth avoids the boundary effect. The adiabatic bulk and shear moduli of the mixture are calculated by the Hashin-Shtrikman method for MgPv and Mw and then arithmetically averaged. The temperature profile was calculated assuming that the lower mantle is adiabatic and T(670 km) = 1873 K. The temperature at the top of D' becomes 2444 K. Being added the temperature increment of 840 K over D' (z = 2741-2891 km) estimated by Stacey and Loper (1983) to our value, the temperature at the core-mantle boundary (CMB) becomes 3284 K in agreement with T(CMB) of 3300 +/- 500 degrees C by Brown and McQueen. The molar ratios of Fe/(Mg + Fe) and (Mg + Fe)/Si become 0.12 and 2.10. The calculated thermal expansivity, (alpha), of the mixture under lower mantle conditions is in agreement with (alpha) of the lower mantle calculated directly from PEM data by Brown and Shankland, and Anderson. For the addition of 5 mol% of CaSiO(sub 3) perovskite to our model, the essential feature of the result is unchanged and the wt% of SiO(sub 2), MgO, FeO, and CaO become 40.7, 44.6, 11.0, and 3.7.

Author's Abstract, COPYRIGHT 1999, Mineralogical Society of America Khmaralite, Ca(sub 0.04)Mg(sub 5.46)Fe(super 3+)(sub 0.12)Fe(super 2+)(sub 1.87)Al(sub 14.26)Be(sub 1.43)B(sub 0.02)Si(sub 4.80)O(sub 40), is a new mineral cosely related to sapphirine from Khmara Bay, Enderby Land, Antarctica. It occurs in a pegmatite metamorphosed at T>820 degrees C, P>10kbar. The minerals surinamite, musgravite, and sillimanite associated with khmaralite at Casey Bay Saturate it in BeO, and thus its BeO content could be close to the maximum possible. Optically, khmaralite is biaxial (-); at lambda = 589 nm, alpha = 1.725(2), beta = 1.740(2), gamma = 1.741(2) 2V(sub meas) = 34.4 (1.8) degrees, v>r strong, and beta // b. The weak superstructure reported using electron diffraction has been confirmed by single-crystal X-ray diffraction. The superstructure corresponds to a doubling of the a axis in monoclinic sapphirine-2M (P2(sub 1)/c setting) with the following unit-cell parameters: a = 19.800(1), b = 14.371(1), c = 11.254(1) Angstroms, Beta = 125.53(1) degrees, Z = 4, D(sub calc) = 3.61 g/cm(super 3). Using a simplified chemical formula of Mg(sub 5.46)Al(sub 14.28)Fe(sub 2.00)Si(sub 4.80)Be(sub 1.46)O(sub 40), the fully anisotropic structure refinement of all site occupancies, including Al vs Mg and Al vs. Si on the 16 octahedral (M) and 12 tetrahedral (T) sites respectively, establishes that the 2Xa superstructure results primarily from ordering in the doubled (T1 > T6, T7 > T12) tetrahedral chain, which is parallel to the a axis. The strongest contribution comes from Al-Si-Be ordering on the T2 (33-16-51%) vs. T8 (0-95-5%) and T3 (4-78-18%) vs. T9 (30-0-70%) sites. Thus, the sequence Al-Si-Al in sapphirine is replaced by the sequence Si-Be-Si in khmaralite; i.e., Be replaces Si on two sites and Si replaces Al on on four adjacent sites, resulting in an indirect replacement of Al by Be by the coupled substitution Be + Si = 2Al. The Be distribution in khmaralite and the strong preference for Be/Al mixing over Be/Si mixing appear to satisfy the bonding requirements of the bridging O atoms by minimizing the number of Be-O-Be and Be-O-Al linkages.

Author's Abstract, COPYRIGHT 1999, Meneralogical Society of America The real structures of clinotobermorite, tobermorite 9 Angstroms, and tobermorite 11 Angstroms were determined through the application of OD approach, which allowed us to explain their peculiar disorder and polytypic features and to derive the main polytypes for each of them. The structural arrangements will be described and discussed for one polytype of each compound: clinotobermorite, triclinic polytype C1, a = 11.274, b = 7.344, c = 11.468 Angstroms, alpha = 99.18 degrees, beta = 97.19 degrees, gamma = 90.03 degrees; tobermorite 9 Angstroms, triclinic polytype C1, a = 11.156, b = 7.303, c = 9.566 Angstroms, alpha = 101.08 degrees, beta = 92.83 degrees, gamma = 89.98 degrees; tobermorite 11 Angstroms, monoclinic polytype B11m, a = 6.735, b = 7.385, c = 22.487 Angstroms, gamma - 123.25 degrees. Common structural features are infinite layers, parallel to (001), formed by sevenfold-coordinated calcium polyhedra. Tetrahedral double chains, built up through condensation of 'Dreiereinfachketten' of wollastonite-type and running along b, link together adjacent calcium layers in clinotobermorite and tobermorite 11 Angstroms, whereas single tetrahedral chains connect adjacent calcium layers in tobermorite 9 Angstroms. The relatively wide channels of clinotobermorite and tobermorite 11 Angstroms host 'zeolitic' calcium cations and water molecules. The present structural results now allow for a sound discussion of the crystal chemical relationships between the various members of the tobermorite group and an explanation of the peculiar thermal behavior of tobermorite 11 Angstroms.

Author's Abstract, COPYRIGHT 1999, Mineralogical Society of America To verify the oxidation-dehydrogenation reaction induced by heat treatment of ilvaite in air, a crystal from Elba Island, Italy, was selected for annealing experiments and crystal chemical study. X-ray intensity data were collected and structure refinement was performed after each heat treatment (temperatures ranges from 400 to 700 degrees C). Oxidation of Fe(super 2+) as well as loss of H atoms were deduced from examination of the structural changes occurring as the heating temperature was increased.

The crystal structure of a Th-U-rich, Y-poor hellandite-(Ce) occurring in a volcanic ejectum from Capranica (Vico volcanic complex, Latium, Italy) was refined to an R index of 1.5% for 2226 observed reflections [I > 3 [Sigma](I)]. Hellandite from Capranica is monoclinic P2/a with a = 19.068 (8), b = 4.745 (2), c = 10.289 (3) [Angstrom], [Beta] = 111.18 (3)[degrees]. The extra-framework (distorted) tetrahedral cavity, usually occupied by H in hellandite, is partially (35-40%) occupied by Be and/or Li. When this is the case, the [[B.sub.4][Si.sub.4][O.sub.22]] tetrahedral chains of hellandite are locally cross-linked to form a sheet, which is built up by the same types of rings as in semenovite but in a different arrangement. The structure refinement and the electron- and ion-microprobe analyses provide constraints on hellandite-group crystal chemistry. The presence of (4)(Be, Li) at the Be site is coupled with the substitution of F and/or O for OH at the O5 site; the amount of extra-framework cations thus constrains the OH content in the general formula. X-ray data exclude the presence of other OH sites in the structure, in agreement with direct H determination by ion probe (1.02 H apfu + 0.98 F apfu = 2.0 apfu of monovalent anions at 05 vs. 4.5-8 apfu proposed previously). The complete chemical analysis also eliminates the need for a vacancy at the REE sites. The general formula of hellandite-group minerals is thus[(Ca,REE).sub.8] (Th,U,Y,REE).sub.4][ Ti,[Fe.sup.3+],Al).s b.2][(O,F,O[H.sub.x]).sub.4][(Be,Li).sub.4-x][Si.sub.8][B.sub.8][O.sub.44], where electroneutrality is assured by the appropriate mixing of heterovalent substituents at the M sites.

Raman spectra were measured and normal mode calculations were performed for phlogopite. Calculated fundamental mode frequencies were fit to observed frequencies assigned to features in the two polarized Raman spectra collected, so that unambiguous assignment of eigenmodes could be made to nearly all Raman spectral features. Average force constant values determined for Si-O and O-Si-O environments as well as for the non-silicate environments in cyclosilicates and gillespite provided an excellent starting point for the phlogopite calculations that quickly converged to a best fit of theoretical mode frequencies to the observed Raman fundamental mode frequencies. Final values of the force constants for T-O stretch and O-T-O bend are within the range of force constant values determined for Si-O and O-Si-O environments in cyclosilicates and gillespite. The force constant similarities extend to the K-O, M(1,2)-O bond stretch and M(1,2)-O-T bond bend environments. Raman-active modes at frequencies greater than 850 [cm.sup.-1] are localized intra-sheet modes dominated by T-O stretch and O-T-O bend motions. Raman-active modes between 350 and 850 [cm.sup.-1] are dominated by mixtures of internal sheet modes with displacements from the M2 sites. Raman-active lattice modes are calculated at frequencies less than 350 [cm.sup.-1]. Calculated frequencies for the IR-active [A.sub.u] and [B.sub.u], modes generally fall within the frequency ranges of broad bands in the IR spectra for phlogopite presented elsewhere. Eigenmodes calculated for phlogopite are generally unique to that mica structure and differ from those determined for the cyclosilicates and gillespite. The few eigenmodes calculated for phlogopite that resemble those calculated for the cyclosilicates are the most localized T-O stretch motions and are assigned to the highest frequency modes.

Akimotoite (Mg,Fe)SiO(sub3) is a new ilmenite group silicate mineral found in the Tenham chondrite. The natural occurrence, chemical composition, crystallography and likely formation mechanism of the (Mg,Fe)SiO(sub3) ilmenite phase is described. Its mode of occurrence differs from that in the Acfer 040 chondrite, suggesting two mechanisms for akimotoite and silicate perovskite formation in the shocked chondrites.

Premelting effects are common in silicates and have been mostly observed as significant heat capacity anomalies beginning 100 to 200 [degrees] C below congruent melting points. To assess the role of cation dynamics in this phenomenon, we collected nuclear magnetic resonance (NMR) spectra and relaxation time data to within 20 to 50 [degrees] C of the melting points of sodium metasilicate ([Na.sub.2]Si[O.sub.3]), which displays a large premelting effect, and isostructural lithium metasilicate ([Li.sub.2]Si[O.sub.3]), which displays little premelting. From 7Li NMR, [Li.sup.+] site hopping is clearly observed in [Li.sub.2]Si[O.sub.3] by a partial averaging of the 7Li quadropolar peak shape, requiring exchange among a few, ordered orientations of Li[O.sub.4] tetrahedra. From 23Na NMR, [Na.sup.+] site hopping in [Na.sub.2]Si[O.sub.3] appears to involve a more liquid-like behavior, implying exchange among many sites with different orientations in a disordered fashion. For this phase, 29Si spectra indicate that in an oxidizing environment, no liquid phase is present at 20 [degrees] C below the melting point, well within the calorimetric premelting regime. However, partial averaging of the low-temperature, biaxial chemical shift powder pattern (typical of Si sites in chain silicates) occurs, suggesting some kind of extensive, librational motion of Si[O.sub.4] tetrahedra that is possibly linked to rapid [Na.sup.+] diffusion near the melting point. In contrast to the simple [Li.sup.+] diffusion in [Li.sub.2]Si[O.sub.3], this process may require considerable nonvibrational energy and may thus be related to the heat capacity anomaly just prior to melting.

Premelting effects are common in silicates and have been mostly observed as significant heat capacity anomalies beginning 100 to 200 [degrees] C below congruent melting points. To assess the role of cation dynamics in this phenomenon, we collected nuclear magnetic resonance (NMR) spectra and relaxation time data to within 20 to 50 [degrees] C of the melting points of sodium metasilicate ([Na.sub.2]Si[O.sub.3]), which displays a large premelting effect, and isostructural lithium metasilicate ([Li.sub.2]Si[O.sub.3]), which displays little premelting. From 7Li NMR, [Li.sup.+] site hopping is clearly observed in [Li.sub.2]Si[O.sub.3] by a partial averaging of the 7Li quadropolar peak shape, requiring exchange among a few, ordered orientations of Li[O.sub.4] tetrahedra. From 23Na NMR, [Na.sup.+] site hopping in [Na.sub.2]Si[O.sub.3] appears to involve a more liquid-like behavior, implying exchange among many sites with different orientations in a disordered fashion. For this phase, 29Si spectra indicate that in an oxidizing environment, no liquid phase is present at 20 [degrees] C below the melting point, well within the calorimetric premelting regime. However, partial averaging of the low-temperature, biaxial chemical shift powder pattern (typical of Si sites in chain silicates) occurs, suggesting some kind of extensive, librational motion of Si[O.sub.4] tetrahedra that is possibly linked to rapid [Na.sup.+] diffusion near the melting point. In contrast to the simple [Li.sup.+] diffusion in [Li.sub.2]Si[O.sub.3], this process may require considerable nonvibrational energy and may thus be related to the heat capacity anomaly just prior to melting.

A study was conducted to analyze the concentration dependence of trans-2-butene diffusion in silicalite-2 zeolite using a jump diffusion model. Molecular dynamic computations were carried out at 623 K in a simulation cell. The site energies and the fundamental rates of the jump diffusion framework were then determined from the molecular dynamics simulations. Results indicated that the jump diffusion model supported the self-diffusivity of butene isomers in zeolite.

The equilibrium conditions hindering the stability fields of clinohumite-OH, chondrodite-OH and phase A were investigated through reversed equilibrium experiments of up to 100 kilobar within the MgO-SiO2-H2O system. Only seven out of 12 Dense Hydrous Magnesium Silicates were known to be synthesized at pressures less than or equal to 100 kbar. The large stability fields of clinohumite-OH and chondrodite-OH went beyond pressure ranges of 80 kbar although breakdown assemblages from the humite-group phases were not distinct and breakdown of phase A was observed at higher temperatures.

The structure of sodium tetrasilicate ([N.sub.2][Si.sub.4][O.sub.9]) glasses containing 0 to 10 wt% water was investigated by a combination of Raman, IR, and NMR methods. Both the 29Si magic angle spinning NMR data and Raman spectra in the Si-O stretching region clearly show that water depolymerizes the silicate network of the glasses. Q-species distributions calculated from Raman spectra, assuming equal scattering cross sections of all bands in the Si-O stretching region, closely agree with results obtained from NMR data. At low total water contents, the silicate network is depolymerized mainly by breaking of [Q.sup.4]-[Q.sup.4] bonds, whereas breaking of [Q.sup.3]-[Q.sup.3] bonds dominates at high water contents. Near IR spectra show the presence of both OH groups and molecular [H.sub.2]O in the glasses. The number of nonbridging O atoms per silicon atom, calculated from the near IR data, closely agrees with the results obtained from Raman and NMR, and confirms the assignment of the 4500 [cm.sup.-1] band in the near IR to a combination mode of Si-OH groups. Moreover, the intensity of the fundamental Si-OH stretching band at 910 [cm.sup.-1] in the Raman spectra varies proportionally to the intensity of the 4500 [cm.sup.-1] near IR band. Both IR and Raman spectra show three main bands in the OH-stretching region, centered at 3580, 3000, and 2350 [cm.sup.-1], due to hydrous species with different hydrogen bond strengths. The relative intensities of these three bands are insensitive to total water content and OH/[H.sub.2]O ratio, suggesting that both OH and [H.sub.2]O contribute to each of these bands. This is consistent with the fine structure of the [H.sub.2]O bending vibration in the IR spectra around 1640 [cm.sup.-1] and with the polarization dependence of the OH-stretching bands in the Raman spectra. Near IR spectra of hydrous sodium tetrasilicate glasses and hydrous aluminosilicate glasses are very similar and show a similar dependence of band intensity on total water content, suggesting that there is no fundamental difference in the dissolution mechanism of water in these systems.

Recent discovery of wollastonite-bearing calc-silicate assemblages adjacent to gneiss-charnockite horizons in the supracrustal terrain of the Kerala Khondalite Belt, southern India, provides an opportunity to evaluate the carbonic fluid infiltration model proposed for charnockite formation. Petrological and fluid inclusion studies across these horizons in three representative localities are presented in this study. The calc-silicate assemblages define peak metamorphic conditions of -800 [degrees]C at 5 kbar and define a low [alphaCO.sub.2]. Adjacent charnockite assemblages developed through dehydration involving the breakdown of garnet, biotite and quartz to produce orthopyroxene under low [alphaH.sub.2]O conditions. Retrograde reactions preserved in the calc-silicate rocks, such as scapolite-quartz symplectites, and the partial breakdown of wollastonite previously has been attributed to a near isothermal decompression during which infiltration of [CO.sub.2]-rich fluids occurred. Fluid inclusion studies indicate that the earliest generation of fluids preserved in the calc-silicate assemblages are aqueous (with salinity 8 wt % NaCl equivalent), consistent with mineral phase equilibria defining low [alpha.sub CO.sub 2]. The estimation of NaCl content in brines coexisting with scapolite, based on the Cl content of the scapolite, indicates the presence of up to 20 wt % NaCl during the formation of scapolite consistent with the saline primary fluid inclusions. Primary carbonic inclusions occur within the retrogressed calcite + quartz assemblage after wollastonite, and are considered to represent the post-peak metamorphic carbonic fluid infiltration event, synchronous with the development of charnockites in the adjacent gneisses. These inclusions have identical characteristics to those in the charnockites. We envisage that the Kerala Khondalite Belt fluid regime was largely internally buffered during the prograde path, and that [CO.sub.2] infiltration post-dated peak metamorphism. Influx of [CO.sub.2] was mostly structurally controlled, and occurred along a near-isothermal uplift path. Graphite-bearing pegmatitic dykes with abundant [CO.sub.2]-rich inclusions in these localities attest to the transfer of carbonic fluids through magmatic conduits.

Two series of leucite group materials, with K, Rb, and Cs as extra-framework cations, have been synthesized by ion exchange from a natural well-ordered analcite and a natural disordered leucite. 29Si and 27Al MAS NMR data for the analcite-derived series provide complementary information on tetrahedral cation ordering. The ordering in terms of the number of Al next-nearest neighbors, Q4(nAl) (short-range order), does not change significantly during ion exchange, indicating that Al and Si remain essentially fixed in their original positions. In contrast, the ordering of Al over T1, T2, and T3 (long-range order) for the analcite-derived series changes dramatically with changing alkali cation; the Al occupancies for the three analcite-derived samples expressed as T1:T2:T3 are approximately 0.25:0.50:0.25 for KAl[Si.sub.2][O.sub.6], 0.40:0.20:0.40 for RbAl[Si.sub.2][O.sub.6], and 0.15:0.70:0.15 for CsAl[Si.sub.2][O.sub.6]. During the ion exchange, at temperatures above the cubic-tetragonal phase transitions, only one symmetrically distinct T site is present. It is proposed that on cooling through the cubic-tetragonal phase transition the structure collapses around the non-framework cations to give the lowest energy Si-Al distribution over the three T sites irrespective of the original T-site ordering in the starting material. Our data suggest that the identity of the cation in the W site affects the orientation of the framework distortions associated with the cubic-tetragonal phase transition and leads to the possibility that a particular tetrahedral cation site can take on the characteristics of a T1, T2, or T3 site. The data and their interpretation have important implications for the mechanism of this type of structural phase transition.

The state of ordering among network-modifier cations in molten silicates has a potentially large effect on their overall configurational entropy, on free energies of mixing, and on viscosity and diffusivity. Most models of thermodynamic and transport properties assume random mixing, but there is relatively little direct microstructural information to constrain the real extent of this disorder. Two-dimensional 17O NMR can produce spectra that are free of quadrupolar broadening and thus in which peak widths for non-bridging oxygen sites directly reflect the extent of disorder in the local structural environment. In this report, we describe new data from triple-quantum magic-angle-spinning (3QMAS) NMR for a series of barium and calcium silicate glasses. Results are best explained by completely random mixing of Ba and Ca, confirming conventional modeling assumptions. Other recent data show, however, that significant ordering may be present (at least at the temperature of the glass transition) for modifier cations with greater differences in size or charge, and among network-forming cations.

Relaxation times of longitudinal strain for five supercooled liquids along the join Si[O.sub.2] (qz)-NaAlSi[O.sub.4] (ne) including the compositions of albite (ab), [ab.sub.50][jd.sub.50], jadeite (jd), [jd.sub.66][ne.sub.33], and [jd.sub.33][ne.sub.66] were measured by photon correlation spectroscopy within the temperature and relaxation time ranges 711-1116 [degrees] C and 35-2 x [10.sup.-4]s, respectively. The measured time correlation functions are fitted to the Kohlrausch-Williams-Watts equation, [[Phi].sup.l] (t) = [Mathematical Expression Omitted], yielding isothermal relaxation functions for longitudinal strain at constant stress. The temperature dependence of the longitudinal strain relaxation times obeys an Arrhenian type equation. The relaxation of the longitudinal strain cannot be described by a single relaxation time but by a relaxation time distribution. The width of the relaxation time distribution, which is represented by the parameter [Beta], is independent over a large temperature range, indicating thermorheological simplicity. The decrease of the parameter [Beta] corresponds to an increase of the excess (configurational) heat capacity with increasing (Na+Al) content, together indicating an increasing fragility of the supercooled liquids toward the composition of NaAlSi[O.sub.4]. Increasing fragility is also shown from the activation energy obtained from longitudinal strain relaxation and shear viscosity measurements, which passes a minimum near 40 mol% NaAlSi[O.sub.4] at low temperatures but decreases toward NaAlSi[O.sub.4] composition at higher temperatures. A comparison between strain, stress, and enthalpy relaxation times shows clearly that the relaxation process resulting from a thermal pertubation is slower than from a mechanical perturbation.

A quantitative model describing the partitioning of rare earth elements (REE) between clinopyroxene and anhydrous silicate melt is presented, based on the Brice equation. Both homovalent and heterovalent substitution were considered, to maintain a general approach. Experimental work should now concentrate on the details of melt-compositional effects, which were lost by the simple melt model in the scatter of partition coefficient data.

The density of silicate liquids is important to the dynamic behaviour and chemical differentiation of magmas. The direct density measurements available for hydrous silicate liquids of Burnham and Davis are currently the only measurements available. A study aimed to give volume measurements of hydrous albitic liquids that could obtain partial molar volume, thermal expansivity, and compressibility of the H20 total component in silicate liquids. Measurements were consistent with Burnham and Davis.

Ordered mesostructured porous silicas that are also macroscopically structured were created by control of the interface on two different length scales simultaneously. Micellar arrays controlled the nanometer-scale assembly, and at the static boundary between an aqueous phase and an organic phase, control was achieved on the micrometer to centimeter scale. Acid-prepared mesostructures of silica were made with the p6, Pm3n, and the [P6.sub.3]/mmc structures in the form of porous fibers 50 to 1 000 micrometers in length, hollow spheres with diameters of 1 to 100 micrometers, and thin sheets up to 10 centimeters in diameter and about 10 to 500 micrometers in thickness. These results might have implications for technical applications, such as slow drug-release systems or membranes, and in biomineralization, where many processes are also interface-controlled., As noted by Israelachvili (1), microemulsions and emulsions occupy a special place in the hierarchy of structures, in that their formation involves long-range forces with an energy of assembly, including [...]

Orthosilicate staurolite weathers slowly under natural conditions due to either the stable kyanite-like ribbons or the low site-energy of the Fe regions. The etch pits are disk-shaped, and are wide parallel to the plane of weak bonding and cleavage (010) and narrow perpendicular to (010). The kinetics of weathering is interface-limited rather than transport limited. Staurolite is unable to form protective covering layers in spite of a sufficient Al content due to slow weathering. The slow weathering restricts the release of Al in concentrations greater than that of secondary minerals.