Band 87, Heft 4

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

Secondary metamorphic biotite grew with sillimanite and garnet at the expense of cordierite during a Pan-African metamorphic event overprinting Grenville age granulite-facies metapelites at the western margin of the Namaqualand Metamorphic Complex, South Africa. Fe-Mg exchange thermometry and average pressure calculations constrain the conditions of the overprinting metamorphism to about 660 °C at 6 kbar. Biotite compositions reflect chemical equilibrium in local domains defined by partly to completely pseudomorphed cordierite grains. They show a continuous range of Ti content from 0.0 to 4.5 wt% TiO 2 depending on the proximity of the domain to a source of Ti such as ilmenite. Thus, they allow an assessment of the Ti substitution that is independent of metamorphic grade variation. Significant correlations between Ti and microprobe analysis total indicate that Ti is accommodated largely through a deprotonation substitution Ti 4+ + 2O 2- + H 2 → M 2+ + 2OH - (where M 2+ is a divalent octahedral cation) rather than by substitutions involving octahedral vacancies. Ti shows a positive correlation with K, and strong negative correlations with octahedral Al and the sum of divalent cations. Ti content also varies with molar Mg/(Mg + Fe) in biotite (XMg), because the process of cordierite replacement involves the mass transfer of Fe into cordierite domains. However, the above trends are also shown by a set of analyses with uniform XMg. In detail, the correlations depend on the method of formula recalculation. If the deprotonation substitution is dominant, normalization should not be made to a constant number of oxygen atoms. A statistical treatment of biotite analyses normalized to 14 tetrahedral and octahedral cations per formula unit suggests that the introduction of Ti involves deprotonation of two OH groups per Ti, and that the replacement of some octahedral Al by Ti is balanced by increases in K and/or Si in other sites. An appropriate Ti-biotite end-member may be K 2 [Mg 4 Ti 2 ]Al 2 Si 6 O 24 , rather than the vacancy-bearing end-members proposed in other studies. In view of the fact that even Ti-free biotite appears to contain a small proportion of octahedral vacancies, a suitable recalculation scheme for microprobe analyses may involve normalizing to 22 + Ti O atoms (neglecting H 2 O).

The enthalpies of drop solution in lead borate (2 PbO·B 2 O 3 ) of four Mg-staurolite samples, synthesized at 720 °C and pressures between 2 and 5 GPa, were measured by high-temperature oxidemelt calorimetry at 702 °C. Staurolite compositions, determined by electron microprobe analysis, Karl-Fischer titration, and thermogravimetry, are: Mg 3.71 Al 18.17 Si 7.60 O 44.31 (OH) 3.69 , Mg 3.87 Al 17.65 Si 7.75 O 43.68 (OH) 4.32 , Mg 3.66 Al 17.76 Si 7.68 O 43.31 (OH) 4.69 , and Mg 3.58 Al 18.05 Si 7.43 O 43.01 (OH) 4.99 . The enthalpy of drop solution of the bulk samples (as well as the calculated values for the enthalpy of formation from the elements of Mg-staurolite) are strongly correlated to the H content of the samples. The enthalpy of formation from the elements is best described by the linear relation Δ f H 0 298 (Mgstaurolite) = (-25357.58 + 87.35 N) kJ/mol, where N = number of H atoms per formula unit in Mgstaurolite. The enthalpy of drop solution of two partially dehydrated Mg-staurolite samples is in a good agreement with the linear relation. Phase-equilibrium data for Mg-staurolite (Fockenberg 1998) were recalculated using the stoichiometric formula Mg 3.5 Al 18 Si 7.75 O 44 (OH) 4 . Based on these mineral equilibria and the internally consistent data set of Berman (1988), a mathematical programming analysis of the thermodynamic data of Mg-staurolite gave Δ f H 0 298 [Mg 3.5 Al 18 Si 7.75 O 44 (OH) 4 ] = -25005.14 kJ/mol, and S 0 298 [Mg 3.5 Al 18 Si 7.75 O 44 (OH) 4 ] = 937.94 J/(K·mol). Thus, for the first time, reliable thermodynamic data for Mg-staurolite, based on experimental constraints, are provided.

Regionally metamorphosed, muscovite-bearing quartzites from Sivrihisar, Turkey, contain coexisting andalusite, kyanite, and sillimanite. Kyanite is the most abundant polymorph and defines a lineation along with prismatic sillimanite, andalusite, staurolite, and elongate quartz. Andalusite is the most Fe-rich of the polymorphs (0.9-1.6 wt% Fe 2 O 3 , compared with 0.6-0.9 wt% for kyanite and sillimanite), and was ductilely deformed. Staurolite has partially pseudomorphed kyanite, and occurs intergrown with sillimanite. Garnet occurs in some metaquartzites and interlayered mica schists. Mica schists lack Al 2 SiO 5 polymorphs. Porphyroblasts in mica schists are chloritoid, chloritoid + staurolite ± garnet, or staurolite ± garnet with inclusions of chloritoid and staurolite. Textural relations and relative deformation features among the three polymorphs suggest the crystallization sequence andalusite → kyanite → sillimanite, with growth of staurolite primarily in the sillimanite stability field. Results of garnet-biotite thermometry and estimates of peak P-T conditions from petrogenetic grids are ~540-560 °C. Maximum pressure is not known, but there is no evidence for the high-pressure - low-temperature conditions that affected similar rocks to the northwest in the Sivrihisar massif. The stable coexistence of staurolite + sillimanite at peak metamorphic conditions suggests a maximum pressure of 5.5 kbar. The three polymorphs grew sequentially along a clockwise P-T path that looped around the Al 2 SiO 5 triple point during incipient subduction of a continental margin sequence (burial: andalusite → kyanite), followed by collision (heating and/or decompression: kyanite → sillimanite).

Massive quartz veins with large andalusite and sillimanite crystals in textural equilibrium were found in a Proterozoic province in the northern Colorado Front Range. The d 18 O values of the andalusite and sillimanite are identical, supporting the idea that these aluminum-silicate (AS) polymorphs crystallized at the same time. These data are consistent with the findings of Sharp (1995), who reported no fractionation associated with polymorphism of the aluminum silicates. Quartz-aluminum- silicate fractionations from veins in textural equilibrium and disequilibrium are 2.63-2.93 and 2.20-2.25‰, respectively. Temperature estimates from quartz-aluminum-silicate oxygen isotope fractionations range from 603-652 °C for equilibrium veins, and geologically unreasonable temperatures of 728-788 °C from veins in textural disequilibrium. Formation temperatures determined from isotope thermometry constrain the location of the veins on the andalusite = sillimanite univariant curve, thus defining a pressure range of 1.2-2.6 kbar for vein formation. The vein P-T estimates from this study agree well with previous host rock P-T estimates from nearby localities and, in conjunction with available Ar thermochronology, indicate that the veins likely formed during a Mesoproterozoic magmatic event in the Colorado Front Range.

Although natural occurrences of corundum + quartz ± aluminosilicate are known, internally consistent thermodynamic databases suggest that they do not represent a stable assemblage. This observation has motivated two sets of experiments. In the first set, the equilibrium kyanite = corundum + quartz has been reversed (±5 °C; ±25 MPa) at 600 °C/320 MPa and 700 °C/535 MPa (externally heated cold-seal hydrothermal autoclaves); 800 °C/775 MPa (gas apparatus; NaCl and CaF 2 furnace assemblies, non end-load piston-cylinder press); and at 900 ∞C/1075 MPa and 1000 °C/1325 MPa (CaF 2 furnace assembly, non end-load piston-cylinder press). These reversals imply an enthalpy of formation from the elements for kyanite of -2594.75 kJ/mol. The slope of the equilibrium curve also confirms both volume and experimental CP data for kyanite. These reversals can serve as a useful calibration for the piston-cylinder press using NaCl furnace assemblies in the 700-800 ∞C and 500- 1000 MPa range and indicate a friction correction for CaF 2 furnace assemblies of 75-100 MPa over 800-1000 °C and 1000-1500 MPa. The second set of experiments (1200 °C and 2000 MPa) investigated the growth of kyanite along corundum-quartz grain interfaces. In experiments where no fluid was present, except adsorbed H 2 O, kyanite did not nucleate and grow. In experiments with 2 wt% H 2 O, kyanite formed and grew preferentially in the pores surrounding the corundum grains parallel to the corundum-quartz interface and along quartz grain boundaries. Due to a large DVR, reaction halos around the corundum grains never become closed to fluid migration. This suggests that in nature, fluids are channeled to these reaction sites via porous reaction halos surrounding the corundum grains and indicates that, under such circumstances, formation of kyanite is self-promoting and probably goes to completion quickly. The stability of sillimanite relative to corundum + quartz is also discussed, from the standpoints of what is predicted by internally consistent mineral databases and what is observed in nature.

At Mazarrón, SE Spain, dacitic lavas of the Neogene Volcanic Province contain numerous xenocrysts and xenoliths with abundant andalusite that displays variable degrees of transformation to both fibrolite and coarse sillimanite. At the onset of replacement, andalusite dissolves along grain boundaries and (110) cleavage planes, probably assisted by fluids or melts. At the same time, fibrolite crystallizes together with plagioclase, cordierite, and graphite in newly formed embayments or in the adjacent matrix. With increasing reaction progress, fibrolite needles coalesce into coarser sillimanite prisms, and direct topotactic replacement of andalusite is observed. The mutual crystallographic orientation of andalusite and sillimanite obtained from TEM investigation deviates slightly from the topotactic relationship proposed in the literature (c And || c Sil , a And || b Sil , b And || a Sil ). The two lattices are rotated by ~2.5° around a And (= b Sil ). With this misorientation, the structurally equivalent {032} And and {302} Sil planes, which exhibit the smallest misfit between the two lattices, become parallel. Macroscopic interfaces with such orientations are rare. Microscopically, however, decomposition of faces into {032} And || {302} Sil and {110} facets are common. The mutual crystallographic orientation of the reactant and the product phases is, therefore, controlled by lattice misfit minimization. The prismatic shape of the final coarse sillimanite crystals, however, is controlled by kinetic factors. The reaction seems to proceed fastest parallel to the octahedral Al chains resulting in the development of crystals elongated along the c axis. The high activation energy and the large overstepping of the equilibrium temperature required for the transformation are probably responsible for the large differences in reaction progress observed in the samples from Mazarrón.

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

Coesite exsolved from supersilicic titanite was discovered in an impure calcite marble at Kumdykol in the Kokchetav UHP (ultrahigh-pressure) metamorphic terrane, northern Kazakhstan. This impure marble consists mainly of calcite, K-feldspar, diopside, and symplectites of diopside + zoisite, with minor amount of titanite, phengite, and garnet. No diamond was found in the marble. Coesite and quartz, which have needle or platy shapes measuring about 20-60 mm in length, occur as major exsolved phases in the cores and mantles of titanite crystals with minor calcite and apatite. The strongest Raman band for the coesite needles and plates was confirmed at about 524 cm -1 with a weak band at about 271 cm -1 . To estimate the initial composition of the titanite before coesite exsolution, exsolved phases were reintegrated by measuring their area fractions on digital images. The highest excess Si in titanite was thus determined to be 0.145 atoms per formula unit (apfu). This composition requires a pressure higher than 6 GPa on the basis of phase relations in the system CaTiSiO 5 -CaSi 2 O 5 . This pressure is consistent with other evidence of high pressure in the same marble, such as 1.4-1.8 wt% K 2 O and over 1000 ppm H 2 O in diopside. Supersilicic titanite and coesite exsolution also indicate that SiO 2 exsolution occurred in the coesite stability field during exhumation of the UHP metamorphic unit.

The composition of synthetic amphiboles was investigated experimentally along the tremolitetschermakite join in the system CaO-MgO-Al 2 O 3 -SiO 2 -H 2 O-Br 2 . Compositions of these amphiboles were studied within the phase assemblages: amphibole-anorthite-quartz-diopside (I), amphiboleanorthite- quartz-talc (II), amphibole-anorthite-quartz-enstatite (III), amphibole-anorthite-talcclinochlore (IV), amphibole-zoisite-talc-quartz (V), and amphibole-zoisite-talc-clinochlore (VI). Assemblages were synthesized from oxide-hydroxide mixtures in the presence of a CaBr 2 -bearing solution between 600-800 °C and 200-2000 MPa. Solid phases were investigated using SEM, HRTEM, EMP, and XRD techniques. EMP data show that the amphiboles produced are solid solutions of the ternary system tremolite-tschermakite-cummingtonite. Enstatite, diopside, talc, and clinochlore showed small deviations from their respective end-member compositions due to incorporation of some Al. The thermodynamic properties of the tschermakite end-member and the mixing properties along the tremolite-tschermakite join were extracted from corresponding exchange reactions of the unreversed synthesized phase assemblages I-VI. Various ideal mixing models were tested for Al-Mg and Al-Si substitution at octahedral M2 and M3 sites and at tetrahedral T1-sites. Best fits were obtained for a two-site coupled model, resulting in Δ f H 0 ts = -12528.3 ± 11.7 kJ/mol and S 0 ts = 556.5 ± 12.0 J/(mol·K) for the tschermakite end-member. Similar calculations were carried out for magnesiohornblende, and values of Δ f H 0 Mghb = -12418.7 ± 5.9 kJ/mol and S 0 Mghb = 562.8 ± 6.1 J/(mol·K) were extracted. Calculated phase relations and amphibole compositions agree well with experimental data if the derived thermodynamic data of tschermakite and a two-site mixing model for Al incorporation in amphibole solid solutions are applied.

Mid-Mesozoic to Tertiary igneous activity reflects arc evolution along the Sierra + White-Inyo sector of the continental margin. The Ca-amphibole + biotite ± augite-bearing Middle Jurassic Barcroft pluton consists of intergradational, quartzose gabbro-diorite, metadiorite, voluminous mafic-to-felsic granodiorite, and rare alaskite-aplite. It was emplaced along the Barcroft structural break-a major ENE-striking suture juxtaposing mid-Mesozoic bimodal metavolcanics on the north against metamorphosed Lower Cambrian platform strata on the south. The Cretaceous, two-mica McAfee Creek Granite intruded both wall rocks and the Barcroft granodioritic pluton on the east. Geologic mapping and petrochemical studies have documented the structure and metaluminous geochemistry of the Barcroft body. Analyses of Ca-clinoamphiboles from 16 Barcroft rocks allow the assignment of P-T conditions of crystallization/recrystallization based on experimentally determined and calculated clinoamphibole thermobarometers. Combined with earlier studies of country-rock metamorphism, and oxygen isotopic investigations of mineral assemblages in both plutons and wall rocks, four stages in the P-T evolution of the Barcroft pluton have been identified: Stage 1, partial melting of a mafic protolith at ~25 km or greater depth, then onset of crystallization of refractory hornblende and other ferromagnesian phases at ~915 ± 25 °C attending magma ascent. Stages 2 → 3, crystallization of progressively more actinolitic hornblende during decompression cooling and stalling of the pluton at a depth of 10-12 km, as temperature declined over the interval ~760-650 °C. Stage 4, subsolidus recrystallization/ exsolution of nearly stoichiometric actinolite ± grunerite attending deuteric alteration over the range ~395-545 °C at a depth of 4-5 km. Mineral assemblages of stages 1 → 4 are disposed progressively inward, indicating that the Barcroft pluton lost heat along intrusive contacts with the wall rocks, cooling, annealing, and re-equilibrating in the process. The Cretaceous McAfee Creek Granite transected this ENE-trending thermal zonation, but exerted little effect on Ti + Al contents (and δ 18 O values) of Barcroft amphiboles.

The abundance and distribution of B and Li in metasedimentary rocks and in the Harney Peak Granite (HPG) in the Black Hills, South Dakota, U.S.A., elucidate the behavior of these two elements during (1) regional metamorphism that began at ~1760 Ma; (2) subsequent contact metamorphism associated with emplacement of the HPG and associated pegmatites at ~1715 Ma; and (3) anatexis leading to production of the granite. There is no clear evidence for changes in B and Li concentrations with progressive regional metamorphism of the metapelites from chlorite-biotite grade up to staurolite grade. Rocks outside the pegmatite aureole that surround the HPG have average B contents of ~80 ppm. There is no correlation of B with other elements in the pelitic schists, which indicates that none of the major phases is the dominant host for B. Boron content is mainly controlled by small amounts of randomly distributed tourmaline that was identified in alpha-track maps of thin sections. In the aureole of the granite, B concentrations are depleted in many samples. The depletion is attributed to consumption of tourmaline during interaction of the rocks with alkalic fluids. The average Li contents in metapelites far away from the HPG are also ~80 ppm. Near the HPG, Li concentrations reach roughly 190 ppm, indicating significant metasomatism by fluids that emanated from the granite and pegmatites. The concentrations of B and Li in the low-grade metapelites are sufficient to explain their abundances in the HPG, if the granite formed by partial melting of the metapelites and if there was total breakdown of tourmaline. Stabilization of tourmaline in leucogranites is not necessarily related to sources enriched in B, but may be related to its competition for Fe and Mg with biotite.

The crystal structure of surinamite from “Christmas Point,” Enderby Land, Antarctica, has been newly refined by single-crystal X-ray diffraction: VI (Mg 2.26 Fe 2+ 0.74 Fe 3+ 0.39 Al 2.61 )O IV (Al 1.00 Be 1.00 Si 3.00 )O 15 (simplified formula), space group P2/n, a = 9.915(2), b = 11.368(2), c = 9.617(2), b = 109.30(2)°, Z = 4, wR(F 2 ) = 0.074 for 4876 independent reflections. The refined site occupancies agree well with the chemical composition determined by electron microprobe analysis and with the Fe 3+ /Fe 2+ ratio estimated from stoichiometry. The surinamite structure is characterized by an ordered Al/Be/Si distribution on the tetrahedral sites and by charge ordering with extensive Mg 2+ -Fe 2+ and Al 3+ -Fe 3+ exchange on the octahedral sites. This cation ordering is distinct from that observed in related phases of the sapphirine series and aenigmatite groups, and the difference is linked to the unique structural topology of the tetrahedral chains in surinamite. Optical and Mössbauer spectra of surinamite have been fully interpreted in terms of an octahedral distribution of Fe 2+ and Fe 3+ cations that agrees very well with the X-ray site populations. Both the X-ray and Mössbauer data establish the absence of significant tetrahedral Fe and the non-uniform distribution of octahedral Fe. Intense and strongly polarized absorption bands caused by IVCT-processes in clusters of Fe 3+ and Fe 2+ iron in edgesharing octahedral sites produce the unusual color and pleochroism observed in surinamite.

Synchrotron micro-XANES (X-ray absorption near-edge structure) spectroscopy (SmX) was used to measure Fe 3+ and Fe 2+ distribution among minerals in standard thin sections with an X-ray beam size of 10 ¥ 15 mm. Measurements were made at beamline X26a, National Synchrotron Light Source, Brookhaven National Lab. Samples studied included metapelites from garnet to upper sillimanite zone rocks that coexist with graphite, graphite/ilmenite, or varying combinations of ilmenite, magnetite, and hematite. SmX results are compared with Mössbauer spectroscopic measurements of Fe 3+ on mineral separates from the same rocks. Results show excellent agreement (within ±5-10%) between Mössbauer and SmX for the Fe-rich phases biotite, chlorite, staurolite, and garnet. Mössbauer spectra of muscovite typically show lower values than SmX for Fe 3+ /SFe (especially at low grades), suggesting contamination of the muscovite separates by fine-grained chlorite. However, heterogeneity of Fe 3+ and Fe 2+ is probably the chief source of discrepancy between the bulk and micro-scale measurements. The %Fe 3+ (relative to total Fe) in these samples ranges from a high of 90% in muscovite to a low of 0-2% in garnet, with Ms > St = Bt = Chl > Tur > Grt in graphite/ilmenitebearing rocks. SmX measurements suggest that the averaged Fe3+/SFe in each mineral species does not change as a function of grade, but varies as a function of buffering assemblage, especially on a very localized scale. This effect shows that the oxygen buffering capacity of mineral assemblages is very large compared to the oxidizing/reducing potential of metamorphic fluids. However, distribution of Fe atoms among phases at each grade also reflects crystal chemistry. Across all grades, the Fe 2+ /Mg ratio is such that Grt >> St > Bt > Chl > Tur > Ms. Fe 3+ /Al is highest in biotite in garnet and sillimanite- zone rocks. For Fe 3+ /Al, Bt > Chl > Tur > St > Ms, and for Fe 3+ /Fe 2+ , partitioning behaves as Ms >> Bt ≈ St ≈ Chl > Tur > Grt in graphite/ilmenite-bearing rocks. Overall, partitioning of Fe3+ in any of the minerals of interest reflects both oxide assemblage and crystal-chemical constraints on the amount of Fe 3+ that can be substituted into the structure.

Two synthetic solid-solution series, analcime to Rb-leucite and analcime to Cs-leucite (pollucite), have been investigated to understand more fully the thermodynamic and structural behavior of analcime- leucite and similar mineral systems. Unit-cell dimensions and volumes in these series expand with the substitution of analcime component in either Rb-leucite or pollucite, as H 2 O molecules structurally replace the smaller entities Rb +1 or Cs +1 , respectively. Unit-cell volumes vary linearly as functions of composition, but with changing slopes over several segments of compositional space, akin to thermal expansion in K-, Rb-, and Cs-end-member materials studied by previous workers. When symmetry changes displacively from tetragonal to isometric, as in the Rb-bearing series, the slope of volume expansion changes. Once structures have reached full expansion, volume slopes flatten and are little affected by additional analcime component. Enthalpies of solution measured at 50 °C in 20.1 wt% hydrofluoric acid show single-slope linear relationships over the entire compositional ranges of both series. Thus, despite positive volumes of mixing, there are no enthalpies of mixing in either series, nor is there energetic evidence of displacive tetragonal/isometric inversion or the various stages of structural expansion. Overall, the data suggest that the analcime-leucite system also can be modeled as close to thermodynamically ideal. The limited solid solution between natural analcime and leucite must be attributed to energetically favored heterogeneous equilibria involving minerals such as feldspars and other feldspathoids, and not to immiscibility between the end-members.

The hydrothermal crystallization of gyrolite was studied dynamically at 190-240 °C using synchrotron- based in situ Energy Dispersive Powder Diffraction (EDPD). The reaction mechanism involves the initial crystallization of a calcium silicate hydrate (C-S-H) gel, which has a sheet structure with well ordered Ca(O,OH) layers and disordered silicate layers. This is followed by the intermediate formation of Z-phase which finally transforms to gyrolite. This process involves ordering of the silicate layers and an increase in the order along c. Kinetics data for all stages of the crystallization process were determined by analyzing the growth and decline of various diffraction peaks with time. The activation energy (Ea) (nucleation) for Zphase is ~39 kJ/mol while that for gyrolite is ~56 kJ/mol. E a (crystallization) of gyrolite is higher at ~80 kJ/mol. The reaction occurs via a two-dimensional, diffusion-controlled mechanism and is a continuous process that suggests that Z-phase is an unstable, transient phase.

Spessartine (Mn 3 Al 2 Si 3 O 12 )-grossular (Ca 3 Al 2 Si 3 O 12 ) solid solutions were synthesized at high pressures and temperatures. Compositionally homogeneous garnets are obtained by crystallizing solidsolution glasses prepared from oxides. The unit-cell parameter, a, for the different solid solutions was determined by X-ray powder diffraction methods and the results give positive deviations from ideal volumes of mixing that can be described with a symmetric mixing model with W v = 0.80 (±0.04) cm 3 /mol. The degree of non-ideality is a function of the difference in size between the Ca 2+ and Mn 2+ cations and is consistent with the range of those observed for the other aluminosilicate garnet binary joins. The crystal structures of synthetic grossular and spessartine were collected at 50 K intervals between 103 K and 498/648 K using single-crystal X-ray diffraction methods. The rotation of the rigid SiO 4 tetrahedra changes slightly by 0.3(1)° for grossular and 0.2(1)° for spessartine between 103 and 648 K. The volume expansions of the polyhedra were calculated and their distortions in grossular and spessartine were analyzed as a function of temperature. The linear thermal expansion coefficients of the Al-O and two X-O bond were also calculated for almandine, pyrope, grossular, and spessartine. The thermal expansion of spessartine is similar to that of grossular. In terms of polyhedral distortion and bond-valence values, spessartine has the most ideal structure of the aluminosilicate garnets. This could explain its large P-T stability field and the ease of synthesis at low pressures.

The crystal structures of becquerelite, Ca[(UO 2 ) 6 O 4 (OH) 6 ](H 2 O) 8 , and Sr-exchanged becquerelite obtained by ion exchange, Sr 1.27 [(UO 2 ) 3 O 3.54 (OH) 1.46 ](H 2 O) 3 , have been refined using diffraction data collected with MoKa X-rays and a CCD-based detector. The structure of becquerelite, orthorhombic, space group Pn2 1 a, a = 13.8527(5), b = 12.3929(4), c = 14.9297(5) Å, V = 2563.2(1) Å 3 , has been refined on the basis of F 2 for 4875 unique reflections to R1 = 3.39%, calculated using 4581 unique observed reflections (|F| ≥ 4σ F ), and a goodness-of-fit (S) of 1.04. Sr-exchanged becquerelite was obtained by placing single crystals of synthetic becquerelite in 2.5 M SrCl 2 solution for 60 h at 160 °C. The structure of Sr-exchanged becquerelite is trigonal, space group P3, a = 7.020(4), c = 6.992(6) Å, V = 298.4(3) Å 3 , and has been refined on the basis of F 2 for 683 unique reflections to R1 = 4.26%, calculated using the 564 unique observed reflections (|F| ≥ 4σ F ), and an S of 1.01. The results for becquerelite confirm the cation polyhedra and structural connectivity reported previously, but collection of data for a high-quality crystal using a CCD-based detector has substantially improved the precision of the atom positions. The structure contains a-U 3 O 8 -type sheets of uranyl pentagonal bipyramids, with a single symmetrically distinct Ca cation and eight symmetrically distinct H 2 O groups located in the interlayer. The structure of Sr-exchanged becquerelite also contains α-U 3 O 8 -type sheets of uranyl pentagonal bipyramids, although the amount of H in the sheets is lower than for becquerelite. The interlayer contains two symmetrically distinct Sr cations and a single H 2 O group. Ion exchange of Sr into the interlayer of becquerelite without destruction of the crystals has potential important implications for the mobility of 90 Sr in contaminated areas, and for the geologic disposal of nuclear waste.

Published measurements of the compression and elasticity of MgSiO 3 orthoenstatite have been reanalyzed and the estimates that they yield of the room pressure bulk modulus and its pressure derivative are now shown to be consistent with one another. New single-crystal compression data is also consistent with the revised EoS parameters. Combining the results of four different experiments (two compression, one Brillouin measurement, and one in situ high-pressure ultrasonic measurement) yields best estimates of K T0 = 105.8(5) GPa and K' 0 = 8.5(3) for a third-order Birch-Murnaghan EoS.

Reidite is a high-pressure polymorph of zircon with the scheelite structure. It has been found in an upper Eocene impact ejecta layer in marine sediments on the upper continental slope off New Jersey and on Barbados. Reidite occurs (epitaxially oriented) in shock-metamorphosed zircons. It is associated with impact glass (tektites), shocked quartz and feldspar with multiple sets of planar deformation features, coesite, and trace amounts of stishovite. This phase was first produced in high-pressure laboratory experiments in 1969 and has also been produced in shock recovery experiments. Reidite is brittle with an irregular fracture, a hardness of 7.5, a calculated density of 5.2 g/cm 3 , a white streak, adamantine luster, and it does not fluoresce. In index oil in transmitted light, shocked zircon grains consisting almost entirely of reidite are transparent. Pleochroism was not observed. Reidite appears to have parallel extinction and is length slow. The maximum birefringence is roughly 0.015. Reidite appears to be uniaxial positive. It is tetragonal, space group I4 1 /a, a = 4.738 (1) Å, c = 10.506 (2) Å, V = 235.84(2) Å 3 . Previous shock-loading experiments on zircons indicate that the transition to reidite starts at about 30 GPa and is completed around 53 GPa. Reidite should be a useful indicator of peak pressure in shock metamorphosed rocks. Reidite is named after Alan Reid who first produced this phase in the laboratory.

X-ray diffraction experiments have been carried out on b-eucryptite (LiAlSiO 4 ) at pressures up to 2.5 GPa and temperatures up to 1073 K in a large-volume apparatus. With room-temperature compression, we observed a phase transition to a new polymorph between 0.83 and 1.12 GPa. This transition is reversible in character. The new phase, referred to here as e-eucryptite, can be indexed according to an orthorhombic unit cell with a = 10.217(4) Å, b = 8.487(4), Å, c = 5.751(3) Å, and V = 498.7(4) Å 3 for XRD data at 2.2 GPa and 298 K. On heating at 2.2 GPa, e-eucryptite and beucryptite were metastable over the temperature interval 298-873 K; at higher temperatures they underwent an irreversible phase transition to a-eucryptite. Both hexagonal α-eucryptite and ε-eucryptite show anisotropic thermal expansion. For α-eucryptite, we obtained α a = 6.71(±0.25) × 10 -6 K -1 , α c = 1.07(±0.05) × 10 -5 K -1 , and α v = 2.42(±0.1) x 10 -5 K -1 at 1.94(2) GPa over the temperature range 298-1073 K. For ε-eucryptite at 2.32(8) GPa, we find larger thermal expansion in a smaller temperature range 298-773 K, with α a = 1.47(±0.15) × 10 -5 K -1 , α b = 6.65(±1.33) x 10 -6 K -1 , α c = 7.83(±0.88) × 10 -6 K -1 , and α v = 2.99(±0.15) × 10 -5 K -1 . In combination with a previous determination of thermal expansion at ambient pressure, the pressure effect on volume thermal expansion of a-eucryptite is determined to be -2.68 × 10 -6 GPa -1 K -1 , and the temperature derivative of the bulk modulus is estimated to be -0.015 GPa/K.

Although titanite (CaTiSiO 5 ) has one of the highest known entropies of fusion among silicates, the corresponding melt is a relatively good glass-former, which provides an opportunity to investigate the structural origin of disorder in the liquid. Here we present an 17 O NMR study of CaTiSiO 5 glass, using detailed new results on the crystalline phase, collected at magnetic fields of 9.4, 14.1, 18.8, and 21.1 Tesla, to interpret the spectra. We find that the glass contains significant concentrations of Si-O-Si and Si-O-Ca sites in addition to the Ti-O-Ti and Ti-O-Si sites of the crystal, suggesting considerable network disorder. Simulations of the spectra for the crystal allow the derivation of isotropic chemical shifts and quadrupolar coupling constants (C Q ) for each of the five O atom sites. In addition the Ti-O-Ti site has an unusually large chemical shift anisotropy and an unusually small C Q value.

We report studies of recently precipitated terrestrial Mn oxides from Pinal Creek (Arizona, U.S.A.) and from Yuno-Taki Falls (Hokkaido, Japan). In addition to chemical analysis [by energy dispersive X-ray spectroscopy (EDS) and proton induced X-ray emission spectroscopy (PIXE)], the following methods were used: X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), contraction and expansion tests, thermogravimetry (TG), derivative thermogravimetry (DTG), differential scanning calorimetry (DSC), infrared spectroscopy (IR), and Mössbauer spectroscopy. Selected area electron diffraction (SAED) was applied in a few cases, but only on synthetic samples. The most useful methods for characterizing the 10 Å Mn minerals were XRD, contraction test, and DSC. Mössbauer spectroscopy was a very useful complementary method to XRD for differentiation of the poorly crystalline Fe compounds. IR spectroscopy was found not to be very useful in distinguishing buserite and birnessite, but a distinct pattern was observed for todorokite. For heterogeneous deposits composed of many minerals, IR was not useful without separation of phases. Three types of samples were identified from Yuno-Taki. One of them, representing a relatively fresh deposit exposed to water, was found to be a nearly ideal buserite with an XRD pattern similar to synthetic samples. An older, non-ideal buserite could be dehydrated but not expanded, suggesting that it had been slightly transformed. The third sample was from a deposit that was dry at the time of sampling. It is a 7 Å mineral (birnessite or takanelite/rancieite). The sample also contains amorphous Fe hydroxide, confirmed by Mössbauer spectroscopy. A test of the possibility of buserite transformation showed that autoclaving a sample of the synthetic 10 Å phyllomanganate, Mg-buserite, resulted in a compound with the XRD pattern and IR spectrum of todorokite. We suggest that this pure synthetic product be used as a model compound for the high-temperature hydrothermal 10 Å manganate with tunnel structure, todorokite. Experiments at other conditions demonstrated that synthetic buserite collapses to birnessite during dehydration. Pinal Creek crusts are composed of quartz, feldspar, Fe minerals (magnetite, maghemite, ilmenite, hematite), and Mn minerals. Resistance to dehydration and expansion tests, and the initial endotherm shown in DSC analysis, suggest that the 10 Å manganate is todorokite. A 7 Å mineral (birnessite or takanelite/rancieite) is also present. Based on comparison of XRD and electron microscopy data for the natural samples with tests done on the synthetic samples, we postulate that the birnessite or takanelite/rancieite and todorokite found in th Pinal Creek crusts may be a transformation products of a buserite precursor.