Band 90, Heft 5-6

The survival of coesite in ultrahigh-pressure (UHP) rocks is most commonly attributed to rapid exhumation, continuous cooling during uplift, and inclusion in strong phases that can sustain a high internal over-pressure during decompression. Exceptions to all of these criteria exist. Perhaps less attention has been paid to the role of fluid infiltration in the preservation of coesite. We used infrared spectroscopy to measure water contents of coesite and coesite pseudomorphs in a variety of UHP rocks. In all cases, OH concentrations in coesite are below the detection limit of ~100 ppm H 2 O. The silica phases surrounding coesite, however, show varying amounts of H 2 O. This is most spectacularly observed in pyrope quartzites from the Dora-Maira massif that contain at least three phases of silica replacing coesite, also distinguished by varying color of cathodoluminescence (CL): palisade-textured quartz (<100 ppm H 2 O, red-violet CL); .mosaic. quartz, which is actually chalcedony (up to 0.4 wt% H 2 O, yellow/brown CL); and a rare, highly hydrated silica phase interpreted to be opal (~7 wt% H 2 O, dark blue CL). Very similar signatures are observed in a grospydite xenolith from the Roberts Victor kimberlite. The quartz replacing coesite in other UHP samples studied contains on the order of 500 ppm H 2 O or less, and most measurements are under the detection limit of our technique. We infer that palisade quartz forms under dry or nearly dry conditions and at high temperatures during dilation of the host phase. The formation of hydrous silica phases such as chalcedony and opal, however, must take place at much lower temperatures, after cracking of the host phase, which allows external fluids to infiltrate. Delay of fluid infiltration to low temperatures, where kinetics are slow even in the presence of water, is the most critical factor in the preservation of coesite.

Laser Raman spectroscopy and cathodoluminescence (CL) images reveal that abundant and complicated mineral inclusions occur in zoned zircon separates from paragneisses in drill holes CCSDPP2, ZK-2304, and CCSD-MH, at Donghai, southwestern Sulu terrane, China. Many low-P mineral inclusions are preserved in inherited zircon cores. Coesite + quartz within a single inclusion, and some low-P minerals (e.g., quartz and albite) are identified in zircon mantles. Textures showing the quartz to coesite transformation in zircon indicate that both core and mantle of the zircon are detrital. Inclusions of coesite, together with garnet, jadeite, and phengite, are common in zircon rims formed during ultrahigh-pressure (UHP) metamorphism. SHRIMP U-Pb analyses of these zoned zircon grains identify three discrete and meaningful age groups. Proterozoic ages of 840 to 658 Ma for zircon cores indicate that the detrital zircons have a variety of sources. An early Paleozoic group of 505 to 455 Ma ages for zircon mantles constrain neither the timing of a discrete UHP metamorphic event, nor the transformation age from quartz to coesite. The 227 ± 9 Ma (the weighted mean age) obtained from coesite-bearing zircon rims represents the UHP metamorphic age. Thus, one Late Triassic UHP metamorphic event, rather than two UHP metamorphic events, is well established in Sulu UHP terrane by the method of U-Pb SHRIMP dating for coesite-bearing zircon rims in garnet-biotite-amphibole-albite gneisses.

The Yangkou garnet-peridotite complex in the Sulu UHP terrane, eastern China, consists of several blocks, tens of meters across, enclosed within gneiss. The blocks consist mainly of garnet lherzolite and garnet clinopyroxenite with small amounts of eclogite. The complex displays a finely layered structure from outcrop to thin-section scales. Lherzolitic garnet is generally homogenous in composition (Prp 64-67 Alm 23-24 Grs 10-12 Sps 1 ). Garnet (Prp 51-63 ) from clinopyroxenite contains more pyrope component than eclogitic garnets (Prp 29-35 ). Porphyroblastic garnet from clinopyroxenite and eclogite contains abundant exsolved rods of rutile and apatite (±Cpx). Lherzolitic garnet exhibits an unusual sinusoidal REE pattern characterized by a decrease in abundance from La to Pr and an increase from Nd to the HREE, and shows Ba and Pb enrichment and negative Nb and Ni anomalies in primitive mantle-normalized trace-element plots. In contrast, diopside shows a normal REE pattern with decreasing abundances from La to Lu, a peak at Ba and Pb, and depletion in Co and Ni. Garnet from eclogite has a REE pattern with increasing abundances from La to Lu, or a maximum at Eu. Eclogitic clinopyroxene shows humped REE patterns with a peak at Nd. Except for slightly lower δ 18 O VSMOW of +4.40. for olivine, O-isotopic compositions (+5.03 to +5.53.) of analyzed garnets and clinopyroxenes from all three rock types lie in the range of mantle δ 18 O values of +5.1 to +6.6.. Small O-isotope fractionations (ΔGrt-Cpx: .0.16 to .0.35) in eclogite suggest isotopic equilibrium. On the basis of petrochemical and isotopic data, this garnet-peridotite complex was initially a mantle peridotite cut by mafic veins. These rocks experienced metasomatism and subduction-zone UHP metamorphism at 750 ± 50 °C and 40 to 45 kbar with a low thermal gradient. The fine-scale layering probably formed as a result of strong shear deformation during exhumation.

Two types of amphibole exsolution lamellae were discovered in garnet from a few garnet peridotites from the North Qaidam ultrahigh-pressure metamorphic belt, northern Tibet, NW China. The amphibole lamellae are strictly oriented in four directions corresponding to the isometric form {111} (i.e., octahedron planes) of garnet. Observations by transmission electron microscopy (TEM) show that amphibole lamellae are topotaxtially concordant with the host garnet and possess 9.8 to 10 Å lattice .c.-spacing. Electron-microprobe analysis reveals that these exsolved amphibole lamellae are high in Na and Ti. The TEM observations and recalculated compositions indicate that the oriented amphibole lamellae exsolved from original supersilicic majorites with high concentrations of Na 2 O(0.3 wt%) and hydroxyl (up to 1000 ppm by weight). These results imply that the host garnet peridotites were formed at depths greater than 200 km and that garnet can be an important reservoir of water at such depths in the mantle.

The Zermatt-Saas ophiolite of the Swiss Alps represents a complete sequence of Mesozoic Tethys oceanic lithosphere. The ophiolite was subducted during early phases of the Alpine orogeny and the mafic rocks were transformed to eclogites and blueschists. Metabasalts locally preserve pillow structures in which glaucophanite forms rims on eclogitic pillow cores. Omphacite-garnet-glaucophane-epidote-ferroan dolomite-Mg-chloritoid-talc-paragonite-chlorite. rutile form characteristic coeval blueschist- and eclogite-facies assemblages. Omphacite + garnet + glaucophane + epidote + rutile represents an equilibrium assemblage that formed during deformation and in the period when the rocks reached the greatest depth of subduction. In rocks containing this assemblage, an additional significant mineral pair is Mg-chloritoid + talc. Coarse chloritoid (X Mg ~ 0.45) and talc formed in dispersed clusters after the last penetrative deformation. The assemblage may require > 2.7 GPa pressure to form. It developed at maximum pressure conditions corresponding to the return-point of the ophiolite in the subduction zone. Coarse paragonite and chlorite replaced parts of the earlier formed assemblages and removed free H 2 O from the rocks. Exhumation of the HP to UHP ophiolite rocks was accompanied by development of symplectite rims and other replacement products along grain boundaries of the eclogite minerals by decompression reactions in a fluid-deficient regime. Particularly noteworthy is the formation of margarite, paragonite, chlorite, albite, barroisite, and preiswerkite. The latter mineral, a very rare Na-biotite, formed as a result of the decomposition of chloritoid + paragonite and is associated with magnetite and hercynite. Omphacite breakdown produced diopside-albite-barroisite symplectites. Calculated equilibrium assemblage phase diagrams for metabasite compositions indicate P-T conditions of ~2.5.3.0 GPa and ~550.600 °C. The conditions of the subduction-related metamorphism denote P and T at the return-point, which coincide with the upper P-T limit of antigorite. Antigoriteserpentinites constitute the largest volume of rocks within the ophiolite. We suggest that the P-T conditions recorded by the exhumed mafic rocks are coupled to those of antigorite breakdown in the serpentinite that released large amounts of dehydration water in the subducted serpentinite slab facilitating exhumation of the Zermatt-Saas eclogites and blueschists.

Coexisting jadeite and omphacite were found as retrograde minerals in a jadeite-bearing lawsonite- eclogite from the Motagua Fault Zone, Guatemala. The lawsonite-eclogite is characterized by the occurrence of garnet porphyroblasts up to 2.5 cm in size, and the eclogite-facies parageneses, almandine- rich garnet + impure jadeite + lawsonite + rutile + quartz; garnet contains inclusions of impure jadeite, phengite, ferroglaucophane, chlorite, lawsonite, rutile, ilmenite, and quartz. Textural relations and parageneses and compositions of minerals indicate that the lawsonite-eclogite experienced two stages of metamorphism: prograde eclogite-facies stage (M 1 ) and retrograde stage (M 2 ). The impure jadeite (Jd-I) of the M 1 eclogite-facies occurs in both the matrix and as inclusions in garnet, and contains considerable amounts of augite and aegirine components (Jd 61-75 Aug 16-24 Ae 0-18 ). It is partly recrystallized to retrograde M 2 jadeite (Jd-II) (Jd 74-87 Aug 9-16 Ae 0-11 ) and omphacite (Jd 42-50 Aug 36-46 Ae 7-16 ); some of these two sodic pyroxenes may have crystallized from fluids. Both M 2 jadeite and omphacite show textural equilibrium and are believed to have grown concurrently. Based on the observed compositions and the phase relations of sodic pyroxenes from Carpenter (1980), the M 1 impure jadeite (Jd-I) may have had a disordered C2/c symmetry at T = ca. 450 °C and P = ca. 1.8.2.4 GPa, and was subsequently crystallized into jadeite (Jd-II) plus ordered P2/n omphacite during retrogression with infiltration of fluids at T < ca. 300 °C and P = ca. 0.7 GPa (M 2 ). The extreme low-T conditions during retrogression may have prevented reaction between eclogitic jadeite and adjacent minerals. Instead, eclogitic impure jadeite (plus fluid) has recrystallized into the retrograde jadeite + omphacite pair with a wide compositional gap.

Three independent net-transfer reactions determine compositional and modal changes for greenschist-, blueschist-, and amphibolite-facies metamorphism of mafic rocks, as different paths are followed though pressure, temperature, and a H ₂ O space. For natural terrains, only one a H ₂ O .dependent reaction is chosen, whereas to interpret experimental studies, where H 2 O is in excess, there is an advantage to choosing two dehydration reactions. Published experimental studies on the greenschist to amphibolite transition and thermodynamic calculation in simple systems and with pseudosections, show that reactions involving Al 2 Mg -1 Si -1 in amphibole and chlorite predominate, that FeMg -1 exchange and Al 2 Mg -1 Si -1 are strongly coupled, and that all reactions and, in particular, the stability of chlorite are affected by Fe 3+ /Fe total . Modal spaces for mafic schist are constructed for different sets of independent reactions, and permit correlation of mineral mode with directions of change of pressure and temperature. These can be used to deduce field gradients in these variables in natural terrains, when only modal change data are available and when compositional and thermodynamic data are limited.

The Qinglongshan O- and H-isotope anomaly occurs within a coesite-bearing, eclogite-facies regional metamorphic belt in Eastern China near Donghai in Jiangsu province. The anomaly is defined by low values of δ 18 O and δD. Garnets from eclogite have δ 18 O as low as .11. and rutiles are .15. (VSMOW). Phengites have δD of .120. (VSMOW). The anomaly is Neoproterozoic in age. Surface outcrops of coesite-eclogite-facies rocks with unusually low δ 18 O and δD values extend over an area of at least 1600 km2. The Chinese Continental Scientific Drilling project has made it possible to investigate the depth of the Qinglongshan anomaly and to measure an O-isotope profile across a garnet peridotite body sandwiched between crustal rocks. New O-isotope analyses of minerals separated from drill core gneisses, eclogites, amphibolite, and quartzite verify that the Qinglongshan O-isotope anomaly extends to a depth of at least 432 m. Crustal rocks with unusually low δ 18 O form both the hanging- and foot-walls of a 100 m thick, fault-bounded, garnet peridotite body intersected by the drill. Minerals of the garnet peridotite body have δ 18 O values indistinguishable from mantle nodules and megacrysts. The garnet peridotites may have originated from enriched mantle sources in sub-cratonic lithosphere, isolated from mantle convection. This origin is consistent with the collision of the Archean Sino-Korean craton with the Yangtze plate, consequent subduction, and UHP metamorphism. The garnet peridotite was transported from the mantle and emplaced into UHP crustal rocks during Triassic subduction and cratonic collision.

We investigated calcite globules and veins in two spinel-garnet peridotite xenoliths from the sub- Sierra Nevada mantle. The studied xenoliths were entrained in a Miocene (11 Ma) volcanic plug. These carbonates are associated spatially with silicate glass inclusions, suggesting that they are primary inclusions-inclusions that formed at high temperature in the mantle and not at or close to the Earth's surface. The host peridotites represent samples of the lithospheric mantle wedge beneath the Mesozoic California magmatic arc, as indicated by radiogenic isotopic ratios measured on clinopyroxene separates [ 87 Sr/ 86 Sr(11 Ma) = 0.7058-0.7061, ε Nd (11 Ma) = -1.9 to -0.7]. Mineral chemistry of the peridotite major phases is typical of a mantle section that was depleted of melt. The δ 18 O values of olivine and orthopyroxene from the two samples are also typical of mantle rocks (δ 18 O = 6-6.5‰). In contrast, calcite veins have δ 18 O of 18-20‰ and δ 13 C of -14‰, arguing for a subducted sedimentary origin for these carbonates. Presumably, the carbonates were expelled from the downgoing slab and fluxed into the overlying mantle wedge as CO 2 - or CO 2 -H 2 O-rich fluids or melts. The trace-element patterns of two analyzed calcite veins are typical of the arc signatures (e.g., depletions in high-fieldstrength elements) seen in calc-alkaline magmatic rocks worldwide. However, the cores of peridotite clinopyroxenes do not show that pattern, suggesting that the arc-like trace element signature was introduced via the recycled carbonate agent. A connection between mantle wedge carbonation and diamond formation in a subduction environment is proposed based on these observations.

Recent seismological investigations into the Hawaiian mantle and geophysical studies of the Hawaiian Swell have led to divergent models regarding the thermal structure and role of the lithosphere in Hawaiian magmatism. Whereas some models require that the Hawaiian plume erodes the lower lithosphere and replaces it with plume-derived residues and cumulates, others do not require any thinning of the lithosphere. In this paper, we develop a model for the physical and thermal characteristics of the oceanic lithosphere beneath Oahu, Hawaii, from the mantle xenoliths included in the rejuvenated stage lavas (Honolulu Volcanics, HV) that erupted through the Salt Lake Crater (SLC) and Kaau-Pali-Kalihi (KPK) vents. Three main xenolith suites are found at Oahu: spinel peridotite, garnet pyroxenite, and dunite. Dunites are generally shallow cumulates, whereas the peridotite and pyroxenite xenoliths are from the mantle. Plagioclase peridotite (harzburgite) is rare, and represents a ~15 km thick lithospheric layer below the ~15 km thick crust (MOR-generated oceanic crust + thickened crust formed by Koolau shield volcano activity). The harzburgite layer is underlain by a spinel peridotite layer. Spinel peridotites from SLC and KPK differ in chemical and isotopic composition. SLC spinel peridotites have unusually enriched 176 Hf/ 177 Hf but highly nonradiogenic 187 Os/ 188 Os isotopic compositions, likely indicating that they are metasomatized, recycled (previously subducted) oceanic lithospheric fragments. On the other hand, KPK spinel peridotites exhibit compositional characteristics of the 90 m.y. old lithosphere beneath Oahu that has been variably metasomatized by passing HV magmas. We suggest that the lower lithosphere beneath SLC and KPK are very different in their petrologic composition. The lower lithosphere (60–90 km) beneath SLC is extensively veined and fragmented by garnet clinopyroxenite intrusives and mixed with spinel peridotite blobs that are recycled subducted lithospheric fragments. For the KPK lithosphere, we favor a model in which the lower part is peridotitic. The thickness of our model lithosphere beneath Oahu is ~90 km. The seismic low-velocity zone beneath Oahu is likely due to the presence of small amounts of hydrous-alkalic and carbonatitic (+kimberlitic) melts. The isotopic data suggest little or no interaction between the basal lithosphere and Koolau shield magmas (mostly plume-derived, particularly those with near bulk-earth Nd- and Sr-isotope ratios), suggesting that during shield-stage volcanism, magmas used well-insulated (i.e., a reaction zone of <2 km radius) narrow conduits that reached down to the base of the lithosphere. All of the deeper (>60 km) xenoliths are isotopically depleted and lack the typical Koolau-like signature. Therefore, the lower lithosphere was not significantly eroded by Koolau magmatism. KPK spinel peridotites give temperatures of 900–1040 °C for the intermediate lithosphere, which is about 200–400 °C hotter than what would be expected of a normal 90 Ma lithosphere. We suggest that such an anomalously high temperature is due to heating of the wall rocks (lithosphere) by ascending HV magmas and not due to simple basal heating of the lithosphere by the hot spot/plume followed by gradual (conductive) upward transport of heat. Thermobarometry of garnet-bearing xenoliths suggests that these rocks equilibrated at ~2−3 GPa and 1200−1350 °C. This temperature range is 50–200° less than that predicted by a recent model (Ribe and Christensen 1999). The plume that generated the Koolau shield magmas was a complex mixture composed predominantly of Koolau source materials at its center and blocks of old recycled lithosphere in its outer fringes that were later rafted into the lithosphere beneath SLC.

Reliable values for mineral-melt trace-element distribution coefficients (D) are essential for constructing realistic models of magma evolution based on trace elements. We have determined D-values for an extensive set of compatible and incompatible trace elements in clinopyroxene, orthopyroxene, plagioclase, and olivine phenocrysts in two moderately evolved (5.4 and 6.6 wt% MgO), tholeiitic lavas from the 1955 eruption of Kilauea volcano, Hawai'i, using laser ablation-ICPMS. Coexisting melt compositions were obtained by analyses of quenched mesostasis. These D-values are consistent with experimental results when major element variations in the host phase are considered. Lattice strain models reproduce many of the partitioning characteristics. The distribution coefficients determined here can be applied to understanding the petrogenesis of evolved tholeiitic magmas from two recent Kilauea eruptions. Trace-element compositions of the 1955 lavas are consistent with 30−40% fractional crystallization of a gabbroic assemblage from an olivine tholeiite parental magma. The reduced influx of melt to Kilauea during the late 19th and early 20th centuries may have allowed the formation of evolved magmas in the rift zone.

A series of low-Ca (1.8–1.65 atoms Ca per formula unit) actinolites were synthesized in the system CaO-FeO-MgO-SiO 2 -H 2 O with bulk compositions of 0, 10, 20, and 30 mol% ferro-actinolite when projected onto the tremolite−ferro-actinolite join for the purpose of determining the cation site occupancies as a function of bulk composition. Syntheses were done by multiple treatments of the oxide-metal mixtures in internally heated gas vessels at 600–800 °C, 6 kbar, ƒ O ₂ near the Co-CoO buffer, which is sufficiently low to prevent the appearance of Fe3+ in this study, and for total durations up to 774 hours. Bulk compositions of the synthesis products were assessed by electron microprobe analysis (EMPA), analytical transmission electron microscopy (AEM), and by using unit-cell dimensions (CD method), whereas the site occupancies were assessed using powder X-ray diffraction (XRD) Rietveld refinements, Fourier-transform infrared (FTIR) spectroscopy, and Mössbauer spectroscopy. Selected-area electron-diffraction (SAED) patterns and high-resolution transmission electron microscopy (HRTEM) images were obtained to study the structure and defects of the samples. A maximum of about 2% non-amphibole chain-multiplicity faults (CMFs) were observed in the sample with 10 mol% ferroactinolite component, indicating the strong reduction in CMFs that occurs with only a small addition of ferro-actinolite to tremolitic amphiboles. This study, as with earlier studies, found no significant preferential partitioning of Fe and Mg relative to the bulk Fe content of the sample at the M1 and M3 sites, weak partitioning of Mg relative to Fe into the M2 site, and strong partitioning of Fe relative to Mg at the M4 site. No ferric iron was detected. The partitioning of Fe and Mg at the M4 site was modeled by a simple ideal-activity, two-site exchange reaction using the actinolites from this study and those of previous studies with bulk Fe/(Fe + Mg) ≤ 0.3 and with low Al, Mn, and Na contents using 1 bar and 298K values of ΔG° = 62 kJ and ΔS° = 54 J/K. The FTIR data were analyzed for the presence of short-range order of Fe and Mg at the M1 and M3 sites but none was found.

Published estimates for the equilibrium relative humidity (RH) at 25 °C for the reaction: bieberite (CoSO 4 ·7H 2 O) = moorhouseite (CoSO 4 ·6H 2 O) + H 2 O, range from 69.8 to 74.5%. To evaluate these data, the humidity-buffer technique was used to determine equilibrium constants for this reaction between 14 and 43 °C at 0.1 MPa. Reversals along five humidity-buffer curves yield ln K = 18.03–6509.43/T, where K is the equilibrium constant, and T is temperature in K. The derived standard Gibbs free energy of reaction is 9.43 kJ/mol, which agrees well with several previously reported values based on vaporpressure measurements. It also agrees well with values calculated from the data derived mostly from calorimetric measurements. Previous studies indicated that the temperature of the invariant point for the assemblage bieberite-moorhouseite-aqueous solution-vapor is near 44.7 °C, and our extrapolated data predict 91.1% RH at this temperature; the predicted position for the invariant point is in excellent agreement with those reported previously.

Unanticipated discoveries of new examples of natural phenomena punctuate the history of science with excitement and afford splendid opportunities for ground-breaking, systematic research. And so it is with the near-simultaneous discoveries of non-mass dependent fractionation of O isotopes (Bao et al. 2000b) and of S isotopes (Farquhar et al. 2000a) in terrestrial surface deposits. The discoveries set the stage for members of the Mineralogical Society of America, including geochemists, mineralogists, mineral physicists, and petrologists, to investigate a geologic record of atmospheric photochemistry across the entire span of Earth's lifetime, from the Archean to the present day. The significance of such breakthrough research to humankind is apparent in the essential role played by atmospheric photochemistry in generating the ozone shield, so necessary for sheltering life from solar ultraviolet radiation, and in its possible part in the origin of life itself by shaping Archean pre-biotic chemistry.

A new procedure has been developed for fast preparation of saponite clays under non-hydrothermal synthesis conditions of 90 °C and 1 atmosphere. Saponites were synthesized from a stoichiometric mixture containing Si/Al 3+ gel, M 2+ -nitrate (M 2+ = Mg, Zn, Ni, Co, or Cu), urea, and water within a few hours. The synthesis products were characterized with XRD, IR, TEM, XRF, N 2 -physisorption, Al-EXAFS, and 27 Al- and 29 Si-MAS-NMR. Incorporation of Mg, Zn, Co, Ni, or a combination of these cations in the octahedral sheet, as well as controlling the Si/Al ratio in the tetrahedral sheet in the range between 5.67 and 39.0, could easily be established. Pure Cu-saponite could not be synthesized due to the preferred formation of chrysocolla, but a combination of Mg 2+ and Cu 2+ resulted in saponite formation. The chemical composition strongly influences the textural properties of the saponites. It is possible to prepare saponite samples of a specific surface area and pore volume of 100 to 750 m 2 /g and 0.15 to 1.05 mL/g, respectively. The lateral size and the amount of stacking of the saponite platelets could be influenced by the composition, synthesis duration, and amount of urea. The new synthesis procedure provides an easy way to prepare large quantities of saponites with far-reaching control on the texture as well as the composition.

Synthetic saponites prepared at atmospheric pressure and at 90 °C within 20 hours with Mg 2+ , Zn 2+ , Ni 2+ , Cu 2+ , and/or Co 2+ in the octahedral sheets have been thermally treated under reducing and nonreducing conditions with or without the presence of steam. The thermal stability of saponite in air is investigated at temperatures up to 900 °C. The nature of the octahedral cation largely determines the thermal stability of the saponite, which increases in the order Zn 2+ , Co 2+ , Mg 2+ , to Ni 2+ from 450 to 800 °C. The stability against reduction of the octahedral cations increases in the order Cu 2+ , Ni 2+ , to Co 2+ from 150 to 600 °C. The recrystallization of saponite at high temperatures is usually topotactic, resulting in the formation of either pyroxene or olivine types of minerals. The reduction of saponites also depends on the nature of the octahedral cation. Cu 2+ -containing saponites are most easily reduced, followed by Ni 2+ -containing saponites. The difficult reduction of Co-saponites is possibly related to the thermal breakdown of the saponite structure. Mg- and Zn-saponites do not show any reduction up to 800 °C. Incorporation of Mg 2+ in Co 2+ - or Ni 2+ -containing saponites enhances the resistance to reduction. A fully reduced Ni-saponite shows a high dispersion of small Ni particles (10 nm) throughout the sample, whereas a fully reduced NiMg-saponite consists of large Ni particles (up to 60 nm) clustered together. The influence of steam on Mg-saponite is small, although incorporation of Zn 2+ corresponding to an Mg/Zn ratio of 2 into the octahedral sheets results in a decrease of the hydrothermal stability. The drop in the stability is evidenced by a decrease in crystallinity and in a movement of Al atoms from sites within the saponite structure to non-framework positions.

Leucite is unique among minerals that have been exploited for K-Ar and Ar-Ar dating, in that it exhibits a major phase change at 645-665 °C, across which Ar diffusion and solubility can be studied. In addition, well-developed twinning occurs only in the low temperature form, offering further opportunity to examine the effects of crystallographic change upon Ar diffusion. Nepheline was studied to compare the effect of its 1-dimensional c-axis oriented lattice channels with the 3-dimensional channels of leucite. The amounts of Ar introduced into leucite were far higher than those observed in feldspars, allowing its analysis by electron microprobe spot traverses and X-ray maps. This has provided the first high spatial resolution 2-dimensional study of Ar distribution in a K-bearing mineral. Extreme changes in Ar solubility were observed across the leucite phase transition, with Ar solubilities jumping from ~70 ppm (at 1 kbar) in the tetragonal (low-temperature) form to ~750 ppm (at 1 kbar) in the cubic (high-temperature) form. Argon penetration profiles were complex and estimates of the diffusion rates show that they are more rapid than in K-feldspar. In addition, spikes of high Ar concentration were observed in both forms, suggesting extended defects or micro-inclusions formed argon traps in the structure. No concentration changes could be correlated to leucite twin planes, suggesting that twin planes did not act as fast pathways for Ar movement. Nepheline yielded a much lower Ar solubility of 0.15-0.31 ppm at 1 kbar.

Dissolution and epitaxial growth on dolomite [CaMg(CO 3 ) 2 ] cleavage surfaces under well-defined aqueous solution conditions were studied in real-time by atomic force microscopy (AFM). Step-retreat speeds in undersaturated solutions were used to predict the growth speed for ordered dolomite utilizing a theoretical step-growth model. Monolayer growth on dolomite under highly supersaturated conditions was observed, but multilayer growth was inhibited due to film disorder enhanced solubility. Ordered dolomite film growth is predicted in supersaturated solutions close to equilibrium, but observed step speeds were immeasurably low under the current laboratory low-temperature conditions.

Rhodonite (ideally MnSiO 3 ) is a triclinic pyroxenoid that typically contains significant concentrations of Ca, Fe, and Mg. A variety called fowlerite contains concentrations of Zn up to 10 wt% and has been found only at Franklin, New Jersey. Data from electron microprobe analyses, single-crystal X-ray structure refinements, and preliminary Mössbauer spectra suggest marked differences in the crystal chemistry of rhodonite and fowlerite. Cations in Zn-poor rhodonites and in fowlerites exhibit distinct substitutional trends. Substitution of Zn in fowlerite occurs entirely at M4, and Ca displays a stronger preference for M5 in fowlerite than in Zn-poor rhodonite. Further, the distinctive compositional variations in fowlerite lead to strongly mutually correlated lattice parameter variations distinct from those in Zn-poor rhodonite. Mean M-O distances in fowlerite, with the exception of <M2-O>, are strongly correlated with all unit-cell edges. For Zn-poor rhodonites, neither <M2-O> nor <M4- O> are significantly correlated with cell edges, and the mean bond lengths for M1, M3, and M5 are correlated strongly with only a and b. The most striking distinctions between fowlerite and typical rhodonite occur in the geometrical details of M4 and M5. Distortion in the M4 site of magnesian rhodonite, typical Mn-rich rhodonite, and fowlerite increases with both increasing electron population and mean electronegativity. The M4 site assumes increasing amounts of tetrahedral character with increasing electron density (i.e., increasing Zn concentration). As M4 becomes more Zn-rich, tetrahedral, and covalent in character, M5 becomes more Ca-rich, less distorted, and more ionic in character. This suggests that .pure. fowlerite (nominally Ca 0.2 Zn 0.2 Mn 0.6 O 3 , or CaZnMn 3 O 15 ) may contain an unequivocally four-coordinated M4 site occupied solely by Zn, and with Ca largely restricted to M5.

The color of variscite from three different localities (Pannecé, France; Sarrabus, Sardinia; Ervedosa, Portugal) and metavariscite from Utah was investigated by diffuse reflectance and EPR spectroscopy. These samples contain low levels of transition elements such as V, Cr, and Fe, the relative concentration of which varies with origin, although they remain in the wt% range. The presence of peculiar antiresonance features indicates that octahedral Cr 3+ ions are responsible for the absorption bands that cause the green coloration of variscite and metavariscite. Trivalent iron ions, which give only a weak absorption band in some samples, and V 3+ ions do not contribute to the green coloration of the samples from these locations. The crystal-field splitting and B Racah parameter of Cr 3+ in variscite and metavariscite are 1590 and 657 cm -1 and 1610 and 575 cm -1 , respectively. These values indicate a weak crystal field and an important Cr-O covalence. Electron paramagnetic resonance spectra show the presence of minor amounts (300.500 ppm) of vanadyl groups substituted for Al. The absence of V 3+ indicates that variscite and metavariscite formed in an oxidizing environment.

Structural imaging of the “odd” m = 17 antigorite polysome is in keeping with the X-ray structure refinement, supplying direct evidence for the presence of 8-reversals. HRTEM also displays the polysomatic and stacking disorder features modeled during X-ray refinement to achieve convergence. Polysomatic faults occur in Mg159 antigorite as (100) lamellae, with wavelength differing from the matrix by only one serpentine module (m = 16 or 18 vs. 17). Mixed ordered polysomes may also occur, such as a regular alternation of m = 16 and m = 17 lamellae, producing a complex polysome with an a repeat distance of 84.5 Å. ”Odd” and .even. antigorite polysomes differ in layer topology. ”Odd” polysomes (m ≠ 2n, with n = integer) have a primitive cell with Pm space group, m tetrahedral and m-1 octahedral modules, and one 6-reversal and one 8-reversal per unit cell. Polysomes with m = 2n have C-centered cells (C2/m), m tetrahedral and m-2 octahedral modules, and two 6-reversals and two 8-reversals per cell.

Tetrahedrite fahlores from the Galena Mine of the Coeur d'Alene Mining District (Wallace, Idaho) have been found to be enriched in Ag by the Ag-Cu exchange reaction: which occurred during cooling following fahlore mineralization. This solid-state reaction produced a distinct population of high-Ag fahlores found in galena-rich samples, quantitatively removed Ag (in an AgSbS 2 component) from galena, and accounts for all of the bournonite mineralization. This reaction has produced the most argentian fahlore yet found [molar Ag/(Ag + Cu) = 0.443] in the district, and forms a secondary overprint on any primary fahlore zoning that may have existed. The results obtained here indicate that the galena and fahlore-siderite stages of mineralization were virtually synchronous, as this reaction has produced the same result despite the opposite relative age relationships of these mineralization stages exhibited in two of the mines studied. Multiple lines of evidence suggest a temperature between 320 and 350 °C for this “stage.” Based on the Ag/(Ag + Cu) of fahlores and Fe-Zn partitioning between fahlore and sphalerite, we estimate that fahlore compositions were frozen in by about 235 °C, roughly 40 °C above corresponding temperatures obtained for the Gold Hunter vein of the Lucky Friday Mine. This result and the absence of readily detectable diaphorite (~Pb 2 Ag 3 Sb 3 S 8 ) in the Galena Mine sample containing the highest Ag fahlore indicate a faster cooling rate for the terrane in the vicinity of the Galena Mine.

The presence of carbon in the Earth makes the search for high-pressure carbon-containing phases essential for our understanding of mineral compositions of the Earth's mantle. In a recent study Isshiki et al. (2004) demonstrated that magnesite transforms into a new phase at lower mantle pressures. However, the structure of the emerging phase remained unknown. Here we show, by means of first principles calculations, that MgCO 3 magnesite can transform into a pyroxene structure at 113 GPa, which further transforms into a CaTiO 3 -type structure at about 200 GPa.

Here we report the results of the first three-dimensional refinement of the 10 Å phase performed with single-crystal X-ray data. The 10 Å phase, Mg 3 Si 4 O 10 (OH) 2 H 2 O, is monoclinic, space group C2/m, a = 5.323(1)Å, b = 9.203(1)Å, c = 10.216(1)Å, β = 99.98(1)°, V = 492.9(2) Å 3 ; the calculated density, assuming Z = 2, is 2.676 g.cm -3 . The structure has been solved by direct methods and refined by least-squares method with anisotropic displacement parameters. The final agreement index (R 1 ) was 0.088 for 54 refined parameters and 499 unique observed reflections collected with a diffractometer with a CCD detector. The structure of the 10 Å phase is very similar to that of a homo-octahedral, 1 M trioctahedral mica: it is a silicate consisting of 2:1 tetrahedral-octahedral layers parallel to (001). The mean Si-O, Mg1-O, and Mg2-O bond lengths are 1.626, 2.082, and 2.081 Å, respectively. The ditrigonal rotation angle α is 0.53°. The interlayer of the 10 Å phase is occupied by water molecules. According to the oxygen occupancy, 1 H 2 O p.f.u. is assumed in the investigated sample. Although the average water oxygen position is in the mid-plane, structural refinement suggests disorder along c*. Twelve hydrogen bonds are located between the water molecule and the 6 + 6 oxygen atoms of the basal rings of adjacent tetrahedral sheets (water-oxygen distances averaging 3.19 Å). Therefore there are six possible orientations for the water molecule, with six hydrogen bonds pointing toward the upper basal ring and six pointing toward the lower ring of tetrahedral sheets. The orientational disorder of water, in agreement with previous Raman spectroscopy data, is a feature relevant to the evaluation of thermodynamic functions and thermal stability of the 10 Å phase, which is a possible water carrier (9.1 wt%) in subducting slabs at high pressure.

The presence of hexagonal silicate perovskite (6H-BaTiO 3 type) was confirmed in the SrSiO 3 compound by in-situ angle dispersive X-ray diffraction at high pressure. The perovskite was crystallized from pressure-induced amorphous SrSiO 3 in a diamond anvil cell by laser heating at 35 GPa. On releasing the pressure, the perovskite also changed into an amorphous state as does CaSiO 3 perovskite. This SrSiO 3 perovskite, with a tolerance factor greater than unity, forms a face-sharing SiO 6 octahedron, which leads to a structure with hexagonal symmetry. Incorporation of Sr into CaSiO 3 perovskite in the early stage of the differentiation in the Earth's mantle might have influenced the symmetry of CaSiO 3 perovskite in the present lower mantle. As far as we know, this is the first report suggesting the existence of hexagonal perovskite in silicates.