Showing papers in "American Mineralogist in 1989"

Coupled substitutions involving REEs and Na and Si in apatites in alkaline rocks from the Ilimaussaq Intrusion, South Greenland, and the petrological implications

Abstract: Ansrnlcr Apatites from the Ilimaussaq augite syenite, the sodalite foyaite, and a qtartz-bearing peralkaline pegmatite have been analyzed using an electron microprobe for the major elements plus Na, Si, and selected rare-earth elements (REEs). In all three rock types, the maximum REETO3 content in the apatite exceeds 16.0 wt0/0. Apatite from the augite syenite and primary apatite from the sodalite foyaite are characterizedby a low Na content that varies independently of the REE content. The composition of these apatites approaches 25 molo/o of the end-member CarREq(SiO4)3(OH,F,Cl) (lessingite) in response to the substitution REE3+ * Sia+ + Ca2+ + Ps+. REE- and Na-enriched apatites from the quartzbearing peralkaline pegmatite are almost Si free, and REEs and Na are positively correlated. The composition of these apatites approaches 25 molo/o of the hypothetical endmember Na5REE5(PO4)6(OH,F,CI)2, in response to the substitution Na* + REE3+ + 2Ca2* . The compositional trend of late-crystalllzing apatite from the sodalite foyaite reflects almost equal contributions of the two substitutions. The relationship between these substitutions in a given apatite and its associated mineral assemblage indicates that the Na component is only present in considerable amounts when apatite crystallizes in a strongly peralkaline environment.

OH- in pyroxene: An experimental study of incorporation mechanisms and stability

Abstract: Most pyroxenes contain trace amounts of OH^- that are lost when heated at 700 °C (1atm) under either air or H_2. When Si-deficient Fe^(3+) -rich pyroxenes (ferrian diopside and esseneite) are heated in H_2, an up to 70-fold increase of the intensity of the infrared OH absorption bands occurs, which is well correlated with Si deficiency in the tetrahedral site. Mossbauer and optical spectroscopy indicate that these samples have substantial amounts of Fe^(3+) in the tetrahedral position. After the uptake of H_2, optical spectra show that the amount of Fe^(2+) in the M(2) site has increased, corresponding to charge compensation according to the reaction Fe^(3+)O^(2-) + 1/2H_2 = Fe^(2+) + OH^-. Heating in air at 600-700 °C decreases or removes the OH^- bands. The OH^- bands from air-heated samples are restored by subsequent heating in H_2. Hydrothermal experiments (600 to 800 °C and Ito 2-kbar H_2O pressure) do not increase the amount of OH^- but do redistribute some of the absorption intensity among the OH^- bands. Experiments performed in D_2O show that OD^- readily replaces OH^-. Therefore the diffusion of H^+ through the pyroxene crystal cannot be a rate-limiting process, and hence a more fundamental change than just a mechanical introduction is required to incorporate OH^- in the pyroxene structure. The thermal stability of the OH^- in pyroxene is comparable to OH^- in Fe-rich amphiboles and suggests that pyroxenes can provide information about the activity of hydrous components prevalent during their crystallization.

Five- and six-coordinated Si in K2Si4O9 glass quenched from 1.9 GPa and 1200 °C

Abstract: It has often been suggested that a high pressure, Si in silicate melts should change from four to higher coordination. Transition states involving five-coordinated Si have often been observed in molecular dynamics simulations even at low pressure and have been given a key in configurational change and viscous flow. Using 29Si MAS NMR, we have definitively observed small amounts of both five- and six-coordinated Si in a K2Si4O9 glass quenched from a liquid at 1200°C and 1.9 GPa. Such results provide new constraints for models of density, viscosity, and thermodynamics of silicate melts. -Authors

OH substitution in garnets: X-ray and neutron diffraction, infrared, and geometric-modeling studies

Abstract: Tetrahedral-site occupations were determined for three titanian andradites (San Benito County, California) and a synthetic deuterated hibschite by using X-ray single-crystal and neutron powder data, respectively. Site refinements reveal the presence of tetrahedral vacancies (4-14%) in all three andradites. Infrared absorption spectra measured for the same material used in the X-ray analysis indicate structurally bound water (as OH^-) in amounts of 0.8 to 5. 7 wt% OH, which is in good agreement with water contents derived from the refinements. These data confirm that the tetrahedral site is not fully occupied and that charge balance can be achieved by the substitution (O_4H_4)^(4-) = (SiO_4)^(4-). The proton position could not be determined because of the small amount of H present. Diffraction maxima in the neutron profile for hibschite exhibited small, well-defined shoulders related to chemical inhomogeneity. The data were fit using multiphase Rietveld techniques assuming four phases with slightly different Si/D ratios. Structural parameters [ɑ= 12.0105(3) A; O(x,y,z) = 0.03561(14), 0.04653(12), 0.64957(12); d site= 0.767(6) Si] refined for the major phase [62(3)% mole fraction) were consistent with X-ray refinements of natural hydrogrossulars. The deuterium atom [0.0965(5), 0.0520(4), 0.6600(5)] was located (ΔF map) outside the tetrahedral volume near the position reported for the Si-free end-member. A significant improvement in R factor was obtained after refinement of a split-atom model to describe the oxygen positional disorder. The short O-D distance [0.744(6)] calculated for the ordered (average) structure can be interpreted within the context of this model. Distance-least-squares (DLS-76) calculations were used to simulate the effect of the (O_4H_4)^(4-) = (SiO_4)^(4-) substitution on the grossular structure. If the tetrahedral d-O distance, calculated from vacancy concentration, is weighted heavily in the geometric refinement, structural variations in the hydrogrossular series [Ca_3Al_2(SiO_4)_(3-)Ca_3Al_2(O-4H_4)_3] can be predicted. Application of DLS to other garnet structures suggests that mantle garnets (rich in pyrope component) may contain only very limited amounts of water.

Corrosion of Fe-Ni alloys by Cl-containing akaganeite (beta -FeOOH); the Antarctic meteorite case

Abstract: A suite of 12 iron and 8 stony meteorites from various environments and locations in Antarctica was studied, representing a range of compositions, metallographic structures, and degrees of weathering. Polished sections of meteoritic metal with adhering corrosion products were examined microscopically, with the electron microprobe and SEM, and were used as samples for X-ray diffraction examination. The major corrosion products are Cl-containing akaganeite (up to 5wtper thousand Cl) and goethite, with minor amounts of lepidocrocite and maghemite. Cl is not native to the meteorites but, owing to the electrochemical nature of the corrosion of Fe-Ni alloys, is attracted from the snow and ice, or rocky soil, environment that contains low amounts of Cl. Akaganeite precipitates near the reaction front, incorporating Cl - ions into ion-exchange sites, where they may be retained, made available for further depassivating, corrosive action, or become flushed from the system. With time and distance from the reaction surface, akaganeite ages and transforms to mainly goethite and maghemite. It is again argued that lawrencite (FeCl 2) is not a mineral in meteorites. In the stormy Antarctic, wind-blown terrestrial particles may become attached to the weathering products on the meteorite and in time themselves decompose, delivering terrestrial ions to the meteorite corrosion system. The ions may move a significant distance from the surface, and in particular Cl may be found along almost invisible cracks in the deep interior. Trace-element work and age determinations ( 36 Cl) should take this problem into account.

Halogen-rich scapolite and biotite; implications for metamorphic fluid-rock interaction

Abstract: Abundant Cl-rich scapolite and biotite indicate high Cl activities during greenschistthrough amphibolite-facies regional metamorphism of metasedimentary rocks northwest of the Idaho batholith. Biotite-zone granofels contain up to 40 modal percent Cl-rich scapolite. The strict stratigraphic control on the occurrence ofscapolite and the fine-scale interlayering of Cl-rich and Cl-poor metasedimentary rocks suggest that scapolite was formed during regional metamorphism by reaction of plagioclase with calcite and halite. Mass-balance constraints on the formation of scapolite show that, prior to metamorphism, sedimentary units containing l0 modal percent halite along restricted sedimentary horizons, 0.5 to 20 cm thick, would have been sufficient to form the most scapolite-rich layers. The abundance ofCl-rich phases decreases abruptly with the appearance ofzoisite-bearing assemblages in the metasedimentary rocks, suggesting infiltration of low-Cl HrO at higher grades. Scapolite compositions range from 32 to 62 equivalent anorthite (EqAn) and 0.7 6 to 0.06 Xo. There is a general increase in EqAn and decrease in X., with increasing metamorphic grade. However, local variations in scapolite composition suggest hat metamorphic fluid compositions were heterogeneous and internally bufered. Biotites coexisting with scapolite contain up to 0.51 wto/o Cl (biotite grade) and 2.58 wto/o F (kyanite + sillimanite grade). Biotite compositions indicate significant differences inlogffir/fr.:) values of fluids in equilibrium with low-grade, biotiteand carbonate-rich granofels. Differences in the inferred metamorphic fluid compositions are observed, in some cases, down t o a s c a l e o f < l c m . Cl is strongly partitioned into an aqueous fluid relative to solid phases and is removed by metamorphic devolatilization and infiltration by dilute fluids. Therefore, the occurrence of bufered activity gradients in Cl, as indicated by scapolite and biotite compositions, suggests that metamorphic fluid flow was highly channelized and aqueous fluid-rock interaction was limited. High and variable Cl activities were not smoothed out by aqueous fluid-rock interaction through two regional metamorphic events.

Order-disorder transition-induced twin domains and magnetic properties in ilmenite-hematite

Abstract: Ansrnacr The transition temperature (4) between disordered, R3c, and ordered, R3, ilmenitehematite solid solutions in the ferrian ilmenite composition range between Ilmuo and Ilm'o has been redetermined by observing the presence or absence of transition-induced cationordered domains and the behavior of pre-existing domains annealed below the transition. The transition was reversed for Ilmro and is bracketed between 1000 and 1050 "C. The domains were shown by dark-field transmission electron microscopy to be twin related by a 180" rotation about an axis parallel to a and to vary in size depending on Z" and the temperature of quench. A model of the twin-domain boundary indicates that such boundaries are disordered and partially Fe-enriched. Magnetic and reiu observations on the same samples show that the room-temperature saturation magnetization is related to the surface area of the twin boundaries. Because the cation-ordered domains are ferrimagnetic with a strong magnetic moment and the disordered boundaries are probably antiferromagnetic with a weak magnetic moment, the quenched samples are essentially mixtures of two magnetic phases. The magrretization is therefore related to the proportion of each phase. The twin boundaries act as the classic "x" phase, which allows ferrian ilmenite to acquire a self-reversed thermoremanent magnetization (rnvr). Our measurements indicate that when the surface area of the twin boundaries in Ilmro exceeds approximately 25 x 106 m2/m3, the quenched samples acquire reverse rnu during cooling in a 0.5-Oe field. When the boundary surface area is less than this critical threshold, the quenched samples acquire a normal rnvr.

Temperature dependence of the speciation of water in rhyolitic melts and glasses

Abstract: The concentrations of molecular water and hydroxyl groups in rhyolitic glasses and melts depend on temperature as well as total water content. Increasing temperature results in an increase in the proportion of water dissolved as hydroxyl groups at a constant total water content. The temperature dependence of water speciation has been determined experimentally and can be described in thermodynamic terms by using a regular-solution formulation for the interactions between water molecules, hydroxyl groups, and oxygens in the rhyolitic glass or melt. This formulation is also consistent with calorimetric data on hydrous haplogranitic glasses. The temperature dependence of water speciation in rhyolitic glasses can be used as the basis of a geothermometer for hydrous volcanic glasses. Obsidians from pyroclastic deposits from the ca. 1340 A.D. eruption of the Mono Craters, California, yield temperatures of about 600 oC. These chips may represent samples of the cool glassy margins and roofs of feeder dikes for this eruption. Glass inclusions in quartz phenocrysts from the Plinian deposit of the Bishop Tuff yield temperatures of ~300-600 oC; inclusions in quartz phenocrysts from the Mono Lobe ash-flow deposit of the Bishop Tuff yielded temperatures of ~200-300 oC. These temperatures are interpreted as closure temperatures and suggest that the initially hotter ash-flow deposit cooled more slowly than the Plinian deposit.

Crystal chemistry of alkali-deficient schorl and tourmaline structural relationships

Abstract: Ansrucr The structure of an aluminous, alkali-deficient schorl has been refined (R : 0.054 for 3084 intensity data) and its data integrated with those of 12 other refined tourmaline structures to provide a better understanding of the structural accommodation of diverse substitutions in the tourmaline group. Alkali-deficient schorl, (Nao rrtro o'IFefiuAlo rnMgo rrTi' 0r)Al6(BO3)3(Si5 s6Al0 ro)Or8 o8((OH), ,rFo ,o), is rhombohedral with a : 15.963(3) and c : 7.148(2) A; its space group is R3ru. The 3a alkali site in the alkali-deficient schorl with an occupancy of (Nao rrEo or) is the largest yet observed in tourmaline. Substitution of cations smaller than Na, notably Ca, afects the configuration of the six-membered tetrahedral ring by decreasing its degree of "crimping" and by increasing its ditrigonality and the distortion of its tetrahedra. The only observed effect ofAl Si substitution is an increase in the size ofthe tetrahedra. Rz+ R3+ substitution in the 9b octahedral site in response to the dehydroxylation-type substitution, (OHf + R2+ : 02+ R3*, and alkali defect-type substitution, R+ + R2+ : 3aE + R3+, results in increased "puckering" (inward rotation of the tetrahedra of the sixmembered ring) and in a marked compression of the smaller l8c octahedra with which the larger 9b octahedra share edges. The volume of the unit cell is highly correlated (r : 0.98) to the weighted-mean size, [9rc + 4(l8c-O)]/5, of the 9b and l8c octahedra, which explains why lattice parameters can be successfully used to estimate octahedral-site occupancies. The chemistries of natural as well as synthetic tourmalines in the systems NarOAlrO3-SiOr-BrOr-HrO and MgO-AlrO3-SiOr-BrOr-HrO are also rationalized on the basis of the observed structural trends.