Showing papers in "American Mineralogist in 1993"

Structural characterization of kerogens to granulite-facies graphite: Applicability of Raman microprobe spectroscopy

Abstract: The usefulness of the laser Raman microprobe (LRM) in characterizing the state of structural order of geologically relevant carbonaceous materials ranging from kerogen to granulite-facies graphite (in our samples, 4A < L a < ∞) is demostrated. The first part of this paper is an overview of the process of graphitization, two techniques used to characterize CM structurally (HRTEM and XRD analysis), and the theory of Raman spectroscopic analysis of CM

Quantitative mineralogical analysis using the Rietveld full-pattern fitting method

Abstract: Ansrnlcr Quantitative phase analysis of a number of multicomponent standard and natural mineral mixtures has been done using an adaptation of the Rietveld method. Binary mixtures (most 50:50 by weight) of corundum with quartz, hematite, ilmenite, magnetite, biotite, analcime, mordenite, or clinoptilolite were analyzed,using digital powder X-ray diffraction (XRD) data. In addition, a suite of standard mixtures of hematite and corundum, a natural feldspar mixture, the G-l standard granite, two natural bauxite samples, and a mixture of biogenic carbonate minerals were also analyzed. Quantitative information was extracted from refined individual scale factors and unit-cell voiumes (derived from refined unit-cell parameters), obtained with a Rietveld refinement program modified to analyze up to ten phases. The quantitative results for standard mixtures were within 2.50/o (absolute) of the true values, with the exception of the hematite, ilmenite, and magnetite mixtures. Results for the latter mixtures using CuKa data were severely afected by microabsorption, but analysis of the hematite mixtures using FeKa radiation gave results with absolute errors <2010. Results for the G-l granite and the natural feldspar agreed well with optically determined modes, and the method facilitated separation of the significant overlaps in the pattern of the carbonate minerals. Quantitative mineralogical analysis by the Rietveld method has several significant advantages over conventional methods of quantitative analysis. The method uses all intensity data in a pattern rather than a few of the most intense reflections, partially compensating for preferred orientation and extinction. In addition, standard dala are calculated for each phase during analysis, overcoming the troublesome requirement of obtaining standards representative of the materials in an unknown. It is also possible to gain a wealth of information from each sample in addition to amounts of phases. Because some of the most troublesome systematic errors, including sample displacement and zero-point shift, can be refined, the method yields unit-cell parameters of an accuracy comparable with that obtained when using an internal d-value standard. The method should find a wide application in geology, including in modal analysis and compositional determinations of individual mineral components using unit-cell parameter systematics.

Interactive software for calculating and displaying X-ray or neutron powder diffractometer patterns of crystalline materials

Abstract: Two computer programs, XPOW and XPOWPLOT, that generate and graph X-ray or neutron powder diffractometer patterns of crystalline materials are described. The input for XPOW requires only the radiation wavelength, cell dimensions, space group, and positional parameters for the atoms in the asymmetric unit. The output includes a listing of the d values, 2θ values, and the relative intensities for the non equivalent Bragg reflections within a given 2θ interval. Using the XPOW output, the XPOWPLOT program creates menu-aided interactive color displays of up to five powder diffractometer patterns simultaneously on the PC monitor

CO2 contents and formation pressures of some Kilauean melt inclusions

Abstract: Of 50 analyzed glass inclusions in olivine phenocrysts from the 1959 Kilauea Iki eruption, 41 formed at pressures 2 kbar. The surprisingly low formation pressures suggest that most 1959 olivines, including most of those with preeruptive equilibration temperatures above 1200 degrees C, crystallized in an upper part of Kilauea) s summit magma storage reservoir. The implication that the parental magma was buoyant relative to stored magma is consistent with an expected preeruptive bulk CO2 content near 0.2 wt% and published evidence for mixing between hot, newly arrived parental and preexisting magma. That the 1959 magma was rich not only in crystals but also in gas, as evidenced by its high lava fountains, suggests that the storage time in a shallow reservoir was too short for either crystals or gas to be lost. Therefore, the 1959 Kilauean magma probably is a near-parental magma that rose and formed a gas- and crystal-rich cap near the top of a shallow body of stored magma beneath Kilauea) s summit region. Whether newly arriving parental magma is buoyant relative to stored magma depends mainly on pressure and magma gas content. Consequently, it seems likely that the eruptive and degassing behavior of Kilauea is regulated in part by an interplay between the CO, content of parental magma and the pressure at which new magma intrudes stored, degassed magma.

A Concise Compilation of Petrologic Information on Possibly Pristine Nonmare Moon Rocks

Abstract: Ansrn.q.cr To facilitate systematic study of the surviving compositionally pristine (endogenously igneous) rocks of the ancient lunar crust, a compilation has been generated of all likely samples, along with key information on the petrologic characteristics and chemistry of each sample. The compilation includes 260 samples. Besides information related to the likelihood of each sample being truly pristine (i.e., mainly its texture and siderophile element abundances), information is compiled on mineral content, listing major phases present as well as basic information on mineral compositions, on size (expressed as mass), and on whether a reasonably comprehensive chemical analysis has been published. The compilation also classifies the samples into seven categories of confidence in the pristine composition of the samples, reflecting an estimation of the relative likelihood that each arguably pristine sample is in fact pristine. For many purposes, it is crucial to avoid inclusion of polymict rocks in a data base. On petrologic diagrams such as a plot of average Mg' [Mg' : 100 x molar Mg/(Mg + Fe)] in a low-Ca mafic silicate vs. average An content in plagioclase, rocks in the top three categories of the confidence in pristine character appear distinctly bimodal in composition, with roughly half belonging to a ferroan suite characterized by high An despite relatively low Mg'. When samples of low to moderate pristine character are included, the bimodality appears less distinct. Sample mass can also be important. With a data base restricted to samples more massive than I g, there is a clear distinction in feldspar content and bulk density between ferroan and nonferroan (Mgsuite) rocks, such that only the ferroan-suite rocks are likely to have formed as flotation cumulates. With a data base including smaller samples, the same basic pattern is seen, but only in a blurred form, as the two rock types show considerable overlap in their modal feldspar contents.

Reassignment of cation site occupancies in tourmaline: Al-Mg disorder in the crystal structure of dravite

Abstract: The crystal structure of a dravite (tourmaline) sample from Osarara, Narok District, Kenya, a = 15.947(2), c = 7.214(1) A, V = 1589.0(6) A3, R3m, has been refined to an R index of 2.0% based on 1136 reflections measured with MoKa X-radiation. Electron microprobe analysis, site-scattering refinement, electronic absorption (published), stereochemical analysis, and Mossbauer spectra show the structural formula to be x (Nao.8l4Cao.oo9Ko.ol4Do.l63)Y (Mgl.30l Mno.o03Fe6.bsl Fe6.16oCro.oo6 Tio.03oAlo.98s)z (AIs.o90M&.9lo)(B03)3Si60l8(O,OH)4. Of particular interest is the assignment of significant Mg to the Z site and Al to the Y site. For published structural refinements of tourmaline, the grand mean size of the Y and Z polyhedra is a linear function of the constituent cation radii. That is not the case for the Y and Z sites individually. However, greatly improved linearity occurs if significant Mg is assigned to the Z site for some compositions when Al > 6.0 atoms pfu; this indicates that the usual assumption that Al completely occupies the Z site before occupying any other site. is not universally correct.

Pressure effects, kinetics, and rheology of anorthositic and related magmas

Abstract: Anhydrous experiments on natural and synthetic starting materials with basaltic to anorthositic bulk compositions show a systematic increase in the albite component of nearJiquidus plagioclase and in the AlrO. content of orthopyroxene with increasing pressure. These results are consistent with crystallization of the highly aluminous orthopyroxene megacrysts and most of the plagioclase in massif anorthosite complexes at lower crustal pressures. Comparison of plagioclase compositions from near-liquidus and subliquidus experiments conducted in this laboratory with plagioclase compositions predicted at I atm for the experimental temperature and liquid compositions by various empirical models indicates that the shift to more albitic plagioclase is predominantly a pressure effect on the partitioning of albite (Ab) and anorthite (An) components between plagioclase and liquid. However, even when pressure terms are added to the models for Ab and An partitioning, there remain statistically significant compositional dependencies that are most apparent when the liquid composition is nepheline normative. These compositional dependencies probably arise from the absence of highly aluminous and nepheline-normative liquids in the data from which the models were constructed. Accordingly, we present empirical adjustments to the plagioclase-liquid models of Drake (1976), Weaver and Langmuir (1990), and Ariskin and Barmina (1990). The positive pressure dependence of AlrO, in orthopyroxene coexisting with plagioclase and liquid is almost entirely the result of changes in orthopyroxene-liquid partitioning and not related to increases in the AlrO, concentration of the liquid. Data for AlrO, partitioning from 46 orthopyroxene-liquid and 45 pigeonite-liquid pairs taken from the literature show that pressure is the most important control on the simple molar partition coefficient for AlrO.. Rapid crystal growth is rejected as an alternative explanation for the high AlrO, contents of orthopyroxene megacrysts because rapid growth leads to low CrrO, concentrations in orthopyroxene, contrary to what is observed. These results support polybaric models for massif anorthosite petrogenesis that entail accumulation of plagioclase in evolved basaltic magma chambers ponded in the lower crust followed by buoyant ascent of plagioclase-rich magmatic suspensions that intrude the upper crust, carrying rafts of orthopyroxene megacrysts. In thick, decompressing suspensions, the interplay of tieline rotation and mass balance prevents plagioclase from becoming significantly more anorthitic. Experimental studies suggest that the transition from liquidto solid-state rheology of plagioclase suspensions occurs at -600lo crystallinity for a homogeneous grain-size distribution and near static conditions. However, both motion of the suspension and uneven grain-size distribution shift the transition to higher crystallinities. Thus the transit of suspensions with leuconoritic composition (65-700/o plagioclase) may be possible with minimal deformation of the entrained plagioclase. Formation of deformed anorthosite masses may then occur as second-stage buoyant segregations within the upper crustal magma chambers.

Dehydration mechanism of clinoptilolite and heulandite: Single-crystal X-ray study of Na-poor, Ca-, K-, Mg-rich clinoptilolite at 100 K

Abstract: Ansrucr The crystal structure of clinoptilolite from Weitendorf, Styria, Austria la : 17.622(2), b : 17.895(2), c : 7.399(l) A, B : 116.45(2), space group C2/ml, (MgonCa,oSro,BaorK,rNaoo)AlrSirrOr2'25H2O, was studied by single-crystal X-ray diffraction. The structures of a fully hydrated form (25 HrO) and three partially dehydrated forms with 75 HrO were refined at 100 K. The dehydration is accompanied by cation difusion within the channel system. Four cation positions (Nal, Ca2, K3, Mg4) were found in the channels of the highly hydrated form. With loss of HrO, cations migrate preferentially to site K3 at the center of a distorted eight-membered ring of tetrahedra, forming channels parallel t o a . I n t h e 5 H , O v a r i a n t I a : 1 7 . 6 1 ( l ) , b : 1 1 . 4 5 6 ( 7 ) , c : 7 . 3 6 0 ( 7 ) A , B : 1 . 1 6 . 7 7 ( 4 ) 1 , HrO is strongly disordered, but the framework remains expanded. With progressing dehydration, disorder in the tetrahedral framework is reflected in increasing atomic displacement parameters and decreased T-O distances. The thermal stiffness of the heulandite-clinoptilolite framework is related to the Al concentration on T2, which governs the bond strength between Ol and Ca2. A small Al concentration on T2 enables cation diffusion to occur from Ca2 to K3 upon heating. High occupancies oflarge cations on K3 prevent the structure from collapsing.

Formation of protective surface layers during silicate-mineral weathering under well-leached, oxidizing conditions

Abstract: Formation of a protective surface layer [a layer of products through which transport (diffusion) of aqueous reactants or products is rate-determin ing] during silicate-mineral weathering requires that (1) elements normally considered immobile (e.g., Al, Fe) behave conservatively; and (2) the volume ratio of product to reactant be greater than one. Mineral suites involving most major rock-forming silicates (feldspars, pyroxenes, amphiboles, and olivines) weathering to common oxides and 1:1 clays are characterized by F prod Freacl (Fprod/FreacI > 1). Weathered almandine and spessartine commonly exhibit laterally continuous, nonporous surface layers underlain by smooth, rounded reactant-mineral surfaces. The rate-determining step during weathering in these instances is diffusion through the surface layer of weathering products. Elemental mobility in the weathering environment and product-reactant volume ratios determine the occurrence of surface features (etch pits vs. protective surface layers) and associated rate-determining mechanisms (interface- vs. transport-controlled kinetics).

Synthesis, stability, and properties of Al2SiO4(OH)2: A fully hydrated analogue of topaz

Abstract: The OH end-member of the F-OH topaz solid solution series, with a composition close to Al 2 SiO 4 (OH) 2 , has been synthesized at pressures between 55 and 100 kbar an dtemperatures up to 1000°C from gels and crystalline starting materials. The results of single-crystal X-ray diffraction [space group Pbnm, a=4.724, b=8.947, c=8.390A] show close agreement with the structure of F-rich topaz but a nearly pure dilatation of both the Al(5%) and the Si(1.5%) sites; the distortion parameters of these sites were slightly affected.

The composition of lizardite 1T and, the formation of magnetite in serpentinites

Abstract: Miissbauer data were obtained from 23 samples 120 Iizardite (1Q samples, two chrysotile samples, and one antigorite samplel taken from five geologically well-characterized serpentinites. These serpentinites represent both hydration and serpentine recrystallization in lizardite-chrysotile serpentinites. Mdssbauer parameters obtained from the samples used in this study were integrated with electron microprobe and HrO extraction yields to generate a comprehensive set of compositional data for lizardite, which was compared with modal magnetite values in the same samples. Lizardite contains up to 700/o of its Fe as Fe3* in both tetrahedral and octahedral coordination. fhs [elfs:+ content shows a positive correlation with t4lFe3+ and a negative correlation with r6lFe2+. Substitutions oftrivalent cations in both sheets are correlated and do not require cation vacancies (including H+) to maintain charge balance. The [alFe/l6lFe and Si/Mg ratios of lizardite, chrysotile, and antigorite are different, with ths t+lfs/telps ratio decreasing, and the Si/Mg ratio increasing from lizardite to chrysotile to antigorite. The Fe,o, content and the t6rFe/(t6lFe + Mg) and Fe,3;f/Fe'", ratios of lizardite in the Jeffrey and Woodsreef serpentinites are inversely correlated with modal magnetite, indicating that f, was internally controlled in these serpentinites. In contrast, the Fe,o, and the [6rFe/(t6rFe + Mg) and Fe."{/Fe,", ratios of lizardite in the Cassiar serpentinite exhibit either direct or inverse correlations with modal magnetite, indicating internal or external control of /o, with /", externally controlled early in recrystallization and internally controlled later in recrystallization. The availability of Si affects internal vs. external control of /o, because Si exchanges with t4lFe3+ either to form or to consume magnetite during serpentine recrystallization.

Crystal chemistry of Fe3+ and H+ in mantle kaersutite: Implications for mantle metasomatism

Abstract: Chemical and crystal chemical analyses have been performed on a suite of subcontinental, mantle-derived hornblende (kaersutite) samples. Mdssbauer techniques have been utilized to investigate Fe valence and site occupancies, U extraction techniques have been used to determine bulk H contents, proton-induced y-ray emission (PIGE) analysis was employed to measure F, and electron microprobe techniques coupled with the above measurements have been utilized to determine major-element contents of hornblende. Similar analyses were performed on a suite of metamorphic amphibole samples from Cosca et al. (1991). Comparison with their wet chemical results on Fe3+/Fe2+ permitted determination of C : 1.22, the correction for differential recoil-free fraction effects, which was used to correct the mantle sample M0ssbauer data. The results of the analyses for the kaersutite samples show a nearly l:l inverse relationship between the Fe3+ and H+ contents. Although the range of Fe3+/H+ in the less oxidized kaersutite samples may be explained by partial H loss during entrainment and ascent, the nearly total dehydrogenation of the Fe3*-rich megacrysts would require time scales significantly longer than what is expected for transport. Thus, it seems likely that these oxykaersutite samples grew in a more oxidized metasomatic fluid, where incorporation of H was not required for charge compensation. As megacrysts from the same location show wide variation in Fe3* and H+, it appears likely that significant variations in the oxidation state of the mantle metasomatic fluid occurred over relatively small temporal or spatial scales.

Syntheses, volume, and structural changes of garnets in the pyrope-grossular join : implications for stability and mixing properties

Abstract: We have synthesized 11 garnet compositions in the pyrope-grossular join from glass starting materials by a combination of hydrothermal synthesis and recycling of most of the products at "-40 kbar, 1400 °C, in graphite capsules. Syntheses of homogeneous crystals were also successful within the compositional range Grso-Grso, which represented a conspicuous gap in earlier studies on synthetic garnets. The lattice parameters of the synthetic garnets have been determined with a precision of 0.0002-0.0004 A. The results show a slightly positive excess volume of mixing, which is asymmetric toward the grossular end, and can be described by a subregular or Margules formulation with WtaMg= 0.36 ::t 0.23 and Wt'tgca= 1.73 ::t 0.3 cm3/mol (12 0 atom basis). Powder X-ray diffraction (XRD) structural refinements show linear dependence of the 0 positional parameters and cation to 0 distance in dodecahedral, octahedral, and tetrahedral sites on the Ca content of garnet. The partial molar volume of grossular in pyrope-grossular and almandine-grossular joins, determined from our data and those of Geiger et al. (1989), varies smoothly and similarly between the terminal compositions and exceeds the molar volume by < 1%, which has an effect of no more than 1.5% on the calculation of equilibrium pressure of the assemblage grossular + alumino silicate + plagioclase + quartz. The partial molar volumes of pyrope and almandine change very similarly as a function of the Ca content of garnet. The v-x relation of the pyrope-grossular join determined in this work has been used to calculate the effect of lattice strain on the enthalpy of mixing. We have also extended the formulation of Ferreira et al. to derive a simple but general expression to account for the effect of multiatom interaction on lattice strain.

The mechanism of Cl incorporation in amphibole

Abstract: The crystal structure of three calcic C2/m amphiboles with different Cl contents have been refined to R indices of ∼2% using MoKα X-radiation. The monovalent anion site is split into two distinct sites, O3 and O3', that are separated by ∼0.5A along [100] and are occupied by (OH,F) and Cl, respectively. The variation in bond lengths tho the O3 and O3' sites as a function of cation and anion site populations can be interpreted in terms of a strong preference for Fe 2+ -Cl short range order. Bond strength arguments and observed interatomic distances indicate a significant bonding interaction between K at the A site and Cl at the O3' site

On the role of Al-Si ordering in the cubic-tetragonal phase transition of leucite

Abstract: The cubic-tetragonal phase transition in leucite has been examined using static lattice energy calculations with realistic, tested interatomic potentials. The calculations have demonstr~ted that the ~ransitio~ is not triggered by AI-Si ordering and can occur without any ordenng. Any AI-Sl order w1l1have only a minor effect on the observed spontaneous strain and structure distortion. Calculations show that any long-range AI-Si order is associated with the energetics of next-nearest AI-O-AI linkages rather than nearest neighbor interactions, leading to a low ordering temperature.

Structure of silicate melts at high temperature : in-situ measurements in the system BaO-SiO2 to 1669°C

Abstract: The structure of glasses, melts, and supercooled melts in the system BaO-SiO 2 with BaO/SiO 2 =0.367 has been investigated in situ with Raman spectroscopy from 25 to 1669°C. For this purpose a technique has been developed that integrates confocal micro-Raman spectroscopy with a microheating stage that fits on the sample stage of a petrographic microscope. The temperatures are precise to ±4°C and accurate to 5-10°C. The excited volume is ∼1μm in diameter by 20-50μm in depth

REE fractionation between scheelite and apatite in hydrothermal conditions

Abstract: The geochemical analysis of hydrothermal apatite and scheelite pairs from various types of W ore deposits (skarns, disseminated scheelite, and quartz veins) provides an insight into REE partitioning between the two minerals. Among the 18 analyzed pairs, only five appear to have grown in equilibrium conditions. Ten other pairs show more or less important departures from equilibrium. The remaining apatite and scheelite pairs have quite different REE patterns, indicating crystallization from different fluids. Both minerals concentrate REE. The relative behavior of HREE and LREE is quite similar in the two minerals. Scheelite is only slightly more enriched in HREE relative to LREE than apatite, with K(ap-sch)La-Yb = 0.86 +/- 0.22. Beside these regularities, some dispersion in the lanthanide content ratios of apatite and scheelite, ranging from 0.6 to 5, may be related to fluid composition. The behavior of Eu can be related to redox conditions, which appear to be more oxidizing in vein associations than in skarn environments. Determination of REE in coexisting scheelite and apatite seems an efficient tool for identification of successive ore-bearing fluids.

Crystal chemistry of ferromagnesian silicate spinels: Evidence for Mg-Si disorder

Abstract: Single-crystal x-ray diffraction data have been obtained for synthetic Fe-Mg silicate spinels, γ-(Mg x Fe 1-x ) 2 SiO 4 (x=1.00, 0.40 and 0.20). The measurements on γ-Mg 2 SiO 4 synthesized at 20 GPa and 1400°C, compared with previous data on a specimen synthesized at 22 GPa and 1000°C, provide evidence that approximately 4% of the Si tot enters octahedral coordination in the sample synthesized at higher temperature. Fe-bearing silicate spinels synthesized at lower pressures display no evidence for Fe-Si disorder