Showing papers on "Silicate minerals published in 1987"

A general equation for estimating Fe3+ concentrations in ferromagnesian silicates and oxides from microprobe analyses, using stoichiometric criteria

Abstract: A simple general equation is presented for estimating the Fe 3 § concentrations in ferromagnesian oxide and silicate minerals from microprobe analyses. The equation has been derived using stoichiometric criteria assuming that iron is the only element present with variable valency and that oxygen is the only anion. In general, the number of Fe 3 + ions per X oxygens in the mineral formula, F, is given by; F = 2X(1 - T/S) where T is the ideal number of cations per formula unit, and S is the observed cation total per X oxygens calculated assuming all iron to be Fe 2 § Minerals for which this equation is appropriate include pyralspite and ugrandite garnet, aluminate spinel, magnetite, pyroxene, sapphirine and ilmenite. The equation cannot be used for minerals with cation vacancies (e.g. micas, maghemite) unless, as in the case of amphiboles, the number of ions of a subset of elements in the formula can be fixed. Variants of the above equation are presented for some of the numerous published schemes for the recalculation of amphibole formulae. The equation is also inappropriate for minerals showing SP += 4H § substitution (e.g. staurolite, hydrogarnet), minerals containing an unknown proportion of an unanalysed element other than oxygen (e.g. boron-bearing kornerupine) and minerals containing two or more elements with variable valency.

Mechanism of diopside dissolution from hydrogen depth profiling

Abstract: The basic processes involved at the reaction interface during the aqueous dissolution of crystalline silicates are a matter of controversy. In particular, the existence of a hydrated or protonated surficial layer which has been invoked to explain dissolution kinetics has not been demonstrated yet. By using a resonant nuclear reaction, which allows hydrogen depth profiling, we present, in the case of diopside, the first direct evidence that dissolution proceeds via a surficial hydration over thicknesses of about 1,000 A. This evidence is combined with X-ray photoelectron spectrometry and secondary ion mass spectrometry data, to show that approximately congruent leaching at energetic portions of the mineral surface allows permeation of the resulting porous structure by water, much of which is strongly bonded to the silicate surface. Thus, we suggest that the migration of molecular water into the surface could be a key step in the dissolution kinetics of diopside and probably of other cationic silicate minerals.

29Si magic-angle NMR spectroscopy of low-temperature ordered plagioclase feldspars

Abstract: Plagioclase feldspars are among the most abundant silicate minerals in the Earth's crust, and yet their subsolidus phase relations remain, perhaps, the most enigmatic1. Their complex behaviour, which involves ordered (commensurate and incommensurate) structures, displacive transitions and three miscibility gaps, is generally attributed to variations in the ordering of aluminium and silicon between tetrahedral sites of the feldspar framework structure as a function of temperature and composition1,2. The similarity in X-ray scattering factors of Al and Si atoms, however, has hindered conventional attempts to elucidate the exact structural relations. In the context of feldspar mineralogy, the recent advent of magic-angle sample spinning nuclear magnetic resonance spectroscopy (MAS NMR) has many implications, because it provides a powerful and direct method for determining the local distributions of Al and Si– in solids3–8. The 29Si-MAS NMR spectra of natural and synthetic plagioclases are equally complex, however, in containing many overlapping peaks and varying according to the chemistry and thermal history of each sample examined8–11. We present here new spectra, obtained for a suite of carefully characterized ordered samples from meta-morphic and slowly cooled igneous rocks, which shed light on: (1) the sensitivity of the feldspar framework to small amounts of Al/Si disorder; (2) the substitution mechanism of Si for Al in the solid solution from pure anorthite (CaAl2Si2O8) towards albite (NaAlSi3O8); and (3) structural variations in the incommensurate ordered structure of intermediate compositions. In the wider context of silicate chemistry, NMR studies on feldspars provide insights into the subtle influences of continuous solid solution on crystal structures with changing composition. This is also the first attempt to examine the local structure of an incommensurate phase by NMR spectroscopy.

Selective weathering of shocked minerals and chondritic enrichment of the Martian fines

Abstract: In a recent paper, Boslough and Cygan reported the observation of shock-enhanced chemical weathering kinetics of three silicate minerals. Based on the experimental data and on those of Tyburczy and Ahrens for enhanced dehydration kinetics of shocked serpentine, a mechnaism is proposed by which shock-activated minerals are selectively weathered on the surface of Mars. The purpose of the present abstract is to argue on the basis of relative volumes of shocked materials that, as a direct consequence of selective weathering, the composition of the weathered surface units on Mars should be enriched in meteoritic material.

Shock-enhanced dissolution of silicate minerals: An important planetary surface process

Abstract: This study was undertaken to examine the effect of shock loading on rock-water interactions and on the dissolution of silicate phases Shock-recovery experiments were carried out on oligoclase, hornblende, and labradorite No additional crystalline phases or glass were produced, although mean grain size was decreased and specific surface area increased Dissolution experiments were conducted; the extracted solutions are being analyzed (DLC)