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A neutron powder diffraction study of cation ordering in high-temperature synthetic amphiboles

Mark D. Welch and Kevin S. Knight
A neutron powder diffraction study of cation ordering in high-temperature synthetic amphiboles
European Journal of Mineralogy (April 1999) 11 (2): 321-331

Abstract

The structures of synthetic fluoro-edenite and pargasite end-members have been refined by Rietveld analysis to R values of 1.4 and 1.9 %, respectively. The distribution of Al and Si over T(1) and T(2) has been determined from mean T(1)-O bond lengths. Pargasite crystallized at 1 kbar, 932 degrees C, has 1.71 + or - 0.11 Al pfu at T(1) and 0.28 + 0.11 Al pfu at T(2), implying > or = 15 % long-range Al-Si disorder. Within experimental error, fluoro-edenite crystallized at 2 kbar, 1006 degrees C, is long-range ordered, with all (super [4]) Al at T(1). The Al-Si distribution of synthetic pargasite is similar to that of natural high-T pargasites. These results indicate that long-range Al-Si disorder in pargasites rather than hornblendes is worth investigating as a geothermometer (as hornblendes have inherently larger Al-Si order parameter errors). Mg-Al ordering on octahedral sites in pargasite has been determined from mean octahedral bond lengths; Al is at M2 and M3 sites, with little or none at M1: M2 = 1.5Mg + 0.5Al and M3 = 0.5Mg + 0.5Al. This difference in M2 and M3 site occupancies between natural and synthetic pargasites may reflect fundamentally different growth mechanisms, though both show qualitatively similar ordering behaviour for octahedral cations, pointing to a shared crystal chemistry.


ISSN: 0935-1221
Serial Title: European Journal of Mineralogy
Serial Volume: 11
Serial Issue: 2
Title: A neutron powder diffraction study of cation ordering in high-temperature synthetic amphiboles
Affiliation: Natural History Museum, Department of Mineralogy, London, United Kingdom
Pages: 321-331
Published: 199904
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 31
Accession Number: 2007-031113
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables
Secondary Affiliation: Rutherford Appleton Laboratory, GBR, United Kingdom
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United KingdomTwickenhamUKUnited Kingdom
Update Code: 200717
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