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

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doi: 10.1127/ejm/11/2/0321
Authors:Welch, Mark D.; Knight, Kevin S.
Author Affiliations:Primary:
Natural History Museum, Department of Mineralogy, London, United Kingdom
Other:
Rutherford Appleton Laboratory, United Kingdom
Volume Title:European Journal of Mineralogy
Source:European Journal of Mineralogy, 11(2), p.321-331. Publisher: Schweizerbart'sche Verlagsbuchhandlung (Nägele u. Obermiller), Stuttgart, Federal Republic of Germany. ISSN: 0935-1221
Publication Date:1999
Note:In English. 31 refs.; illus., incl. 4 tables
Summary: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°C, has 1.71 ± 0.11 Al pfu at T(1) and 0.28 + 0.11 Al pfu at T(2), implying ≥ 15 % long-range Al-Si disorder. Within experimental error, fluoro-edenite crystallized at 2 kbar, 1006°C, is long-range ordered, with all [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. [R.A.H.]
Sections:Crystal structure
Subsections:Specific structures: silicates
Subjects:Amphibole group; Chain silicates; Clinoamphibole; Colloidal materials; Crystal chemistry; Crystal structure; Edenite; Fluorine; Gels; Geologic thermometry; Halogens; High temperature; Neutron diffraction data; Order-disorder; Pargasite; Powder method; Rietveld refinement; Silicates; Synthetic materials; Temperature; Unit cell; Fluoro-edenite
Abstract Numbers:99M/3587
Record ID:2007031113
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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