Cation distribution in amphiboles synthesized along the Ga analogue of the tremolite–aluminotschermakite join |
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| Authors: | D M Jenkins K Ishida V Liogys D Ghosh |
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| Institution: | (1) Department of Geological Sciences and Environmental Studies, Binghamton University, Binghamton, New York 13902-6000, USA Tel.: +1-607-777-2264 Fax: +1-607-777-2288 E-mail: dmjenks@binghamton.edu, US;(2) Kyushu University Graduate School of Social and Cultural Studies, 4-2-1 Ropponmatsu, Chuo-ku, Fukuoka, 810, Japan, JP;(3) Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0420, USA, US |
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| Abstract: | Amphiboles were synthesized from bulk compositions prepared along the join Ca1.8Mg5.2Si8O22(OH)2–Ca1.8Mg3Ga4Si6O22(OH)2 hydrothermally at 750–850 °C and 1.0–1.8 GPa, and along the join Ca2Mg5Si8O22F2–Ca2Mg3Ga4Si6O22F2, anhydrously at 1000 °C and 0.7 GPa to document how closely the tschermak-type substitution is obeyed in these analogues
of aluminous amphiboles. Electron-microprobe analyses and Rietveld X-ray diffraction structure refinements were performed
to determine cation site occupancies. The extent of Ga substitution was found to be limited in both joins, but with the fluorine
series having about twice the Ga content (0.6 atoms per formula unit, apfu) of the hydroxyl-series amphiboles (0.3 apfu).
The tschermak-type substitution was followed very closely in the hydroxyl series with essentially equal partitioning of Ga
between tetrahedral and octahedral sites. The fluorine-series amphiboles deviated significantly from the tschermak-type substitution
and, instead, appeared to follow a substitution that is close to a Ca-pargasite substitution of the type: 6]Ga3++24]Ga3++1/2A] Ca2+ = 6]Mg2++24]Si4++1/2A]□. Infrared spectroscopy revealed an inverse correlation between the intensity of the OH-stretching bands and the Ga content
for the hydroxyl- and fluorine-series amphiboles. The direct correlation between the Ga and F content and inverse relationship between the Ga and
OH content may be a general phenomenon present in other minerals and suggests, for example, that high F contents in titanite
are controlled by the Al content of the host rock and that there may be similar direct Al–F correlations in tschermakitic
amphiboles. Evidence for the possibility that Al (Ga) might substitute onto only a subset of the tetrahedral sites in tschermakitic
amphiboles was sought but not observed in this study.
Received: 5 March 2001 / Accepted: 31 July 2001 |
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| Keywords: | Amphibole Tschermak Tremolite Fluorine Gallium |
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