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New data on welshite, e.g. Ca2Mg3.8Mn0.62+Fe0.12+Sb1.55+O2[Si2.8Be1.7Fe0.653+Al0.7As0.17O18], an aenigmatite-group mineral

Published online by Cambridge University Press:  05 July 2018

E. S. Grew ,
U. Hålenius ,
M. Kritikos  and
C. K. Shearer
E. S. Grew*
Affiliation:
Department of Geological Sciences, University of Maine, 5790 Bryand Center, Orono, Maine 04469-5790, USA
U. Hålenius
Affiliation:
Department of Mineralogy, Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden
M. Kritikos
Affiliation:
Division of Structural Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91, Stockholm, Sweden
C. K. Shearer
Affiliation:
Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico 87131, USA
*
*E-mail: esgrew@maine.maine.edu
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Abstract

Electron and ion microprobe data on two samples of welshite from the type locality of Långban, Sweden, gave analytical totals of 99.38–99.57 wt.% and BeO contents of 4.82–5.11 wt.%, corresponding to 1.692–1.773 Be/20 O. Mössbauer and optical spectra of one of these samples gave [iv]Fe3+/ΣFe = 0.91, [vi]Fe2+/ΣFe = 0.09, and no evidence of Mn3+. The resulting formula for this sample is Ca2Mg3.8Mn0.62+Fe0.12+Sb1.55+O2[Si2.8Be1.7Fe0.653+Al0.7As0.17O18, and that for the second sample, Ca2Mg3.8Mn0.12+Fe0.12+Fe0.83+Sb1.25+O2[Si2.8Be1.8Fe0.653+Al0.5As0.25O18], is related by the substitution involving tetrahedral and octahedral sites: 0.59[vi,iv](Fe,Al)3+ ≈ 0.42[vi](Mg,Mn,Fe)2+ + 0.21([vi]Sb,[iv]As)5+, i.e. 3[vi,iv]M3+ = 2[vi]M2+ + [vi,iv]M5+. Welshite is distinctive among aenigmatite-group minerals in the high proportion of Fe3+ in tetrahedral coordination and is unique in its Be content, substantially exceeding 1Be per formula unit. Given the cation distributions in other minerals related to aenigmatite, we think it is reasonable to assume that at least one tetrahedral site is >50% occupied by Be and that one octahedral site is >50% occupied by Sb, so that welshite should be retained as a distinct species with its own name in the aenigmatite group.

Keywords

welshiteaenigmatite groupLångban depositSweden
Type
Research Article
Information
Mineralogical Magazine , Volume 65 , Issue 5 , October 2001 , pp. 665 - 674
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2001

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