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Disorder and compositional variation in the lillianite homologous series

Published online by Cambridge University Press:  05 July 2018

A. Pring
Affiliation:
Department of Mineralogy, South Australian Museum, North Terrace, Adelaide, South Australia 5000, Australia
M. Jercher
Affiliation:
Department of Mineralogy, South Australian Museum, North Terrace, Adelaide, South Australia 5000, Australia
E. Makovicky
Affiliation:
Mineralogical Institute, University of Copenhagen, Øster Voldgade 5-7, DK-1350 Copenhagen, Denmark

Abstract

High resolution transmission electron microscopy studies on the lillianite group minerals from the Ivigtut cryolite deposit, Ivigtut, South Greenland revealed the existence of disordered intergrowths of lillianite/gustavite-like blocks (N − 4) and heyrovskyite-like (N = 7) structural blocks. One disorder sequence is examined in detail, which gave an average homologue number N = 4.92 corresponding to a composition of Pb3.922xBixAgxS6.92 with x ≈ 1.2. An Axial Next-Nearest Neighour Ising model was used to follow the fluctuations in the average homologue number N across the crystal. This yielded compositional fluctuations of the order of 70–170 Å over a 1800 Å region of the crystal, with a 202 Å lamella of ordered vikingite. Trends in the randomness of the gustavite-vikingite intergrowth were evaluated and the dominant slab sequence was found to be 4,4,4,7 and 4,4,7,7, suggesting that some longer period homologues may be stable. A number of defects were noted in which changes in slab widths were accommodated. The origin of these partially ordered/disordered lillianite homologues is discussed.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1999

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Footnotes

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Current address: Anger Straße 27, 40593 Düsseldorf, Germany.

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