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Disorder and compositional variation in the lillianite homologous series

Published online by Cambridge University Press:  05 July 2018

A. Pring ,
M. Jercher  and
E. Makovicky
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
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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 Pb3.922xBi2·xAgxS6.92 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.

Keywords

lillianite group mineralsdisorderHRTEMlvigtutGreenland
Type
Research Article
Information
Mineralogical Magazine , Volume 63 , Issue 6 , December 1999 , pp. 917 - 926
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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