Disorder and Compositional Variation in the Lillianite Homologous Series

A. Pring1, M. Jercher1, * and E. Makovicky2
1 Department of Mineralogy, South Australian Museum, North Terrace, Adelaide, South Australia 5000, Australia
2 Mineralogical Institute, University of Copenhagen, Øster Voldgade 5-7, DK-1350 Copenhagen, Denmark
*Current address: Anger Straße 27, 40593 Düsseldorf, Germany.

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 Pb3.92 2xBixAgxS6.92 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 minerals • disorder • HRTEM • Ivigtut • Greenland

Mineralogical Magazine; December 1999 v. 63; no. 6; p. 917-926; DOI: 10.1180/002646199548907
© 1999, The Mineralogical Society
Mineralogical Society (www.minersoc.org)