Disorder and compositional variation in the lillianite homologous series

A. Pring, M. Jercher, E. Makovicky

Research output: Contribution to journalArticle

39 Citations (Scopus)

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 Pb(3.92-2x) Bi(2+x)Ag(x)S6.92 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.

Original languageEnglish
Pages (from-to)917-926
Number of pages10
JournalMineralogical Magazine
Volume63
Issue number6
DOIs
Publication statusPublished - 1 Jan 1999
Externally publishedYes

Keywords

  • Disorder
  • Greenland
  • HRTEM
  • Ivigtut
  • Lillianite group minerals

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