Disorder and compositional variation in the lillianite homologous series.

• Published in: Mineralogical Magazine
• Main Authors: Jercher, M., Makovicky, E., Pring, A.
• Format: Article in Journal/Newspaper
• Language: English
• Published: MINERALOGICAL SOCIETY 1999
• Subjects: Greenland
• Online Access: http://hdl.handle.net/2440/13815; https://doi.org/10.1180/minmag.1999.063.6.11

<jats:title>Abstract</jats:title> <jats:p>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 (<jats:italic>N</jats:italic> − 4) and heyrovskyite-like (<jats:italic>N</jats:italic> = 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<jats:sub>3.92</jats:sub><jats:sub>2<jats:italic>x</jats:italic></jats:sub>Bi<jats:sub>2·<jats:italic>x</jats:italic></jats:sub>Ag<jats:sub><jats:italic>x</jats:italic></jats:sub>S<jats:sub>6.92</jats:sub> with x ≈ 1.2. An Axial Next-Nearest Neighour Ising model was used to follow the fluctuations in the average homologue number <jats:italic>N</jats:italic> 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.</jats:p>