Disorder and compositional variation in the lillianite homologous series
ABSTRACT I ttigh resolution transmission electron microscopy studies on the lillianite group minerals from the Ivigtul cryolite deposit, Ivigtut, South Greenland revealed the existence of disordered intergrowths of lillianite/gustavite-like blocks (N- 4) and beyrovskyite-like (N = 7) structural bloc...
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| Format: | Text |
| Language: | English |
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.604.8555 http://www.minersoc.org/pages/Archive-MM/Volume_63/63-6-917.pdf |
| Summary: | ABSTRACT I ttigh resolution transmission electron microscopy studies on the lillianite group minerals from the Ivigtul cryolite deposit, Ivigtut, South Greenland revealed the existence of disordered intergrowths of lillianite/gustavite-like blocks (N- 4) and beyrovskyite-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:~.92 x Bi2, ~Ag,$6.92 with x ~ 1,2. An Axial Next-Nearest Neighour lsing modcl 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 A over a 1800 A region of the crystal, with a 202 A lamella of ordered vikingite. Trends in the randomness of the gustavite-vikingite ntergrowth 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 i discussed. |
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