학술논문
Coulsellite, CaNa3AlMg3F14, a rhombohedral pyrochlore with 1:3 ordering in both A and B sites, from the Cleveland Mine, Tasmania, Australia
Document Type
Academic Journal
Author
Source
American Mineralogist. 95(5-6):736-740
Subject
Language
English
ISSN
0003-004X
Abstract
Coulsellite, CaNa3AlMg3F14, from the Cleveland tin mine at Luina, western Tasmania, has a rhombohedral distortion of the cubic pyrochlore A2B2X6Y structure, with a=7.1756(1) Å, α=59.867(1)°, space group R3̄m, Z=1. The corresponding hexagonal cell parameters are a=7.1620(1) Å, c=17.5972(3) Å. The crystals are multiply twinned about threefold axes of the pseudocubic cell. The structure was determined using X-ray data collected on a twinned crystal and refined to Robs=0.027 for 452 observed reflections with I>2σ(I). The structure is possibly unique among published structures of pyrochlore-like minerals in having full 1:3 ordering of Ca:Na in the A sites and Al:Mg in the B sites. Transmission electron photomicrographs show a nanodomain structure due to twinning on a scale of approximately 5 nm.