Crystal chemistry of Ba-rich trioctahedral micas-\M
Brigatti, Maria Franca; Poppi, Luciano
European Journal of Mineralogy Volume 5 Number 5 (1993), p. 857 - 872
published: Jan 1, 1992
manuscript accepted: May 11, 1993
manuscript received: Feb 9, 1993
ArtNo. ESP147050505015, Price: 29.00 €
Abstract Ten single-crystal X-ray structure refinements of igneous and metamorphic Ba-bearing micas were determined to define the exchange vectors required to compensate for the excess in positive layer charge caused by [XII]Ba2+ for [XII]K+ substitution. The determined exchange vector is BaAlICiSLi. The micas have a Ba2+ content in the range 0.01-1.09 apfu (atoms per formula unit) with octahedra mostly populated by Mg2+ (2.21 < Mg2+ < 3.07 apfu) and Fe2+ (0.13 < Fe2+ < 1.53 apfu). All samples are IMpolytype and the structures were refined in space group Cllm (0.020 < R < 0.034). Increasing the substitution of [IV]Al3+ for [IV]Si4+ distorts the tetrahedra by progressively increasing the dimensions of the basal edges. This increase in the lateral dimensions of the tetrahedral sheet allows for a greater rotation angle α (5.9 < α < 11.5). Octahedral sheet crystal chemical features (ordering pattern, size and distortion of both M(l) and M(2) sites) are similar to those reported for micas in the phlogopite-annite join. The most notable variation involves the length of M(l) unshared edges, which decreases in samples with the highest [XII]Ba2+ content.