CRYSTAL-CHEMISTRY OF BA-RICH TRIOCTAHEDRAL MICAS-1M

Ten single-crystal X-ray structure refinements of igneous and metamorp hic Ba-bearing micas were determined to define the exchange vectors re quired to compensate for the excess in positive layer charge caused by [XIII]Ba2+ for [XIII]K+ substitution. The determined exchange vector is BaAlK-1Si-1. The micas have a Ba2+ content in the range 0.01-1.09 a pfu (atoms per formula unit) with octahedra mostly populated by Mg2+ ( 2.21 less-than-or-equal-to Mg2+ less-than-or-equal-to 3.07 apfu) and F e2+ (0.13 less-than-or-equal-to Fe2+ less-than-or-equal-to 1.53 apfu). All samples are 1M polytype and the structures were refined in space group C2/m (0.020 less-than-or-equal-to R less-than-or-equal-to 0.034) . Increasing the substitution of [IV]Al3+ for [IV]Si4+ distorts the te trahedra by progressively increasing the dimensions of the basal edges . This increase in the lateral dimensions of the tetrahedral sheet all ows for a greater rotation angle alpha (5.9 less-than-or-equal-to alph a less-than-or-equal-to 11.5). Octahedral sheet crystal chemical featu res (ordering pattern, size and distortion of both M(1) and M(2) sites ) are similar to those reported for micas in the phlogopite-annite joi n. The most notable variation involves the length of M(1) unshared edg es, which decreases in samples with the highest [XIII]Ba2+ content.