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 BaAlK−1Si−1. 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 1M polytype and the structure were refined in space group C2/m (0.020 ≤ R ≤ 0.034). lncreasing 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(1) 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(1) unshared edges, which decreases in samples with the highest [XII]Ba2+ content