Crystal chemistry and structure refinements of barian muscovites from the Berisal Complex, Simplon region, Switzerland

The structures of seven muscovite-2M₁ crystals from the Berisal Complex, with Ba between 0.04 and 0.35 pfu, were refined from single-crystal X-ray data. The crystal with the highest Ba content has the composition [Si_2.82Al_1.18O₁₀](OH)₂, space group C2/c, Z = 2, a = 5.198(1), b = 9.017(3), c = 19.895(5) A, β = 95.79(2)°, V = 927.7 A³. Ba is incorporated into dioctahedral micas mainly by the substitution Ba + ^[IV]Al ↔ K + ^[IV]Si and to a lesser extent by Ba + ^[VI](Mg,Fe²⁺) ↔ K + ^[VI]Al. Both types of substitution cause a lateral expansion of the sheets, observed as an increase of the a and b cell dimensions. The distortions of tetrahedral and octahedral sheets are very similar to ordinary muscovites leading to tetrahedral rotations of α = 10.6-11.5° for both Ba-poor and barian muscovites. Because of the similar ionic radii of Ba²⁺ and K⁺, all crystals investigated have I - O_inner average distances of 2.83 Å that is achieved by a reduction of the interlayer separation with increasing Ba content. Thus the lateral expansion, which would increase I - O_inner, is counterbalanced by decreasing the interlayer separation. Extrapolation to the hypothetical Ba end-member yields an interlayer separation of about 3.1 Å, very similar to paragonite, whereas muscovite has an interlayer separation of about 3.45 Å. The reduced interlayer separation in barian muscovites is also reflected in the lattice spacing perpendicular to (001), expressed by c X sin β, if a correction for the paragonite content (Na on 1) is applied.