Kinoshitalite, Ba(Mg)(3)(Al2Si2)O-10(OH,F)(2), a brittle mica from a manganese deposit in Oman: Paragenesis and crystal chemistry

Kinoshitalite, ideally Ba(Mg)(3)(Al2Si2)O-10(OH,F)(2), was studied in a gra nulite-facies manganese ore associated with calcium-bearing magnesian tephr oite, hausmannite, calcite and manganoan clinochlore, and in a second assem blage together with hausmannite, altered tephroite, manganoan diopside. and calcite. The metamorphic rocks occur enclosed in peridotites of the Semail Ophiolite, Sultanate of Oman and were metamorphosed during ophiolite obduc tion. Single-crystal X-ray data, collected on an inclusion-free kinoshitalite of Ba0.99K0.66Na0.01 (Mg2.64Mn0.31Al2.01Si2.03)O-10(OH1.61F0.37Cl0.02). compos ition, yielded a C-centered lattice of monoclinic symmetry with a = 5.316(1 ), b = 9.230(2), c = 10.197(2) Angstrom, beta = 100.06(1)degrees V = 492.6 Angstrom(3), and Z = 1 characteristic of the I M polytype. The structure wa s refined in two models assuming complete Si, Al ordering in the space grou ps C2 and C (1) over bar allowing for additional twinning. The data clearly suggest that in spite of the Si/Al ratio of 1 assumption of complete Si, A l ordering can be rejected. Two models, both in agreement with space group Gum, causing Si, Al disorder, are discussed. Three-dimensional Si, Al disor der would lead to violation of the Al avoidance rule and substantial Ba dis placement. The rather well defined Ba position capped by two six-membered r ings of Si3Al3O18 composition suggests one dimensional disorder where compl etely Si, Al ordered layers exist parallel to (001). The disorder occurs pe rpendicular to (001) and may be interpreted as random stacking faults.