The crystal structure of santaclaraite,l CaMnn[SisOr4(OH)l(OH).HrO: the role of hydrogen atoms in the pyroxenoid structure

A new mineral, santaclaraite (CaoqoMn?LM&orFe2o*0,)s[Si5Or4(OH)XOH). H2O, is triclinic with a: 10.273(4),0: l l.9l0(4), c: 12.001(6)A, a: 105.77(3), F: l l0.6a(3), r: 8?.13(3)", V: l3l7.0(8)A'; Z:4 for the 1T unit-cell setting. The crystal structure consists of alternat-ing tetrahedral and octahedral layers. The tetrahedral layer is made up of infinite single chains of silicate tetrahedra with a repeat period of five tetrahedra. The octahedral layer in-cludes rows of ten octahedra with adjacent octahedral rows displaced along their length to form bands two or three octahedra wide. As isolated units, the tetrahedral chain and octahe-dral band of santaclaraite are similar to the corresponding portions of the rhodonite struc-ture. The structure of santaclaraite, however, ditrers in that (l) two adjacent chains (or bands) in a given layer are displaced by a half c translation, and (2) the octahedral layer is rotated by a half turn in the plane parallel to the layer with respect o the adjacent tetrahedral layer. The three roles of hydrogen as hydrogen bond, hydroxyl group, and water molecule are respon-sible for the above half-translation and half-rotation. Three octahedral sites, Ml, M2, and M3, are essentially occupied by Mn atoms. The Ca atoms are ordered in M5, and the small amount of Mg is probably concentrated in M4. Differential thermal analysis and thermo-gravimetric analysis indicate that the dehydration of santaclaraite occurs at approximatel