Lithiomarsturite, LiCa₂Mn₂Si₅O ₁₄(OH)

Lithio-marsturite, ideally LiCa₂Mn₂Si ₅O₁₄(OH), is a member of the pectolite-pyroxene series of pyroxenoids (hydro-pyroxenoids) and belongs to the rhodonite group. A previous structure determination of this mineral based on triclinic symmetry in space group P1̄ by Peacor et al. [Am. Mineral. (1990), 75, 409-414] converged with R = 0.18 without reporting any information on atomic coordinates and displacement param-eters. The current study redetermines its structure from a natural specimen from the type locality (Foote mine, North Carolina) based on single-crystal X-ray diffraction data. The crystal structure of lithio-marsturite is characterized by ribbons of edge-sharing CaO₆ and two types of MnO₆ octa-hedra as well as chains of corner-sharing SiO₄ tetra-hedra, both extending along [110]. The octa-hedral ribbons are inter-connected by the rather irregular CaO8 and LiO6 polyhedra through sharing corners and edges, forming layers parallel to (1̄11̄ ), which are linked together by the silicate chains. Whereas the coordination environments of the Mn and Li cations can be compared to those of the corresponding cations in nambulite, the bonding situations of the Ca cations are more similar to those in babingtonite. In contrast to the hydrogen-bonding scheme in babingtonite, which has one O atom as the hydrogen-bond donor and a second O atom as the hydrogen-bond acceptor, our study shows that the situation is reversed in lithio-marsturite for the same two O atoms, as a consequence of the differences in the bonding environments around O atoms in the two minerals.