Abstract
Leucophoenicite, a 10.842 (19), b 4.826 (6), c 11.324 (9) Å, β 103.93° (9), P21/a, possesses the crystallochemical formula Mn7 [SiO4]2[(SiO4)(OH)2], with two formula units in the crystal cell. The atomic arrangement was deciphered from Patterson synthesis; atomic coordinate and isotropic temperature factor refinement by least-squares techniques led to Rhkl=0.07, using 1207 non-zero reflections.
The structure is based on hexagonal close-packed oxygen anions stacked parallel to {010}, with an octahedral two-layer repeat. The octahedral populations define a new kind of kinked serrated chain equally apportioned in the two octahedral levels of the b-axis repeat. These chains run parallel to the z-axis, explaining the frequent twinning by reflection on {001}. A family of kinked serrated chains can be defined by a simple algorithm which utilizes a particular octahedral cluster as its component. Leucophoenicite actually belongs to a homologous series distinct from the closely related humite mineral group, although both series have in common the olivine structure type as their simplest member.
The octahedrally populated chains place restrictions on the tetrahedral populations. For leucophoenicite, there is a set of fully occupied tctrahedra with point symmetry 1 and a set of disordered half-occupied tetrahedra; these latter occur as edge-sharing tetrahedral pairs, with the mid-point of the common edge possessing point symmetry 1. This pair has average composition [(SiO4)(OH)2] and its presence results in unusual but explicable polyhedral distortions.
