H positions in leucophoenicite, [Mn.sub.7][Si.sub.3][O.sub.12][(OH).sub.2]: A close relative of the hydrous B phases

The proton positions in leucophoenicite, ideally [Mn.sub.7][Si.sub.3][O.sub.12][(OH).sub.2], have been determined by neutron powder diffraction under ambient conditions on a natural sample from Franklin, New Jersey. Refinement in the P[2.sub.1]/a space group gave [R.sub.p] = 2.0% (w[R.sub.p] = 2.1%), [chi square] = 4.04 for 110 refined parameters. The two non-equivalent protons form a pair of hydroxyl groups and make hydrogen bonds to the same O7 atom of the Si1 tetrahedron at distances of 1.99(1) and 2.09(1) [Angstrom]. The H.... H distance in leucophoenicite is 2.16(1) [Angstrom], which is more than twice the Van der Waal's radius of H (>2 [Angstrom]) and so no proton positional disorder is expected in leucophoenicite. A comparison of the H environments is made between leucophoenicite and Phase B, Superhydrous B, and Phase A.