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H positions in leucophoenicite, Mn₇Si₃O₁₂(OH)₂: A close relative of the hydrous B phases

¹ Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, U.K.
² ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, U.K.
³ Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, U.S.A.

Correspondence: ^* E-mail: mdw{at}nhm.ac.uk

The proton positions in leucophoenicite, ideally Mn₇Si₃O₁₂(OH)₂, have been determined by neutron powder diffraction under ambient conditions on a natural sample from Franklin, New Jersey. Refinement in the P2₁/a space group gave R_p = 2.0% (wR_p = 2.1%), ² = 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) Å. The H····H distance in leucophoenicite is 2.16(1) Å, which is more than twice the Van der Waal’s radius of H (>2 Å) 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.

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