Refinement of hydrogen positions in synthetic hydroxyl-clinohumite by powder neutron diffraction

The structure of synthetic hydroxyl-clinohumite [Mg7Si4O14. 2Mg(OH)O] was r efined for a deuterated sample using powder neutron diffraction data and th e Rietveld technique [(P2(1)/b; Z = 2; a = 4.7488(1) Angstrom; b = 10.2875( 2) Angstrom; c = 13.6967(3) Angstrom; alpha = 100.63(1)degrees; V = 657.65( 2) Angstrom (3))]. The H atoms of the hydroxyl groups are disordered over t wo positions with an occupancy of approximately 0.5. This removes the possi bility of a strong H-H interaction that was thought to destabilize the stru cture. The limited occurrence of hydroxyl-clinohumite in nature is the resu lt of compositional rather than crystal chemical constraints.