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The crystal structure of perhamite

¹ Museum Victoria, GPO Box 666E, Melbourne Victoria 3001, Australia
² CSIRO Minerals, Bayview Avenue, Clayton, Victoria 3169, Australia
³ CNRS Laboratoire de Cristallographie, BP 166X, 38042 Grenoble, France

^* E-mail: ian.grey{at}csiro.au

The crystal structure of perhamite, (Ca,Sr)₃Al_7.7Si₃P₄O_23.5(OH)_14.1.8H₂O, from the Emmons mine, Maine, USA, has been determined using single crystal X-ray data. The average structure has trigonal symmetry, Pm1, with cell parameters a = 7.021(1) Å and c = 20.218(1) Å. It was refined to R₁ = 0.044 for 618 observed reflections. The structure comprises ordered blocks of crandallite-type structure, centred at z = 0, intergrown parallel to (001) with disordered aluminosilicate structure blocks centred at z = 1/2 to form a microporous structure containing large channels along [100]. These channels are bounded by 8-member rings of 6 tetrahedra (2 SiO₄, 2 AlO₄ and 2 PO₄) and 2 AlO₆ octahedra. Calcium atoms and water molecules are distributed in the [100] channels. A model was developed for the local ordering of silicon into the fractionally occupied sites in the (001) layer at z = 1/2 and this model was refined in space group P321 to R₁ = 0.041 for 933 observed reflections. The dominant contributors to the local order are Si₃O₉ rings of corner-shared tetrahedra, together with Si₂O₇ pairs of tetrahedra. These units corner-link to (Al,Si)O₄ tetrahedra above and below the plane at z = 1/2 to form 4-member rings, which in turn corner-share to PO₄ tetrahedra in the crandallite blocks to give the 8-member rings. The analysis suggests that 5-coordinated Si may also be present in the (001) plane at z = 1/2.

KEYWORDS: crystal structure, perhamite, Maine, USA, XRD