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The crystal structure of perhamite

Published online by Cambridge University Press:  05 July 2018

S. Mills ,
G. Mumme ,
I. Grey  and
P. Bordet
S. Mills
Affiliation:
Museum Victoria, GPO Box 666E, Melbourne Victoria 3001, Australia
G. Mumme
Affiliation:
CSIRO Minerals, Bayview Avenue, Clayton, Victoria 3169, Australia
I. Grey*
Affiliation:
CSIRO Minerals, Bayview Avenue, Clayton, Victoria 3169, Australia
P. Bordet
Affiliation:
CNRS Laboratoire de Cristallographie, BP 166X, 38042 Grenoble, France
*
*E-mail: ian.grey@csiro.au
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Abstract

The crystal structure of perhamite, (Ca,Sr)3Al7.7Si3P4O23.5(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, Pml, 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 A1O4 and 2 PO4) and 2 A1O6 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 structureperhamiteMaineUSAXRD
Type
Research Article
Information
Mineralogical Magazine , Volume 70 , Issue 2 , April 2006 , pp. 201 - 209
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2006

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