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Refinement of the crystal structure of a synthetic non-stoichiometric Rb-feldspar

Published online by Cambridge University Press:  05 July 2018

A. Kyono  and
M. Kimata
A. Kyono*
Affiliation:
Institute of Geoscience, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki, 305-8571, Japan
M. Kimata
Affiliation:
Institute of Geoscience, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki, 305-8571, Japan
*
*E-mail: kyono@esca.geo.tsukuba.ac.jp
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Abstract

The crystal structure of hydrothermally synthesized Rb-feldspar (monoclinic, space group C2/m, a= 8.839(2)Å, b= 13.035(2)Å, c= 7.175(2)Å, β = 116.11(1)8, V= 742.3(3)Å3, Z= 4) has been refined to a final R of 0.0574 for 692 independent X-ray reflections. Microprobe analyses of the Rb-feldspar suggest deviation from stoichiometry, with excess Si and Al, resulting in a unit formula of Rb0.811□0.127Al1.059Si3.003O8. Infrared (IR) spectra indicate the structural occupancy of large H2O content, which implies that the □Si4O8 substitution favours the structural incorporation of the H2O molecule at the M-site. The mean T–O distances are 1.632 Å for T1 and 1.645 Å for T2, revealing highly disordered (Al,Si) distribution with Al/Si = 0.245/0.755 (T1 site) and 0.255/0.745 (T2 site).

There are two geochemical implications from this refinement: (1) identification of both rubicline triclinic with (Al,Si) ordered distribution and synthetic monoclinic RbAlSi3O8 with (Al,Si) disordered distribution implies that Rb cannot be one of factors disrupting the (Al,Si) ordered and disordered distributions in feldspars; and (2) natural and synthetic feldspars capable of accommodating the large cations tend to incorporate □Si4O8, excess Al and H2O components in their crystal structures.

Keywords

Rb-feldspar□Si4O8Al-excess substitutionH2O molecule
Type
Research Article
Information
Mineralogical Magazine , Volume 65 , Issue 4 , August 2001 , pp. 523 - 531
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2001

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