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Refinement of the Crystal Structure of Phengite-2M1

Published online by Cambridge University Press:  02 April 2024

Audrey C. Rule  and
S. W. Bailey
Audrey C. Rule
Affiliation:
Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, Wisconsin 53706
S. W. Bailey
Affiliation:
Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, Wisconsin 53706
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Abstract

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The crystal structure of phengite-2M1 from Rio de Oro, Spanish Sahara, was refined in space group C2/c to a residual of 3.3% with 1267 independent X-ray diffraction reflections. The composition of the mica determined by electron microprobe analysis is (K0.948Na0.051Ba0.027)(Al1.510Mg0.273Fe0.144Cr0.095Ti0.010Mn0.003)2.035(Si3.253Al0.747)O10(OH)2. The cell dimensions are a = 5.2153(5), b = 9.043(2), c = 19.974(9) Å, β = 95.789(9)°, and V = 937.2(3) Å3. The substitution of Si for Al in the tetrahedral sheet and larger divalent cations for Al in the octahedra allows the amount of distortions that are generally required to alleviate tetrahedral-octahedral sheet lateral misfit in muscovite, such as tetrahedral rotation and octahedral flattening, to be reduced. The O…H vector is nearly horizontal and points slightly into the octahedral sheet where it may be involved in hydrogen bond contacts inside M(1). In contrast to previously reported refinements of all but one other phengite, no ordering of tetrahedral cations was found in the study specimen. This disorder is considered to be due to the low-pressure, high-temperature amphibolite facies environment of crystallization.

Keywords

Cation orderingCrystal structureMicaPhengite
Type
Research Article
Information
Clays and Clay Minerals , Volume 33 , Issue 5 , October 1985 , pp. 403 - 409
Copyright
Copyright © 1985, The Clay Minerals Society

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