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GEOREF RECORD

Thermal expansion of alunite up to dehydroxylation and collapse of the crystal structure

M. Zema, A. M. Callegari, S. C. Tarantino, E. Gasparini and P. Ghigna
Thermal expansion of alunite up to dehydroxylation and collapse of the crystal structure
Mineralogical Magazine (June 2012) 76 (3): 613-623

Abstract

The high-temperature (HT) behaviour of a sample of natural alunite was investigated by means of in situ HT single-crystal X-ray diffraction from room temperature up to the dehydroxylation temperature and consequent collapse of the crystal structure. In the temperature range 25-500 degrees C, alunite expands anisotropically, with most of the contribution to volume dilatation being produced by expansion in the c direction. The thermal expansion coefficients determined over the temperature range investigated are: alpha (sub a) = 0.61(2)X10 (super -5) K (super -1) (R (super 2) = 0.988), alpha (sub c) = 4.20(7)X10 (super -5) K (super -1) (R (super 2) = 0.996), alpha (sub c) /alpha (sub a) = 6.89, alpha (sub V) = 5.45(7)X10 (super -5) K (super -1) (R (super 2) = 0.998). At approximately 275-300 degrees C, a minor discontinuity in the variation of unit-cell parameters with temperature is observed and interpreted on the basis of loss of H (sub 3) O (super +) that partially substitutes for K (super +) at the monovalent A site in the alunite structure. Increasing temperature causes the Al(O,OH) (sub 6) sheets, which remain almost unaltered along the basal plane, to move further apart, and this results in an expansion of the coordination polyhedron around the intercalated potassium cation. Sulfate tetrahedra act as nearly rigid units, they contract a little in the lower temperature range to accommodate the elongation of the Al octahedra.


ISSN: 0026-461X
EISSN: 1471-8022
Coden: MNLMBB
Serial Title: Mineralogical Magazine
Serial Volume: 76
Serial Issue: 3
Title: Thermal expansion of alunite up to dehydroxylation and collapse of the crystal structure
Affiliation: Universita di Pavia, Dipartimento di Scienze della Terra e dell'Ambiente, Pavia, Italy
Pages: 613-623
Published: 201206
Text Language: English
Publisher: Mineralogical Society, London, United Kingdom
References: 36
Accession Number: 2012-092172
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201248
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