Skip to Main Content
Skip Nav Destination
GEOREF RECORD

The high-temperature behaviour of riebeckite; expansivity, deprotonation, selective Fe oxidation and a novel cation disordering scheme for amphiboles

Roberta Oberti, Massimo Boiocchi, Michele Zema, Frank C. Hawthorne, Guenther J. Redhammer, Umberto Susta and Giancarlo Della Ventura
The high-temperature behaviour of riebeckite; expansivity, deprotonation, selective Fe oxidation and a novel cation disordering scheme for amphiboles
European Journal of Mineralogy (May 2018) 30 (3): 437-449

Abstract

The HT behaviour of a well-characterized sample of riebeckite has been examined by combining X-ray diffraction, FTIR and Mossbauer spectroscopy. The crystal-chemical composition of the crystal studied is: (super A) (K (sub 0.05) Na (sub 0.04) ) (super B) (Na (sub 1.86) Ca (sub 0.09) Fe (super 2+) (sub 0.05) ) (super C) (Fe (super 2+) (sub 2.94) Mg (super 2+) (sub 0.22) Zn (super 2+) (sub 0.01 ) Fe (super 3+) (sub 1.74 ) Al (super 3+) (sub 0.06) Ti (super 4+) (sub 0.01) ) (super T) (Si (sub 7.95) Al (sub 0.05) )O (sub 22) (super W) (OH (sub 1.9) F (sub 0.10) ). The onset of the deprotonation process is detected at approximately 700 K by single-crystal XRD analysis of the unit-cell parameters, but starts at 623 K as indicated by Mossbauer spectroscopy on powders (and by changes in the cation distribution observed by structure refinement). FTIR analysis shows that a completely deprotonated oxo-amphibole is obtained after annealing at 893 K. Room-T single-crystal structure refinements of the deprotonated phase shows a very peculiar cation disorder, which has never been observed in amphiboles until now: there is significant depletion of B and C cations coupled with an increase in A cations, which implies the presence of vacancies at the M(3) and the M(4) sites in double-chain silicates. FTIR data collected at 873 K confirm both this conclusion and the onset of the cation exchange before completion of deprotonation. Axial and volume thermal expansion coefficients were etermined in the T range 298-698 K for riebeckite (alpha a = 1.40(2) . 10 (super -5) K (super -1) , alpha b = 0.67(1) . 10 (super -5) K (super -1) , alpha c = 0.17(2) . 10 (super -5) K (super -1) , alpha beta = -0.07(1) . 10 (super -5) K (super -1) and alpha V = 2.27(2) . 10 (super -5) K (super -1) ) and in the 298-1173 K range for the oxo-amphibole (alpha a = 1.53(2) . 10 (super -5) K (super -1) , alpha b = 0.77(1) . 10 (super -5) K (super -1) , alpha c = 0.25(1) . 10 (super -5) K (super -1) , alpha beta = 0.10(1) . 10 (super -5) K (super -1) and alpha V = 2.52(2) . 10 (super -5) K (super -1) ).


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 30
Serial Issue: 3
Title: The high-temperature behaviour of riebeckite; expansivity, deprotonation, selective Fe oxidation and a novel cation disordering scheme for amphiboles
Affiliation: Universita degli studi di Pavia, Istituto di Geoscienze e Georisorse, Italy
Pages: 437-449
Published: 201805
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Germany
References: 47
Accession Number: 2018-088867
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 6 tables
Secondary Affiliation: University of Manitoba, CAN, CanadaUniversity of Salzburg, AUT, AustriaUniversita di Roma Tre, ITA, Italy
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201848
Close Modal

or Create an Account

Close Modal
Close Modal