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Temperature-dependent Al order-disorder in the tetrahedral double chain of C2/m amphiboles

Roberta Oberti, Luciano Ungaretti, Elio Cannillo, Frank C. Hawthorne and Isabella Memmi
Temperature-dependent Al order-disorder in the tetrahedral double chain of C2/m amphiboles
European Journal of Mineralogy (October 1995) 7 (5): 1049-1063

Abstract

The crystal structures and site populations of eight calcic amphiboles (tremolite, magnesio-hornblende, pargasite) from a leucogabbro at Punta Falcone, Sardinia, have been refined to R approximately 2%. These amphiboles are unusual in that they show a significant degree of disorder of (super [4]) Al over the T(1) and T(2) sites in the tetrahedral double chains of the C2/m amphibole structure. New relationships are derived between the <<<$I>T-O>><$I> distances and the Al contents of the T(1) and T(2) tetrahedra; it is also shown that the <<<$I>T(2)-O>><$I> distance is affected by variation in occupancy of other sites in the structure. The normal limit of (super [4]) Al approximately 2 per formula unit found in amphiboles occurs so as to avoid Al-O(7)-Al linkages that are unfavourable from a bond-valence viewpoint, and to avoid the occurrence of Al at T(2). When (super [4]) Al >><$I> 2 pfu, the additional (super [4]) Al normally occurs at the T(2) site. Medium-T (high-P) amphiboles from the Liset eclogite pod [Norway] show all (super [4]) Al to occur at T(1) up to a limit of 2.0 pfu, with all additional (super [4]) Al occurring at T(2). High-T amphibole from Punta Falcone have (super [4]) Al <<<$I> 2.0 and yet have Al at both T(1) and T(2). Intermediate-T amphiboles from potassic volcanic rocks at Monte Vulture, Italy, have (super [4]) Al >><$I> 2.0 pfu with (super [4]) Al at T(1) and T(2). It is thus apparent that long-range (super [4]) Al disorder over T(1) and T(2) in amphiboles is induced by high T of crystallization.


ISSN: 0935-1221
Serial Title: European Journal of Mineralogy
Serial Volume: 7
Serial Issue: 5
Title: Temperature-dependent Al order-disorder in the tetrahedral double chain of C2/m amphiboles
Affiliation: CNR Centro di Studio per la Cristallochimica de la Cristallografia, Pavia, Italy
Pages: 1049-1063
Published: 199510
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 30
Accession Number: 1996-001697
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: 8 tables
N38°49'60" - N41°30'00", E08°15'00" - E09°45'00"
Secondary Affiliation: Dipartimento di Scienze della Terra, ITA, Italy
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United KingdomTwickenhamUKUnited Kingdom
Update Code: 199601
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