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The crystal chemistry of the scapolite-group minerals; I, Crystal structure and long-range order

Elena Sokolova and Frank C. Hawthorne
The crystal chemistry of the scapolite-group minerals; I, Crystal structure and long-range order (in A tribute to Joseph V. Smith (1928-2007), Ross J. Angel (editor), Ian Parsons (editor) and Robert F. Martin (editor))
The Canadian Mineralogist (December 2008) 46 (6): 1527-1554

Abstract

Scapolite-group minerals are a group of rock-forming framework aluminosilicates common in a wide variety of igneous and metamorphic rocks. They have the general formula M (sub 4) [T (sub 12) O (sub 24) ]A, where M represents Na, K, Ca, Sr, Ba and Fe (super 2+) , T stands for Si, Al, (Fe (super 3+) ), and A represents Cl, (CO (sub 3) ), (SO (sub 4) ). They constitute a non-binary solid-solution series involving the end members marialite (Na (sub 4) [Al (sub 3) Si (sub 9) O (sub 24) ]Cl, Ma) and meionite (Ca (sub 4) [Al (sub 6) Si (sub 6) O (sub 24) ]CO (sub 3) , Me). The crystal structures of eighteen scapolite-group minerals from Me (sub 4.9) to Me (sub 92.8) [Me = 100 Sigma M (super 2+) /Sigma (M (super 2+) +M (super +) )] have been refined with MoKalpha X-radiation. The crystals, a 12.045-12.208, c 7.564-7.586 Aa, V 1099.7-1130.1 Aa (super 3) , I4/m or P4 (sub 2) /n, Z = 2, were analyzed with an electron microprobe subsequently to collection of the X-ray intensity data. There are subtle differences between the behavior of the distinct T tetrahedra in the I4/m and P4 (sub 2) /n structures. For Si and Al, determinative curves for the assignment of site populations from <T-O> distances were developed for each T site in each structure type, and T-site populations were assigned from these curves. In I4/m marialite, there is no Al at the T(1) site. In P4 (sub 2) /n scapolite, Al is strongly ordered at T(2), and Si is strongly ordered at T(3), but this order is never complete. The A-site cation (Na, Ca, K) is [8]-coordinated in end-member marialite and is (on average) [8.5]-coordinated in end-member meionite. Each (Mphi (sub n) ) polyhedron shares two edges with adjacent (Mphi (sub n) ) polyhedra, forming a three-dimensional framework that interpenetrates the [T (sub 12) O (sub 24) ] silicate framework. In (CO (sub 3) )-bearing structures, the model proposed by previous workers, four (CO (sub 3) ) groups arranged about the 4 axis (I4/m) or 4 axis (P4 (sub 2) /m), is consistent across the series from marialite to meionite. Any (SO (sub 4) ) present is disordered by rotation of 90 degrees about the central [Formula: see text]-fold axis; in I4/m structures, each arrangement is 50% occupied, whereas in P4 (sub 2) /n structures, the occupancies of the two arrangements are not required to be equal.


ISSN: 0008-4476
EISSN: 1499-1276
Coden: CAMIA6
Serial Title: The Canadian Mineralogist
Serial Volume: 46
Serial Issue: 6
Title: The crystal chemistry of the scapolite-group minerals; I, Crystal structure and long-range order
Title: A tribute to Joseph V. Smith (1928-2007)
Author(s): Sokolova, ElenaHawthorne, Frank C.
Author(s): Angel, Ross J.editor
Author(s): Parsons, Ianeditor
Author(s): Martin, Robert F.editor
Affiliation: University of Manitoba, Department of Geological Sciences, Winnitoba, MB, Canada
Affiliation: Virginia Polytechnic Institute and State University, Blacksburg, VA, United States
Pages: 1527-1554
Published: 200812
Text Language: English
Publisher: Mineralogical Association of Canada, Ottawa, ON, Canada
References: 40
Accession Number: 2009-031297
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 10 tables
Secondary Affiliation: University of Edinburgh, GBR, United KingdomMcGill University, CAN, Canada
Country of Publication: Canada
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Association of Canada. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200917
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