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Molybdophyllite; crystal chemistry, crystal structure, OD character and modular relationships with britvinite

U. Kolitsch, S. Merlino and D. Holtstam
Molybdophyllite; crystal chemistry, crystal structure, OD character and modular relationships with britvinite
Mineralogical Magazine (June 2012) 76 (3): 493-516

Abstract

A detailed crystal-chemical study of the complex layered silicate molybdophyllite was conducted using single-crystal X-ray diffraction (XRD) methods, supplemented by powder XRD, infrared (IR) and Raman spectroscopic studies, chemical analyses by energy-dispersive spectrometry (EDS) on a scanning electron microscope (SEM), and electron probe microanalysis (EPMA). The results, based on several samples from both Langban and Harstigen, Filipstad, Sweden, show that the crystal structure of molybdophyllite has an order-disorder (OD) character. The latter is especially evident in specimens from Langban which display a complex diffraction pattern characterized by the simultaneous presence of sharp spots, diffuse reflections and continuous streaks. The sharp reflections define the unit cell of the family structure (a = 3.124, c = 41.832 Aa, space group R32). Two main polytypes (maximum degree of order structures) are indicated by the OD approach: a trigonal one and a monoclinic one; the latter polytype is the most common in the samples that were studied and has space group C2, with a = 16.232(6), b = 9.373(2), c = 14.060(3) Aa, beta = 97.36(4) degrees and V = 2121.5(10) Aa (super 3) . The crystal structure determination [R (sub 1) = 0.096], together with the EPMA, IR and Raman data, reveal that molybdophyllite is built up by a regular alternation of complex layers with a composition {Mg (sub 9) [Si (sub 10) O (sub 28) (OH) (sub 8) ][OPb (sub 4) ] (sub 2) } (super 6+) and simple layers with a composition [(CO (sub 3) ) (sub 3) .H (sub 2) O] (super 6-) , leading to the ideal crystal-chemical formula Pb (sub 8) Mg (sub 9) [Si (sub 10) O (sub 28) (OH) (sub 8) O (sub 2) (CO (sub 3) ) (sub 3) ].H (sub 2) O (Z = 2). This contribution is mainly devoted to the results obtained for molybdophyllite sensu stricto, but new data for britvinite [i.e. "molybdophyllite-18 Aa"] are also presented and its modular relationship with molybdophyllite is discussed.


ISSN: 0026-461X
EISSN: 1471-8022
Coden: MNLMBB
Serial Title: Mineralogical Magazine
Serial Volume: 76
Serial Issue: 3
Title: Molybdophyllite; crystal chemistry, crystal structure, OD character and modular relationships with britvinite
Affiliation: Naturhistorisches Museum, Mineralogisch-Petrographische Abteilung, Vienna, Austria
Pages: 493-516
Published: 201206
Text Language: English
Publisher: Mineralogical Society, London, United Kingdom
References: 40
Accession Number: 2012-092164
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 6 tables
Secondary Affiliation: Universita degli Studi di Pisa, ITA, ItalySwedish Museum of Natural History, SWE, Sweden
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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