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GEOREF RECORD

Reaction sequences, polysomatic faults and chemical compositions of manganese pyroxenoids from Campiglia Marittima skarn

Gian Carlo Capitani, Henry Bernard Grobety and Marcello Mellini
Reaction sequences, polysomatic faults and chemical compositions of manganese pyroxenoids from Campiglia Marittima skarn (in Special issue dedicated to the memory of Luciano Ungaretti, Maria Chiara Domeneghetti (prefacer))
European Journal of Mineralogy (April 2003) 15 (2): 381-391

Abstract

The late crystallisation stages of Campiglia Marittima skarn are characterised by an increase in Mn-activity at the metasomatic front. Mn-pyroxenoids became stable with respect to johannsenite. The pyroxene-pyroxenoid textures indicate either pyroxenoid overgrowths on johannsenite or pyroxenoid pseudomorphs after johannsenite. TEM investigation of the replacement sites reveals that the pyroxene-to-pyroxenoid replacement mainly consists of a topotactic solid-state reaction involving the movement of a reaction front through the original johannsenite crystal (bulk reaction mechanism). Due to the fast kinetics of skarn formation, pyroxenoids are chemically heterogeneous and highly defective, with numerous chain periodicity faults. The crystallographic relationship (001) (sub Pxd) //(111) (sub Jh) is confirmed. Johannsenite-pyroxenoid solid-state transformations appear to occur at temperatures lower than 260 degrees C. In contrast with previous assumptions, AEM data and crystal chemistry indicate that the maximum calcium content of pyroxmangite is 2/7, rather than 1/7. As a consequence, the AEM data plot along a normal and chemically collinear polysomatic trend. Moreover, it is suggested that minor concentrations of iron and magnesium in the system can affect the pyroxene --> pyroxenoid transformation, determining which pyroxenoid is stable in the replacement sequence.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 15
Serial Issue: 2
Title: Reaction sequences, polysomatic faults and chemical compositions of manganese pyroxenoids from Campiglia Marittima skarn
Title: Special issue dedicated to the memory of Luciano Ungaretti
Author(s): Capitani, Gian CarloGrobety, Henry BernardMellini, Marcello
Author(s): Domeneghetti, Maria Chiaraprefacer
Affiliation: Universita di Siena, Dipartimento di Scienze della Terra, Siena, Italy
Pages: 381-391
Published: 200304
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 51
Accession Number: 2003-032481
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables
N42°19'60" - N44°19'60", E09°40'00" - E12°25'00"
Secondary Affiliation: Universitat Fribourg, CHE, Switzerland
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200311
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