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The aenigmatite-rhonite mineral group

Thomas Kunzmann
The aenigmatite-rhonite mineral group
European Journal of Mineralogy (August 1999) 11 (4): 743-756


The aenigmatite-rhonite mineral group consists of eight minerals: Aenigmatite, rhonite, serendibite, krinovite, welshite, dorrite, wilkinsonite and hogtuavite. The general chemical formula of the minerals in this group may be written as {X2} [Y6] (Z6) O (sub 20) , with {X} eightfold coordinated Na (super +) , Ca (super 2+) and [Y] sixfold coordinated Mg (super 2+) , Fe (super 2+) , Fe (super 3+) , Ti (super 4+) , Al (super 3+) , Mn (super 2+) , Cr (super 3+) , Ti (super 3+) , Ca (super 2+) , Sb (super 5+) , Nb (super 5+) and As (super 5+) , and (Z) fourfold coordinated Si (super 4+) , Al (super 3+) , Fe (super 3+) , Be (super 2+) and B (super 3+) . There are two subgroups: a sodic group, including the minerals aenigmatite, krinovite and wilkinsonite, and a calcic group with rhonite, serendibite, dorrite, welshite and hogtuavite. The general features of the crystal structure are common to all the minerals of this group. These minerals occur in a wide range of rock types, e.g. alkaline lavas, sodium-rich intrusives, granitic gneisses, skarns, limestone-basalt contacts and meteorites, but mostly as accessories. Experimental data on stability are available only for aenigmatite and rhonite. Aenigmatite was synthesized at 700 degrees C/1000 bars and 750 degrees C/500 bars by Thompson & Chisholm (1969) and Lindsley (1969). The oxygen fugacity is constrained be lower than the faylite-quartz-magnetite = FQM buffer. Rhonite is stable from 850 degrees -1000 degrees C/1 bar to at least 5 kbar, 900 to 1100 degrees C (Kunzmann, 1989). There is no limit on oxygen fugacity. In alkali-basaltic rocks, the stability is restricted to pressures lower than 600 bars and temperatures from 840 to 1200 degrees C (Kunzmann, 1989). The chemistry of this group is complex, due to the flexibility of the structure. The structural formulae of 192 available analyses can be described in terms of seven substitutions: 1: Si (super IV) +Na (super VIII) <--> Al (super IV) +Ca (super VIII) ; 2: Si (super IV) +Mg (super VI) <--> Al (super IV) +Al (super VI) ; 3: Ti (super VI) +Mg (super VI) <--> 2Al (super VI) ; 4: Mg (super VI) <--> Fe (super 2+VI) ; 5: Al (super IV) <--> B (super IV) ; 6: Si (super IV) +Be (super IV) <--> 2Al (super IV) ; 7: Sb (super 5+VI) +2Mg (super VI) <--> 3Fe (super 3+VI) . The theoretical number of end-members (and names) resulting from these seven substitutions is immense. A simplified nomenclature is proposed here based on three substitutions. I: 2 Si (super IV) +2Na (super VIII) <--> 2Al (super IV) +2Ca (super VIII) ; II: 2Si (super IV) +2(M (super 2+) ) (super VI) <--> 2Al (super IV) +2(M (super 3+) ) (super VI) ; III: 2Ti (super 4+VI) +2(M (super 2+) ) (super VI) <--> 4(M (super 3+) ) (super VI) . This results in a rectangular polyhedron for the aenigmatite-rhonite group, in which ten sub-volumes can be assigned to ten end-members.

ISSN: 0935-1221
Serial Title: European Journal of Mineralogy
Serial Volume: 11
Serial Issue: 4
Title: The aenigmatite-rhonite mineral group
Author(s): Kunzmann, Thomas
Affiliation: Ludwig-Maximilians-Universitaet Muenchen, Institut fuer Mineralogie, Munich, Federal Republic of Germany
Pages: 743-756
Published: 199908
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 33
Accession Number: 1999-066207
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 199922
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