Manganiferous phyllosilicate assemblages: occurrences, compositions and phase relations in metamorphosed Mn deposits

Autor: Jürgen Abrecht
Rok vydání: 1989
Předmět:
Zdroj: Contributions to Mineralogy and Petrology. 103:228-241
ISSN: 1432-0967
0010-7999
Popis: Mn-rich members of the pyrosmalite-family [(Mn, Fe)8Si6O15(OH, Cl)10], friedelite and schallerite have been identified as rock-forming minerals together with caryopilite, in several metamorphosed carbonate Mn-deposits. The phase assemblages and mineral compositions are described for eight of these localities each of which represents a distinct geologic situation. Friedelite is always Cl-bearing and occurs both as a prograde phase in low-grade metamorphic rocks (Pyrenees, Haute-Maurienne) and as a secondary phase formed by retrogressive replacement of primary anhydrous phases in higher-grade rocks. Schallerite, an Asbearing relative of friedelite, occurs in the greenschist metamorphic deposit of the Kodnitztal (Austria) together with other As-minerals. In these deposits, caryopilite is typically formed during retrograde metamorphism by alteration of, generally anhydrous, Mn-silicates. Based upon these occurrences, a qualitative petrogenetic grid for the system MnO-SiO2-CO2-H2O with the phases friedelite, caryopilite, pyroxmangite/rhodonite, tephroite, rhodochrosite, quartz, CO2, and H2O is proposed. The phase relations imply that Cl- (or As-) free friedelite is not stable in hydrous systems with respect to caryopilite. From the mineral assemblages containing hydrous Mn silicates, waterrich fluids are inferred during the retrograde metamorphic evolution of the investigated deposits. Chemical data for Mn-rich chlorites, which are basically members of the clinochlore-pennantite series which coexist with the pyrosmalite minerals, show the absence of intermediate Mn/Mg ratios. This supports the existence of a miscibility gap as previously hypothesized by other authors.
Databáze: OpenAIRE
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