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Triphylite-sarcopside miscibility gap in the Feo-MnO-Li (sub 2) O-P (sub 2) O (sub 5) -H (sub 2) O system; experimental investigation and thermometric application to granitic pegmatites

Frederic Hatert, Encarnacion Roda-Robles, Luisa Ottolini, Peter Schmid-Beurmann, Maxime Baijot and Fabrice Dal Bo
Triphylite-sarcopside miscibility gap in the Feo-MnO-Li (sub 2) O-P (sub 2) O (sub 5) -H (sub 2) O system; experimental investigation and thermometric application to granitic pegmatites (in Mineralogy, crystallography, petrology, economic geology, geochemistry; pegmatite special issue dedicated to Drs. William B. (Skip) Simmons and Karen L. Webber, Alexander U. Falster (prefacer) and Pietro Vignola (prefacer))
The Canadian Mineralogist (July 2016) 54 (4): 827-845

Abstract

In order to assess the stability of the primary triphylite + sarcopside assemblage, we performed hydrothermal experiments between 400 and 700 degrees C (Ni/NiO oxygen fugacity buffer, P = 1 kbar), starting from the Li(Fe (super 2+) (sub 2.5-) (sub x) Mn (super 2+) (sub x) ) (PO (sub 4) ) (sub 2) (x = 0.0, 0.5, 1.0) compositions, which represent the ideal compositions of triphylite + sarcopside assemblages in which both minerals occur in a 1:1 molar ratio. The triphylite + sarcopside assemblage is observed in all experiments, associated with other phosphates like (Fe (super 2+) ,Mn (super 2+) ) (sub 2) P (sub 2) O (sub 7) , (Fe (super 2+) ,Mn (super 2+) )Fe (super 3+) (sub 2) (PO (sub 4) ) (sub 2) (OH) (sub 2) .nH (sub 2) O, or Fe (super 3+) (sub 4) (Fe (super 2+) ,Mn (super 2+) ) (sub 3) (PO (sub 4) ) (sub 6) Electron-microprobe and SIMS analyses show a progressive decrease of the Li contents in the triphylites, balanced by an increase of their Fe (super 2+) -contents, when the temperature increases. These compositional changes are due to the increase of the triphylite-sarcopside miscibility along the Li (sub 2) (Fe (super 2+) ,Mn (super 2+) ) (sub 2) (PO (sub 4) ) (sub 2) -Fe (super 2+) (Fe (super 2+) ,Mn (super 2+) ) (sub 2) (PO (sub 4) ) (sub 2) solid solution; the experimental phase diagrams can consequently be used as a geothermometer to calculate the exsolution temperatures of the assemblages. A linear fit of the experimental data leads to the general equation: T ( degrees C) = (-142 * XFe) - (773 * Li pfu) + 1131, where XFe = Fe/(Fe + Mn). The uncertainty is around + or -15 degrees C, and the influence of pressure is assumed to be negligible. By using this equation, exsolution temperatures were calculated for nine triphylite-sarcopside assemblages from pegmatites; these temperatures do not represent the crystallization temperatures of the phosphate nodules, but correspond to the closing temperature of the triphylite-sarcopside element exchange. Nevertheless, these temperatures, between 276 and 397 degrees C, are in fairly good agreement with those generally accepted for the crystallization of primary phosphate assemblages in granitic pegmatites.


ISSN: 0008-4476
EISSN: 1499-1276
Coden: CAMIA6
Serial Title: The Canadian Mineralogist
Serial Volume: 54
Serial Issue: 4
Title: Triphylite-sarcopside miscibility gap in the Feo-MnO-Li (sub 2) O-P (sub 2) O (sub 5) -H (sub 2) O system; experimental investigation and thermometric application to granitic pegmatites
Title: Mineralogy, crystallography, petrology, economic geology, geochemistry; pegmatite special issue dedicated to Drs. William B. (Skip) Simmons and Karen L. Webber
Author(s): Hatert, FredericRoda-Robles, EncarnacionOttolini, LuisaSchmid-Beurmann, PeterBaijot, MaximeDal Bo, Fabrice
Author(s): Falster, Alexander U.prefacer
Author(s): Vignola, Pietroprefacer
Affiliation: Universite de Liege, Departement de Geologie, Liege, Belgium
Affiliation: Maine Gem and Mineral Museum, Bethel, ME, United States
Pages: 827-845
Published: 201607
Text Language: English
Publisher: Mineralogical Association of Canada, Ottawa, ON, Canada
References: 43
Accession Number: 2017-080333
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 8 tables
Secondary Affiliation: Universidad del Pais Vasco, ESP, SpainCNR, Istituto di Geoscienze e Georisorse, ITA, ItalyUniversitaet Muenster, DEU, Germany
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: 201742
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