The Formation of Chrysoberyl in Metamorphosed Pegmatites |
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Authors: | FRANZ G; MORTEANI G |
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Institution: | 1Institut für Angewandte Geophysik Petrologie und Lagerstättenkunde, Technische Univ Berlin, Str. d. 17. Juni 135, D-1000 Berlin 12, EB 309
2Lehrstuhl für Angewandte Mineralogie und Geochemie der Technischen Univ München, Lichtenbergstr. 4, D-8046 Garching |
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Abstract: | Mineral assemblages in pegmatite samples from Kolsva, Swedenand Marikov, Czechoslovakia show that chrysoberyl is alwaysaccompanied by quartz, and is a breakdown product of primarypegmatitic beryl. Textures and the mineral-forming process forthe Kolsva pegmatite are explained by the reactions beryl +K-feldspar + H+ = chrysoberyl + quartz + SiO2, aq + K+ + H2Oor alternatively beryl Kfeldspar + H2O = chrysoberyl+ quartz + melt. Mineral assemblages from mica-rich parts ofthe pegmatite include sillimaniteKfeldspar, muscoviteKfeldsparsillimanite,and annitemagnetitespinelsillimanitegarnet.Details about the composition and the textural relationshipsof these minerals are given; they indicate a post-pegmatiticmetamorphic event at PT conditions near to the anatecticregime. The samples from Marikov show textures, which are explainedby the reactions beryl + albite + H+ = chrysoberyl + quartz+ Na+ + H2O or alternatively beryl + albite + H2O = chrysoberyl+ quartz + melt. Breakdown of muscovite produces sillimaniteaccording to the reactions beryl + albite + muscovite + H+ =chrysoberyl + quartz + sillimanite + Na+ + K+ + H2O or alternativelyberyl + albite + muscovite + H2O = chrysoberyl + quartz + sillimanite+ melt. Similar reaction textures and mineral assemblages were foundin other chrysoberyl-bearing pegmatites (Maroankora, Madagascar;Helsinki, Finland; Haddam, Greenfield, Greenwood, U.S.A.). Hydrothermal experiments located the reaction beryl + alkalifeldspar + H2O = chrysoberyl + phenakite + melt at PTconditions between the KfeldsparquartzH2Osolidus and the Kfeldsparalbitequartz-H2Osolidus. It is concluded that the formation of Al-rich minerals likechrysoberyl and sillimanite in pegmatites is due to a post-pegmatiticevent at high PT conditions. The question as to whichof the alternative set of reactions is more likely, the ionicequilibria or the anatectic chrysoberyl formation, must be leftopen. The previous hypothesis of a desilification of a pegmatitewhich intruded into SiO2-poor country rocks, or of the assimilationof Al2O3-rich country rocks, cannot explain the mineral assemblagesof the two pegmatites. |
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