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Large pseudomorphous trapezohedra of analcime and pumpellyite after leucite, Aghda area, Central Iran

Große pseudomorphe Trapezoeder von Analcim im Pumpellyit nach Leuzit, Aghda-Gebiet Zentraliran

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Summary

Spectacular large (3 cm) euhedral trapezohedra composed of single crystals of analcime or aggregates of pumpellyite occur in three stratigraphically adjacent phonolitic lavas from Aghda. The trapezohedra contain concentrically arranged inclusions of plagioclase, pyroxene, titanomagnetite and apatite which are fresh in the analcime but extensively altered in the pumpellyite. The analcime crystals are remarkably homogeneous in composition and are interpreted as having formed by ion-exchange pseudomorphous replacement of primary leucite. In the lower phonolite, however, the analcime became unstable and was replaced by pumpellyite which has a wide compositional range, reflecting variable input of Fe, Mg, Ca and Al from the precursor analcime and alteration of inclusions. The occurrence of analcime and pumpellyite at Aghda is indicative of zeolite facies conditions with low fCO 2and H2O activity and probably high Pfluid.

Zusammenfassung

Ungewöhnlich große idiomorphe Trapezoeder, die aus einzelnen Kristallen von Analcim oder aus Aggregaten von Pumpellyit bestehen, kommen in drei stratigraphisch benachbarten phonolitischen Lagen von Aghda vor. Die Trapezoeder enthalten konzentrisch verteilte Einschlüsse von Plagioklas, Pyroxen, Titanomagnetit und Apatit, die im Analcim frisch, im Pumpellyit jedoch intensiv umgewandelt sind. Die Analcim-Kristalle sind von bemerkenswerter Homogenität in ihrer Zusammensetzung und werden auf pseudomorphe Verdrängung von primärem Leucit durch Ionenaustausch zurück-geführt. Im unteren Phonolit ist der Analcim jedoch instabil geworden, und wurde durch Pumpellyit ersetzt, der ein weites Spektrum von Zusammensetzungen erkennen läßt und auf variierende Zufuhr von Fe, Mg, Ca, and Al vom Vorläuferanalcim und auf Umwandlung von Einschlüssen hinweist. Das Vorkommen von Analcim und Pumpellyit bei Aghda ist ein Hinweis auf Zeolithfazies-Bedingungen mit niedrigen fCO 2 und H2O und hohem Pfluid.

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  1. School of Geosciences, University of Wollongong, Northfields Avenue, 2522, Wollongong, NSW, Australia

    P. F. Carr, M. Perkins & A. Moradian

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  1. P. F. Carr
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  2. M. Perkins
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Carr, P.F., Perkins, M. & Moradian, A. Large pseudomorphous trapezohedra of analcime and pumpellyite after leucite, Aghda area, Central Iran. Mineralogy and Petrology 58, 23–32 (1996). https://doi.org/10.1007/BF01165761

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