Phase Relations of Aenigmatite Minerals in a Syenitic Ejectum, Wonchi Volcano, Ethiopia

Mario Gaeta and Annibale Mottana
Dipartimento di Scienze della Terra, Università, di Roma ‘La Sapienza’, Piazzale A. Moro, 5, I-00185 Roma RM, Italy

Abstract: Wonchi volcano (Ethiopia) is the second locality for wilkinsonite, the new mineral related to aenigmatite via the coupled substitution 2Fe3+ = TiFe2+. Wilkinsonite occurs as relics surrounded by rims of riebeckite and biotite in a syenitic ejectum which also contains microphenocrysts of aenigmatite. The two minerals do not represent an equilibrium pair. They both formed in the magma chamber under plutonic to hypabyssal conditions, but during two following stages of crystallisation separated by a major drop in ƒO2 conditions.

Keywords: aenigmatite • wilkinsonite • syenitic ejectum • Wonchi volcano • Ethiopia

Mineralogical Magazine; December 1991 v. 55; no. 381; p. 529-534; DOI: 10.1180/minmag.1991.055.381.05
© 1991, The Mineralogical Society
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