Ænigmatite from an Ægirine—Riebeckite granite, Liruei Complex, Nigeria

Abstract

Ænigmatite occurs as a trace constituent of the Ægirine-Riebeckite Granite of the Liruei Complex, Younger Granite Province of Nigeria. It is enriched in Ti and Mg and depleted in Si, Al, and Ca relative to many other ænigmatites. Relative to coexisting ægirine and arfvedsonitic riebeckite it is enriched in Ti and depleted in Fe3+. Conditions required for formation of the mineral appear to have been high Ti concentration in a peralkaline magma of low T and fo2. The composition of the Nigerian ænigmatite confirms that there is only limited miscibility between rhönite and aenigmatite although in certain circumstances the two may have a paragenetic relationship.