Compositional zoning in hydrothermal aegirine from fenites in the Proterozoic Gardar Province, south Greenland

Compositionally zoned Ti-bearing aegirine is a common constituent in fenites and hydrothermal veins and vugs associated with nepheline syenites from the Gardar Province. A typical assemblage in these mineralizations includes quartz, albite, microcline, aegirine, calcite, stilpnomelane, apatite, monazite and Nb-Fe-Ti-Mn oxides, with a wide range of unusual REE-Zn-Be-Ba-Mn-silicates as accessory phases. Individual aegirine crystals display evidence of sequential growth, concentric- and sector-zoning that represent large variations in wt.% TiO₂, Na₂O, CaO, FeO_t, and ZrO₂. Concentrations of Al₂O₃ and MgO are both very low. Textural relationships, together with the chemical inhomogeneities of the pyroxenes, suggest rapid nonequilibrium crystallization from a hydrothermal fluid of changing composition and/or temperature. -from Authors