Some Garnets, Pyroxenes, and Amphiboles From Nepheline Gneisses in Ghana

Nepheline gneisses, predominantly leucocratic, but grading into mafic and feldspar-poor varieties, occur in a thin layer in the Dahomeyan shield of southeastern Ghana. Two garnets, one pyroxene, and three amphiboles were separated for chemical analyses and optical determinations from three specimens of nepheline gneiss. Optical data of two additional pyroxenes are given.

Garnets are andradite-rich grossularite, and occur only in the mafic and feldspar-poor nepheline gneisses. Aegirine-augites in the leucocratic nepheline gneisses contain up to 0.63 Fe³⁺ ions per formula unit, but in the mafic and feldspar-poor gneisses the aegirine-augites are closer to soda augite (0.298 Fe³⁺ ions per formula unit). On the basis of optical properties the amphiboles appear to range in a continuous series from hastingsite to iron-rich alkaline amphibole. Zoned amphiboles invariably have hastingsite cores and alkaline amphibole rims. An unzoned alkaline amphibole, optically similar to the rims of the zoned amphiboles, was analyzed. This amphibole is optically and chemically distinctive, having normal-symmetric orientation of the optic plane (Y:c = 9°), dark olive-green (γ) to dark greenish-blue (β) pleochroism, two aluminum atoms in tetrahedral positions, a Ca/Na+K ratio of 0.44, and a high Fe³⁺/Fe²⁺ ratio.

Textures indicate the zoned amphiboles result from the reaction:

calcite+nepheline+aegirine-augite+hastingsite→grandite+iron-rich alkaline amphibole+CO₂.