Solid solution between potassic-obertiite and potassic-fluoro-magnesio-arfvedsonite in a silica-rich lamproite from northeastern Mozambique

Abstract

A fine-grained dike rock was collected during regional geologic mapping in the Xixano region, northeastern Mozambique. Observation in thin section and SEM and EMP analyses showed a fine-grained igneous texture dominated by optically unusual zoned amphibole and Fe 3+ -bearing low sanidine, with Sr-bearing fluorapatite, rutile, Sr-bearing barite, a silica mineral, hematite and zircon. The mineralogy and major element data indicate an unusual high-silica lamproite. The amphibole is concentrically zoned and pleochroic from pale brown to pale greenish blue, with common abnormal blue and brown interference colours, indicative of high dispersion commonly associated with Fe 3+ . Results of 29 EMP analyses, including two traverses across zoning, were formulated in different modes. The most satisfactory formulae were obtained assuming high Fe 3+ /(Fe 2+ + Fe 3+ ) ratios and significant oxo component (O 2− at the O(3) site substituting for (OH, F)). The most Ti-rich core composition approximates: K Na 2 (Mg 3 Fe 2+ 0.5 Fe 3+ 1 Ti 0.5 ) Si 8 O 22 (F 1 O 1 ), and can be classified as 1:1 solid solution between potassic-obertiite and potassic-fluoro-magnesio-arfvedsonite. Zoning toward the rims increases the potassic-fluoro-magnesio-arfvedsonite component up to K Na 2 (Mg 3.5 Fe 3+ 1.5 ) Si 8 O 22 (F 1 (OH) 0.5 O 0.5 ).