Summary: An amphibole occurring in the Mbozi syenite-gabbro complex in SW. Tanganyika approaches very closely to the theoretical end-member Na2CaFe3‥Fe2…Al2Si6O22(OH)2; it has α 1·705, pale to light yellow, β 1·713, deep blue-green, γ 1·715, deep blue to green to black, with γ ‖ [010] and β : [001] about 7 to 14°; absorption γ > β ≫ α, with the maximum absorption on (010) at 9 to 12° to [001]. The mineral shows anomalous bright orange-red interference colours and incomplete extinction in sections parallel to (010) even in monochromatic light. X-ray powder data are given. A very similar amphibole occurs in the Darkainle nepheline-syenite complex, Borama district, Somali Republic.