Zn-rich staurolite from the Uvete area, central Kenya

Abstract

AbstractZn-rich staurolite occurs in a pelitic gneiss from the Uvete area, central Kenya. The rock was metamorphosed to staurolite-kyanite grade during the Mozambique metamorphism. Microprobe analyses of the staurolite gave the following values (in wt. %): SiO2 26.2–27.3, Al2O3 52.4–53.5, FeO 8.3–9.2, MgO 2.2–2.6, and ZnO 6.0–7.5. Cell dimensions are a = 7.874(± 0.003), b = 16.620(±0.006), c = 5.656(±0.002) Å and β = 90.00 (± 0.002)°. Optical properties are 2Vz = 82–85°, α = 1.742 (±0.002), β = 1.747(±0.002) and γ = 1.753(±0.002). The optical dispersion is moderate and r > v. Density is 3.83(±0.01) g/cm3. Infra-red absorption spectra were also recorded. The cation correlation suggests that the principal substitutions in the staurolite are Zn ⇆ Fe in the tetrahedral sites in the hydroxide sheet and Al ⇆ Mg in the octahedral sites.