Ti-poor hoegbomite in kornerupine-cordierite-sillimanite rocks from Ellammankovilpatti, Tamil Nadu, India

Abstract

Hoegbomite occurs sparingly in minute (mostly 0.1 mm) grains with fine-grained hercynite, magnetite, and rutile in two coarse-grained kornerupine-cordierite-sillimanite rocks from Ellammankovilpatti, Tamil Nadu, India. The hoegbomite is Ti-poor (2.5 wt% TiO2), Fe-rich (25–26% Fe as FeO), and contains 6.2–6.8% MgO, 59.8–60.1% Al2O3, 1.0–1.3% ZnO, 0.3–0.7% Cr2O3 and 0.02% Li2O. Minor amounts (estimated not to exceed 0.2 wt% oxide) of V, Co, Ni, Ga, and Sn were detected on the electron microprobe, but Be, Nb, and Zr were not detected with the ion microprobe mass analyser. Assuming the crystal structure refined by Gatehouse and Grey (1982) to be applicable to the Ellammankovilpatti hoegbomite, the analyses were recalculated on a basis of 22 cations, 30 oxygens, and two hydroxyls, resulting in 49 to 53% of the iron being ferric. Identification of hoegbomite was confirmed by X-ray powder diffraction. Associated cordierite (Fe/(Fe+Mg)=0.14) and kornerupine (Fe/(Fe+Mg)= 0.27) contain 0.02 weight % Li2O and 0.05–0.07% BeO, while only the kornerupine contains B2O3 — 1.57% (ion microprobe analyses). Hoegbomite and the other oxides may have crystallized at temperatures between 680 and 720° C (P≈6.5 kbar) following attainment of peak conditions by the reaction: kornerupine+sillimanite±rutile+ZnO+H2O+O2 =cordierite+chlorite+hercynite+hoegbomite +magnetite+B2O3.