Mineralogy and chemistry of the opaques of Cox's Bazar (Bangladesh) beach sands and the oxygen fugacity of their provenance

Abstract

The heavy minerals of Cox's Bazar beach sands (Bangladesh) are dominated by hornblende, opaque minerals, garnet and epidote. Opaque minerals consist primarily of ilmenites (both unaltered and altered) with magnetite, rutile, pyrite and some hydroxides. A large part of ilmenites (about 50% of the unaltered ilmenites) exsolves hematite in different textural patterns. Exsolutions of ilmenite-hematite-rutile and ilmenite-rutile are insignificant. In hematite host, the first-generation coarser ilmenite lamellae contain less FeTiO 3 ( ∼ 84 mol%) than the second-generation finer ilmenite lamellae ( ∼ 93 mol% FeTiO 3). No significant difference in chemical composition is observed in the coarse and fine lamellae of hematite exsolved in ilmenites. Exsolution features and compositions of ilmenites (e.g., MgO% 0.5) point to an igneous source. Titanomagnetite (± exsolved ilmenite) and optically homogeneous magnetite are common. A regular partitioning of Cr, V, Al, Mg and Mn between coexisting titanomagnetite and ilmenite suggests chemical equilibrium. The distribution of MnO between coexisting ilmenite and titanomagnetite and the plot of MgO vs. TiO 2 of ilmenite intergrowth in titanomagnetite suggest their origin in intrusive igneous rocks. The approximate temperature and oxygen fugacity as estimated from the microprobe analyses of coexisting titanomagnetite-ilmenite phases range from 645° to 850°C and 10 −13 to 10 −19, respectively, suggesting a crustal origin.