Chemical composition of chromite and intergrown chlorite in metamorphosed ultramafic rocks (serpentinite and talc schist) of the Egbe-Isanlu schist belt, southwest Nigeria: genetic implications.
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Journal of Mining and Geology
In the Egbe-Isanlu schist belt serpentinite of dark and light greenish colours and whitish-grey talc schist occur. In these rocks black spots consisting of chromite are disseminated. Chromite is always intimately intergrown with chlorite . In serpentinite, chromite occurs first as compact and irregular crystals (up to 200 ìm) that may be associated with magnetite, ilmenite and sulphides or secondly as tiny crystals (5 ìm or smaller) which are concentrated within rectangular to rounded shaped outlines that represent the original shape of chromite grains. The latter type is predominantly developed in talc schist where tiny chromite crystals arranged in the herringbone structure are of subordinate abundance. The main end-members of the chromite solid solutions in serpentinite and talc schist are 2+ 3+ FeCr 0 (= chromite) and Fe Fe 0 (= magnetite) with the chromite-magnetite mole ratios generally higher in talc 2 4 2 4 schist than serpentinite. Chlorite has the composition of clinochlore and shows low level of chemical variability. However, Cr O and NiO occur in anomalously enhanced concentrations. Thermometric calculations suggest that 2 3 chlorite was formed or re-equilibrated within the greenschist metamorphic spectrum. This investigation revealed that, chromite originally contained in ultramafic rocks (= peridotite), inferred to be similar in composition to chromite of stratiform deposits, suffered chemical alterations during metamorphism. The original peridotite was transformed into serpentinite and talc schist while the altered chromite suffered a near complete loss of Al and Mg and the enrichment of Cr, Ti, Zn and Fe-concentrations.
concentration , Chromite , crystals , intergrown
Olobaniyi, S. B., & Mücke, A. (2011). Chemical composition of chromite and intergrown chlorite in metamorphosed ultramafic rocks (serpentinite and talc schist) of the Egbe-Isanlu schist belt, southwest Nigeria: genetic implications. Journal of Mining and Geology, 47(2), 115-134.