57Fe Mossbauer Study of Low Aluminous (Low Ca) Orthopyroxene from Sukinda, Orissa (India) and its Petrogenetic Implication

A low Al, low Ca enstatite from within an orthopyroxenite dyke rock in the Sukinda chromite field was physico-chemically investigated by use of ⁵⁷Fe Mossbauer spectroscopy XRF and XRD. The parameters of the fitted ⁵⁷Fe Mossbauer spectra show the presence of Fe³⁺ in the octahedral M1 site of pyroxene structure.

The room temperature Mossbauer results show three doublets corresponding to Fe²⁺ (Ml), Fe²⁺ (M2) and Fe³⁺ (M1) sites. The QS values in two samples are found to be 2.80 and 2.96 mm/s for Fe²⁺ , (M1), and 1.89 and 2.08 mm/s for Fe²⁺ (M2) doublets. In both samples Fe³⁺-(M1) gives IS values 0.38 and 0.56 mm/s and QS values 0.48 and 0.49 mm/s. From their dependence on X^Fe21_M¹and X^Fe21_M2 distributions the bond distances of tetrahedral and octahedral sites have been measured.

In the present case Fe³⁺ in the orthopyroxene structure is presumed to have been formed primarily at the stage of its crystallisation from a melt having requisitely high fO₂. The temperature recorded by the Sukinda orthopyroxenile body has been estimatcd as 800°C from the distribution of Fe²⁺ - Mg over Ml and M2 sites. From the Fe³⁺/Fe²⁺ ratio the oxygen fugacity has been calculated as log fO₂ = -6.189, which suggests that crystallisation of Sukinda orthopyroxene took place in the lower crustal regions rather than in the mantle.