Four petrologically distinct transitional zones are recognized in the metamorphosed Barakar coal seams (Permian)--natural coke, coked coal, affected coal, and unaffected coal. Distance from the contact to the unaffected coal is similar to the thickness of the sill. Metamorphism obliterated the volatile-rich organic layers, destroying the banding and forming vacuoles. Inorganic matter, principally apatite and calcite, was introduced from the sills.

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