A Note on the Petrology of Apatite-Biotite-Monchiquite from Giridih Coalfield, Bihar

A re-examination of Holland's collection of mica peridotites from Giridih coalfield is presented with a detailed petrographic description of the rocks which entitles them to be specifically termed as 'apatite-biotite-monchiquites', recorded as such for the first time from the Lower Gondwana coalfields of India.

The order and periods of crystallisation of the constituent minerals are given in a graphical form by plotting the modal analysis to discuss the petrogenesis. The authors consider that the first genetic class of minerals, viz. olivine and augite crystallised and accumulated at place before the emplacement of the magma into the country rock. After emplacement, the minerals of the second genetic class, viz. apatite, biotite and perovskite formed either from the condensation of a volatile phase or hydrous environment with quick lowering of temperature. With the abrupt termination of favourable temperature conditions, the last remnants of the liquid froze into interstitial glass, most of which soon devitrified into analcite.