Chemical Mineralogy of Hornblendes and Biotites from the Charnockitic Rocks of Kondapalli, India

Complete chemical analyses, structural formulae and trace elementdata are presented for 9 hornblendes and 12 biotites from thecharnockitic and associated rock types of Kondapalli (AndhraPradesh, India), some 200 miles north of the type area nearMadras. The chemical characteristics of the hornblendes are:very high [Al]⁴, nearly constant Ca, very high and nearly constanttotal alkalis (Na₂O > K₂O in the hornblendes of ultrabasiccharnockites and Na₂O < K₂O in those of basic charnockites),very high F and Cl, low total water, and Ni in amount greaterthan Co. The biotites are characterized by high Ti and F, lowMn, total absence of Ca, exceptionally high amounts of Cl, lowtotal water, and have Ni > Co; the biotites from differentgroups of host rocks have different Al and Fe³+ contents. Thevariations in composition of these minerals are interpretedas due to the change in bulk rock chemistry and mineralogy andnot to the change in temperatures of crystallization. The element distributional relationships in hornblende-biotite,hornblende-orthopyroxene, hornblende-clinopyroxene, biotite-orthopyroxene,and biotite-clinopyroxene pairs from the Kondapalli charnockitesare evaluated, with particular attention to the partition ofFe²+, Mg, Mn, Ti, Cr, Ni, Co, and V, and comparisons are madewith similar data available for corresponding mineral pairs.Highly consistent and uniform distribution coefficients forFe²+ and Mg (and a small range of distribution coefficientsfor many of the trace elements) are obtained for most of thedifferent sets of mineral pairs from Kondapalli, suggestingthat equilibrium distribution of the elements between the phaseshad been attained.