Alkali Amphiboles and Pyroxenes from Siwana Granite and the Associated Acid Volcanics, Rajasthan

Compositional characteristics and substritutional schemes of the amphiboles and pyroxenes of the alkali granites and the associated volcanics around Goliya Bhaylan area are studied. In the alkali granite, the amphiboles evolve from ritchterite to arfvedsonite, in trachytes from winchite to riebeckite and in the case of rhyolites from ritchterite through arfvedsonite to riebeckite with dominant substitution of the type Na^M4 +R³⁺ for Ca + R²⁺. The pyroxenes in alkali granite evolve from hedenbergite to degirine through Aegirine-Augite representing the acmitic hedenbergite trend with dominant NaFe³⁺= Ca(Mg,Fe)²⁺substitution and a minor amount of NaAl = CaFe²⁺substitution , whereas the pyroxenes in the acid volcanics are aegirine representing the acmitic trend, with the same substitution characteristics as those found in the alkali granites. The occurrence of TI aegrine and aenigmatite is also reported. The chemical trends of the pyroxenes and amphiboles indicate that they are formed from a highly evolved peralkaline residual melt.