Petrographic observations on nepheline syenites from the Eastern Ghat province of India indicate that alkali-exchange reactions wherein potassium replaces sodium in a silica-deficient environment may convert plagioclase to perthite plus nepheline. A pervasive vapor phase is postulated as the vehicle for diffusion of alkali ions along temperature and/or pressure gradients, K moving down (and Na up) any temperature gradients.

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