Barth's thermometer has been applied to three perthites from hypersolvus alkali syenites of India. Chemical analyses and X-ray data are discussed. Subsolvus temperature as indicated by the perthites may not be applied directly to estimate the feldspar inversion temperature due to migration of A1, but assuming the two processes to be closely related, the inversion temperature in the syenite body may be of the order of 350°- 4500C. The chemical composition of alkali feldspars seems to play a dominant role in the inversion problem.
Perthite, a possible indicator of subsolvus temperature