gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1964.033.266.03 The Lattice Parameters of High-Temperature Triclinic Sodic Feldspars Carmichael I. S. E. M.A., Ph.D. 1 Department of Geology, Imperial College of Science London, S.W. 7 Mackenzie W. S. M.A., B.Sc., Ph.D., F.R.S.E. Department of Geology, The University, Manchester, 13

Present address: Dept. of Geology and Geophysics, Univ. of California, Berkeley.

 9 1964 33 266 949 962 Copyright © 1964, The Mineralogical Society 1964 Summary

The lattice parameters of twenty calcium-bearing anorthoclases and potassium-bearing acid plagioclases have been determined by powder methods. The 201¯ spacing of the homogenized anorthoclases gives a good measure of the Or content irrespective of the amount of calcium present ; this spacing combined with α* (010): (001) may be used to determine the ternary composition of homogeneous feldspars more potassic than approximately Or15. The lattice parameters of feldspars less potassic than Or15 cannot be used for the determination of their ternary composition.