Studies in felspar equilibria at the Geophysical Laboratory, Washington

Evidence for the existence of four series of alkali-felspars has been obtained from X-ray studies of over forty analysed felspars.1 Optical studies confirm these findings, but suggest that transitional forms may occur. The four series are: high-sanidine- high-albite; sanidine high-albite; orthoclase low-albite;microcline-low-albite. Highsanidinc has the optic axial plane parallel to (010) in the potash-rich members, whereas it is perpendicular to (010) in sanidine.