Absence of optically positive potash feldspar in the inyo mountains, California-Nevada

An unusual, untwinned potash feldspar of low refractive index and positive sign was reported by Anderson (1937, p. 11) to be abundant in the Boundary Peak granite of the Northern Inyo Range, California-Nevada. The mineral, tentatively identified as the “isorthoclase” of Duparc (1904), was studied in more detail by Anderson and Maclellan(1937). By x-ray, universal stage and spectroscopic techniques this mineral was found to be an optically positive, triclinic potash feldspar (Or₇₂Ab₂₆An₂) with 2Vγ = 64-89°, α=1.518, β= 1.520, γ=1.524. This description more closely fits the “isomicrocline” of Kazakov (1959).