The Lattice and Twinning of Microcline and Other Potash Feldspars

The literature on potash feldspar (including the modern textbooks) gives one the impression that, within the limits of observation, the several varieties of KAlSi3O8 have the same apparent lattice "symmetry" (α and γ of microcline being quoted as 90° within the limits of ±10′). Two principal opinions exist to explain the different modifications: (1) The atomic arrangement (structure) of all potash feldspar is the same, and this structure is triclinic. The differences in optical behavior are explained by differences in the size and shape of twinned portions in the material (Mallard's hypothesis). (2) Two different modifications of potash feldspar exist: triclinic microcline and monoclinic orthoclase. It has been suggested that this difference is due to a different degree of order-disorder between the Si and the Al atoms, triclinic microcline having a greater degree of order than monoclinic orthoclase (Barth's hypothesis). In this investigation it is shown that Mallard's hypothesis can be excluded, although it was here found that, optically, "monoclinic" potash feldspar composed completely of finely twinned microcline does exist. However, a new X-ray determination of the lattice of microcline (α = 89½°, γ = 92½°) and the determination of the twin laws (albite, pericline) and their somewhat complex relations to each other, here discovered in microcline, indicate that such material originally grew as a monoclinic modification. In addition, it has been found that another triclinic modification of KAlSi3O8 exists (α = 90°, γ = 90½°), which is here called "triclinic adularia." The structural relations between microcline, triclinic adularia, and "monoclinic" orthoclase are discussed, making use for the most part of intensity differences in the X-ray photographs of these three substances. Interpretation of the X-ray photographs of all "monoclinic" orthoclase investigated indicates that the monoclinic modification of potash feldspar is unsTable at low temperatures. These photographs show diffuse streaks that are explained by partial transformation to a triclinic modification, which may be either microcline, triclinic adularia, or both.