The lamellar structure of potash-soda felspars

Abstract

The structures of some typical members of the felspar group were determined by Taylor, Darbyshire and Strunz (1934), and an attempt was made to explain the characteristic twinnings and the nature of the lamellar inter-growths known as perthites and microperthites. These authors emphasized that the Carlsbad, Manebach, and Baveno types of twin, which appear in monoclinic as well as in triclinic felspars, should be regarded as quite distinct from the albite and pericline types which are of frequent occurrence in the triclinic felspars. For in these last types the twinned triclinic individuals are merely simulating the higher symmetry of the monoclinic structures: thus for example in an albite twin the individuals are related by reflexion in the plane (010) which is actually a symmetry plane of the monoclinic crystal, while in a pericline twin the individuals are related by rotation about theb-axis [010] which is a digonal axis in the monoclinic structure. The formation of Carlsbad, Manebach, or Baveno twins is a consequence of certain special features of the structure, and is not due to the simulation of higher symmetry.