Orientation of feldspar intergrowths : application of lattice misfit theory to cryptoperthites and e-plagioclase

Abstract

The lattice misfit and coherent elastic theories for intergrowth morphology are reviewed with particular reference to their application to feldspar intergrowths. Precise alignment of phase boundaries in feldspar intergrowths appears to be determined by minimisation of lattice misfit. Good agreement between the two theories is only obtained when the area misfit is low. In complex cryptoperthite, the interface between the fine albite twin lamellae and the K-rich matrix is a fine-scale [106] zone zigzag, which is superimposed on the characteristic coarse-scale (diagonal association) zigzag. Lattice misfit calculations with sanidine (0r50)-anorthoclase (Or25) lattice parameters precisely reproduce the (36-1), (-361) orientations of the fine zigzag. By adding progressively increasing fractional amounts of the triclinic distortion of the anorthoclase lattice to the equivalent monoclinic parameters for Or25, the orientations of the parallel, sinuous, coarse zigzag and fine zigzag interfaces are sequentially reproduced. The most likely interpretation of these data is that the coarse zigzag develops in the early stages of the monoclinic → triclinic inversion of the Na-rich phase, and the fine albite twin lamellae form subsequently. Lattice misfit calculations show that the orientation of the plane of modulation of the e-plagioclase structure results from the competing effects of albite-Na plagioclase and anorthite-Ca plagioclase domain interfaces. Because stresses are required to maintain the interfaces, the resultant orientation is a bulk property dependent on the relative sizes of the albite and anorthite domains. The limiting albite-Na plagioclase interface orientation may be reproduced with both high albite and intermediate albite lattices, thus extending the possible temperature stability of e-plagioclase.