Petrofabric Study of the Lamellar Structures in Quartz

The lamellar structures of quartz so far described in the literature have been classified in-to four types, that is, type L1, type L2, type L3 and type L4 (see p. 56). The lamellae of type L1 and of type L2 have been concluded to be relict lamellae which have been evolved from the lamellae of type L3 by annealing. As to the formation of the lamellae of type L3, the basic lamellae, deformation features of quartz grains have been analyzed in detail. The lamellae of type L3 have been regarded as a deformation band inclined at high angles to the glide line parallel or subparallel to the crystallographic c. axis Undulatory extinction bands developed in the grains containing lamellae have been classified into three types: a) undulatory extinction bands with wide width displayed as banding of parts with lamellae and those without lamellae in a grain, b) those with narrow width developed only in parts showing lamellae in a grain, and c) marginal undulatory extinction bands developed parallel or subparallel to the grain bound-aries. On the basis of detailed examination of the undulatory extinction bands of the second type, it is pointed out that the boundaries of them in the host crystal coincide with the glide plane, which is responsible for the formation of lamellae, and that the formation of the undulatory extinction bands of the second type may be attributed to inhomogeneous rotation of the secondary glide plane parallel or subparallel to the lattice plane (0001) in each of intervening spaces bounded by the glide plane responsible for the formation of lamellae.