MICROPEGMATITE

quartz and alkali feldspar intergrowth so fine that it can be resolved only under the microscope; it is otherwise indistinguishable from the coarser intergrowths known as graphic granite. The quartz-feldspar interfaces are planar, and the quartz areas tend to be triangular in cross section. These patches of quartz are in parallel optical orientation, and it was long supposed that they were actually continuous, either above or below the plane of observation; a detailed study of the coarser intergrowths common in many pegmatites indicated, however, that the various quartz patches usually are separate individuals. Both the angular (runic, or letterlike) character of the quartz areas and their optical parallelism are evidently crystallographic responses to growth conditions. Micropegmatite often occurs in granite and diabase and composes all or a large part of granophyre. Where the intergrowth is coarse enough and sufficiently abundant to permit chemical analysis, the bulk composition averages about 25 percent quartz and 75 percent feldspar. This composition often is that of granites and rhyolites. In diabase, micropegmatite may be interstitial, forming pods (roughly cylindrical cigar-shaped aggregates), seams, or veins.