Simultaneous crystallization of quartz-feldspar intergrowths from granitic magmas

Simultaneous growth of quartz and alkali feldspar may occur in a coupled or a noncoupled manner. Coupled growth appears to result in sectoral (hourglass) or radial (spherulitic) intergrowths, depending upon the growth behavior of the feldspar. Sectoral textures are produced as a result of faceted feldspar growth, whereas nonfaceted growth favors a radial arrangement. Micrographic and micropoikilitic intergrowths appear to be caused by a noncoupled growth mechanism. Transitions from coupled to noncoupled growth usually occur during crystallization, probably because of an increased tendency for one or both of the constituent phases to facet as H₂O builds up at the interface.