Simultaneous crystallization of quartz-feldspar intergrowths from granitic magmas

Coupled or noncoupled growth. 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 H (sub 2) O builds up at the interface.--Modified journal abstract.