Na-rich plagioclase and K-feldspars alter readily in the presence of hydrous fluids to the white micas muscovite (K) or paragonite (Na).  Incipient alteration appears as a cloudiness in the feldspars, and in more advanced stages, the mica grains are coarse enough to be identified as such (although muscovite and paragonite are difficult to distinguish from one another).  The Na and K feldspars are not always altered to the same extent.  In the example shown here, the plagioclase grains are partially sericitized and appear cloudy in plane light (A) due to partial replacement by fine mica, identifiable as such in B. Quartz does not alter, and one clear grain is visible at the right margin in A, though extinct in B.  Sample is a granite from an unknown location.  Both views are 2.2 mm across.  A ppl, B x-nicols.