Figure 3. The optically continuous nature of the enclosing alkali feldspar crystal is demonstrated by the uniform reflection of the cleavage planes of the feldspar. In many cases (but not all - see Whalstrom, 1939) the quartz crystals are also optically continuous. Relationships like this one led to theories that posited the origin of graphic granite by replacement of alkali feldspar along cleavage planes.
Last updated on October 14, 2008