Fig. 63.-Cotectic or simultaneous crystallization of two minerals

A---Cotectic texture of K-feldspar with quartz at final stage of crystallization. Branch-like K-feldspar penetrates quartz as if the former replaces the latter.

B---Cotectic or simultaneous crystallization of K-feldspar with quartz. The grain size of both cotectic minerals became bigger from their interiors outwards.

C---Relicts of quartz in calcite indicate that quartz is replaced by calcite. Plagioclase is rimmed by quartz grains just as quartz is also replaced by plagioclase. However, the quartz-plagioclase relationships are probably due to cotectic crystallization.

D---Two euhedral allied crystals of K-feldspar with albite. K₁, K₂ has the same orientation as that of Ab₁, Ab₂. They are primary rather than metasomatic.