Tonalite mineralogy is similar to that of granodiorite, but with little or no K-feldspar. The photos above (crossed polars on left, plane light on right) show a large grain of zoned, altered plagioclase in the lower right (partially altered to sericite and epidote), and large pleochroic crystals of biotite. The extinct (dark) areas in the upper left corner and lower left corner of the photo on the left are quartz crystals at extinction.

Occurrence:

Tonalite is a major component of many I-type batholiths.