Intermediate Rock

igneous rock containing 56–65 percent silica. This category includes primarily feldspathic rocks with small admixtures of such ferromagnesian minerals as pyroxene, hornblende, and, more rarely, biotite. Intermediate plagioclases (oligoclase, andesine) are typical of feldspars. According to the weight percentage of the constituents of intermediate rocks, a distinction is made between the sodium series (diorites, andésites, porphyrites) and the calcic series (syenites, trachytes).

Intermediate rocks are found mainly among extrusive rocks in which andésites and porphyrites predominate over trachytes and porphyries; the intrusive rocks (diorite, syenite) are much less widely distributed.