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a melanocratic veinstone consisting of medium plagioclase (andesine), hornblende, or monoclinic pyroxene (diopside-augite). Apatite, iron minerals, and, frequently, zircon are present as secondary minerals.
Spessartite is a fine-grained and uniform rock, although individual large crystals (porphyritic impregnations), indicating crystallization in several stages, are sometimes encountered. Spessartite belongs to the lamprophyre group. It is formed in the final stages of crystallization of intrusive granitic mountain masses of low depth and, apparently, is the product of crystallization of a relatively low-temperature, water-rich magma.