A rock of intermediate composition, diorite has 55 to 65 percent total silica content. Essentially composed of plagioclase feldspar (oligoclase or andesine) and hornblende. Biotite mica and pyroxene may also occur in diorite.

TEXTURE The grain size of diorite is medium to coarse (sometimes pegmatitic). It may be equigranular or porphyritic with phenocrysts of feldspar or hornblende.

ORIGIN Sometimes forms as independent intrusions, such as dykes, but usually comprises parts of major granitic masses.

Group: IGNEOUS

Origin: Intrusive

Grain size: Medium to coarse

Crystal shape: Anhedral, Euhedral

Classification: Intermediate

Occurrence: Pluton, Dyke

Color: Medium, Dark