Diorite ( ) is an intrusive igneous rock composed principally of the silicate minerals plagioclase feldspar (typically andesine), biotite, hornblende, and/or pyroxene. The chemical composition of diorite is intermediate, between that of mafic gabbro and felsic granite. Diorite is usually grey to dark-grey in color, but it can also be black or bluish-grey, and frequently has a greenish cast. It is distinguished from gabbro based on the composition of the plagioclase species; the plagioclase in diorite is richer in sodium and poorer in calcium. Diorite may contain small amounts of quartz, microcline, and olivine. Zircon, apatite, titanite, magnetite, ilmenite, and sulfides occur as accessory minerals. Minor amounts of muscovite may also be present. Varieties deficient in hornblende and other dark minerals are called "leucodiorite". When olivine and more iron-rich augite are present, the rock grades into "ferrodiorite", which is transitional to gabbro. The presence of significant quartz makes the rock type "quartz-diorite" (>5% quartz) or tonalite (>20% quartz), and if orthoclase (potassium feldspar) is present at greater than 10 percent, the rock type grades into "monzodiorite" or granodiorite. A dioritic rock containing feldspathoid mineral/s and no quartz is termed "foid-bearing diorite" or "foid diorite" according to content.