biotitediorite

Biotite diorite is a coarse-grained, intrusive igneous rock of the diorite family that contains biotite mica as a prominent dark mineral. It is typically composed of plagioclase feldspar as the light-colored mineral with dark ferromagnesian minerals such as biotite, hornblende, or pyroxene, and it may contain quartz in varying amounts. When quartz is abundant, the rock is often referred to as quartz diorite, while biotite remains a key diagnostic mineral.

The mineral assemblage of biotite diorite commonly includes plagioclase feldspar (ranging from andesine to labradorite), biotite, and one or more dark mafic minerals such as hornblende or augite. Quartz may be present in some samples, and accessory minerals such as magnetite, ilmenite, zircon, and apatite can occur. The dark biotite gives a characteristic speckled appearance in hand specimens.

Biotite diorite is typically phaneritic, with visible crystals resulting from the slow cooling of magma within the crust. It forms from intermediate-silicic magmas that crystallize in plutons or similar intrusive bodies. The presence of biotite reflects particular oxidation and crystallization histories and can influence the rock’s color, durability, and weakly magnetic properties.

Geologically, biotite-bearing diorites occur in various continental crust settings and are commonly associated with related intrusive rocks such as quartz diorite and granodiorite. They are used locally as decorative or dimension stone in some regions and are valued for their dense texture and distinct light-dark mineral contrast.