Saturday, May 15, 2010


Diorite is a grey intermediate intrusive igneous rock composed mainly of plagioclase feldspar, biotite, hornblende, and/or pyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite, sphene, magnetite, ilmenite and sulfides occur as accessory minerals. Diorite has about the same structural properties as granite but, perhaps because of its darker color and more limited supply, is rarely used as an ornamental and building material. It is one of the dark gray rocks that is sold commercially as black granite. 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 ten percent the rock type grades into monzodiorite or granodiorite.

Diorite has a medium grain size texture, occasionally with porphyry. Diorite is very hard, making it difficult to carve and work with. It has been used as structural stones and pavement cobblestones. It has also been used for statuary, and can take a high polish. Diorites may be associated with either granite or gabbro intrusions, into which they may subtly merge. Diorite results from partial melting of a mafic rock above a subduction zone. It is commonly produced in volcanic arcs, and in cordilleran mountain building such as in the Andes Mountains as large batholiths. The extrusive volcanic equivalent rock type is andesite.

Diorites are truly igneous; they have crystallized from molten material. Occassionally, we find others that are products of reactions between magma and included fragments of foreign rock (xenoliths). Many have been chemically transformed (metasomatized) in the solid state from some pre-existing rock, such as gabbro, by the loss of certain constituent atoms and the gain of others.