Diorite

Lore & Background

Diorite results from the partial melting of mafic rock above a subduction zone and is found in volcanic arcs and cordilleran mountain building, such as the Andes. While its extrusive equivalent andesite is common, diorite is a minor component of plutonic rocks, which are mostly granodiorite or granite. Diorite also makes up some stocks intruded beneath large calderas. Source localities include Leicestershire and Aberdeenshire, UK; Thuringia and Saxony in Germany; Finland; Romania; central Sweden; southern Vancouver Island; the Darran Range of New Zealand; the Andes; and Concordia in South Africa. Hornblende diorite is common in the Henry, Abajo, and La Sal Mountains of Utah, where it was emplaced as laccoliths.

Reader's Guide

Human use of diorite dates at least to the Middle Neolithic, when it was used in a passage grave at Le Dolmen du Mont Ubé, Jersey. The Akkadian Empire of Sargon of Akkad used diorite for funerary sculptures after sources came under their control. Diorite was used to depict rulers or high officials in ceremonial poses or attitudes of prayer. The Inca civilization used diorite as structural stone, and medieval Islamic builders used it for water fountains in the Crimea. In later times, diorite was commonly used as cobblestone; many diorite cobblestone streets remain in England and Guernsey. Guernsey diorite was used in the steps of St Paul's Cathedral, London. Today, diorite is often sold commercially as 'black granite' and used for construction aggregate, curbing, dimension stones, cobblestone, and facing stones.

Did You Know?

• Diorite was first named by René Just Haüy from Ancient Greek διορίζειν, 'to distinguish', due to its easily identifiable large hornblende crystals.
• Orbicular diorite shows alternating concentric growth bands of plagioclase and amphibole surrounding a nucleus within a diorite porphyry matrix.
• A ferrodiorite is a dioritoid enriched in iron and titanium, common in the lower oceanic crust.