Diorite is a coarse-grained intermediate plutonic rock. Intermediate are rocks whose composition lies between felsic and mafic.
Felsic rocks are silica-rich and high in lighter elements like silicon, oxygen, potassium, and sodium. Examples are dacite, granite, and rhyolite.
On the other hand, mafic rocks are silica-poor rocks rich in iron and magnesium. Such include basalt and gabbro.
Therefore, this rock is mesocratic, meaning its color, mineral, and chemical composition lies between mafic and felsic rocks.
We mentioned that diorite is a plutonic, also called intrusive igneous rock. It means that this forms deep beneath the Earth’s surface, where cooling is slow. The slow cooling histories allow times and conditions for coarse-grain texture.
Considering mineral and chemical composition, the entrusive equivalent of diorite is andesite. Andesite rock forms on the Earth’s surface where cooling is fast.
The quick cooling in andesite results in a fine-grained texture, not a coarse one like diorite. This marks their main differences.
Properties and texture
Diorite is a massive, hard, coarse-grained rock with a 2.8g/cm3 density and a Mohs hardness scale of 5-6.
Its color is medium to dark gray with a salt and pepper appearance from the dark and light crystals. However, some have a green, blue, or brown tint.
Typically, diorites have a color index ‘M’ of less than 35. Those with M more than 35 are melanodiorites, while leucodiorites have M between 10-25.
The dark and lighter varieties are due to more and less percentage of mafic minerals like hornblende and biotite.
Diorite rock has a phaneritic texture characterized mainly by equigranular, interlocking crystals. These crystals are mostly hypidiomorphic, i.e., partly well-formed faces.
However, this rock may show other textures and fabrics like porphyritic, pegmatitic, comb layering, and orbicular textures.
Porphyritic diorite has large crystals called phenocrysts in a finer but phaneritic groundmass or matrix. It forms due to two cooling stages.
On the other hand, pegmatitic diorite has abnormally large crystals, often 3-4 inches, with some up to tens of meters. It forms from the water-rich, the last portion of magma to crystallize.
What about orbicular diorite? It happens when this rock has ovoid structures with concentric shells with alternating texture and/or mineral composition.
Lastly, comb layering happens when this rock has elongated, subparallel crystals oriented perpendicular to the layering plane or planar boundary.
Chemical composition
Diorite is an intermediate rock with 52-63 wt.% silica. Also, it has intermediate amounts of alkali oxides (Na2O and K2O) and calcium, iron, and magnesium oxides.
Diorite mineral composition
Diorite is mainly composed of plagioclase, biotite, and hornblende. Also, it may have a smaller amount of pyroxenes, quartz, alkali feldspar, and olivine.
Accessory minerals are sphene, ilmenite, apatite, zircon, and sulfides.
The plagioclase is sodium-rich and calcium-poor, with less than 50 mol.% anorthite. It is usually andesine or oligoclase, while alkali feldspar is microcline.
On the other hand, pyroxenes are augite and orthopyroxene, while quartz and biotite fill interstitial spaces. Thus, their crystals are tiny to see.
On the QAPF classification, diorite is a coarse-grained plutonic rock in which quartz is less than 5% of QAPF content by volume, and plagioclase is over 90% of the total plagioclase.
This rock is one of the dioroids. Dioritoids are rocks that, on the QAPF classification, have less than 10% feldspathoids and less than 20% quartz of the QAPF content by volume, with plagioclase more than 65% of the total feldspars.
Quartz diorites are coarse-grained, in which quartz is 5-20% of the QAPF content by volume, while plagioclase is over 90% of the total feldspar. If quartz exceeds 20%, you will have tonalite.
On the other hand, foid-bearing diorites have up to 10% feldspathoids of the QAPF content by volume, and more than 90% of the feldspar is plagioclase.
Monzodiorite has a composition that lies between diorite and monzonite. In this coarse-grained rock, quartz is less than 5% of the QAPF content by volume, and plagioclase is 65-90% of the total feldspar.
An increase in quartz to 5-20% of the QAPF content by volume grades monzodiorite to quartz monzodiorite, while those with up to 10% feldspathoids are foid-bearing monzodiorite.
How does diorite rock form?
Dolerite forms deep beneath the Earth’s surface from andesitic magmas. Cooling is slow, allowing large crystals to grow. Thus, it has a coarse-grained texture.
Andesitic magma forms from 1) fractional crystallization of basaltic magmas with or without assimilation of crustal rocks, 2) partial melting of subcrustal rocks, 3) magma mixing, and 4) partial melting of the metasomatized mantle.
Where is diorite found?
They occur in igneous intrusions on continental volcanic arcs above subduction and mountain-building belts. It occurs in sills, dikes, or as part of granitoid and granodioritic batholiths.
Some occur also occur as stocks beneath large calderas or in smaller massifs.
Some places diorite occurs in North America include the Rocky Mountains range. They are also found in Utah, Arizona, New England, Washington, and California, including Yosemite Valley in the US and Victoria and Vancouver Island in Canada.
More places are the Andes mountain range in South America, Leicestershire and Aberdeenshire in the UK, and Thuringia and Saxony in Germany.
They also occur in central Sweden, Concordia in South Africa, and Darran Range in New Zealand, among other places.
Diorite uses
Diorite is a hard and durable rock perfect for many uses in the dimensional stone industry and construction aggregate. Despite its good engineering properties, it is rarely used, as it is uncommon.
Crushed diorite to make aggregate for the roading industry, railway ballast, subbase, and as a fill material. However, it rarely makes concrete due to the relatively high silica content.
As a dimensional stone, this rock is sold as black granite, while the leucocratic varieties are sold as white granites. Cut and polished diorite stones are used in building and construction. More uses include making slabs, tiles, monuments, pavers, sculptures, etc.
Some prehistoric and middle age uses include passage graves, sculptures, vases, stelae, water fountains, and steps.
Frequently Asked Questions (FAQs)
Microdiorites are medium-grained diorites formed at shallower depths, usually less than 2 km (1.2 mi) depth. These plutonic rocks are called subvolcanic, and their grains are medium in size due to their relatively faster cooling than plutonic rock.
They occur mainly in the lower continental crust and as plutons in the continental crust above subduction zones (convergent plate boundaries). Erosion and uplift may expose these rocks to the surface.