English: Diorite from the Ordovician of Massachusetts, USA.

Igneous rocks form by the cooling & crystallization of hot, molten rock (magma & lava). If this happens at or near the land surface, or on the seafloor, they are extrusive igneous rocks. If this happens deep underground, they are intrusive igneous rocks. Most igneous rocks have a crystalline texture, but some are clastic, vesicular, frothy, or glassy.

Diorite is an intrusive igneous rock. It is similar to granite and gabbro in that is has phaneritic texture, with all or almost all crystals between 1 millimeter and 1 centimeter in size each. Phaneritic texture forms as a result of relatively slow cooling of magma deep underground. The sample seen here is somewhat porphyritic.

Diorite has a chemical composition between that of granite's felsic chemistry and gabbro's mafic chemistry. Diorite is an example of an intermediate igneous rock, which has 52 to 65% silica (= SiO2 chemistry) (it has also been defined as 55 to 65% silica). Intermediate igneous rocks are sometimes light-colored, sometimes dark-colored, and sometimes have medium colors.

The mineral content of diorite is dominated by feldspar with little to no quartz. Diorite is defined as plagioclase feldspar-rich, with little to no alkali feldspar. The minor alkali feldspar component is often intergrown with the plagioclase feldspar to form perthite.

The petrologic definition of diorite has varied over the years, but a reasonable working definition seems to be a plagioclase-rich intrusive igneous rock with one or more mafic minerals (not to be confused with the idea of anorthosite, however - it has far more plagioclase & a far-smaller mafic mineral component).

The diorite seen here has moderately large brownish-gray plagioclase feldspar crystals, with a dark gray groundmass dominated by finely-crystalline plagioclase plus hornblende amphibole.

Geologic unit: Salem Gabbro-Diorite (a.k.a. Salem Diorite-Gabbro), Middle Ordovician, 460 Ma

Locality: unrecorded locality at or near the town of Salem, southern Essex County, northeastern Massachusetts, USA