Dictionary:

dolerite

  (dŏl'ə-rīt')

A dark, fine-grained igneous rock; diabase.

[French dolérite, from Greek doleros, deceitful (from its easily being mistaken for diorite), from dolos, trick.]

A fine-textured, dark-gray to black igneous rock composed mostly of plagioclase feldspar (labradorite) and pyroxene and exhibiting ophitic texture. It is commonly used for crushed stone. Its resistance to weathering and its general appearance make it a first-class material for monuments. See also Stone and stone products.

The most diagnostic feature is the ophitic texture, in which small rectangular plagioclase crystals are enclosed or partially wrapped by large crystals of pyroxene. As the quantity of pyroxene decreases, the mineral becomes more interstitial to feldspar. The rock is closely allied chemically and mineralogically with basalt and gabbro. As grain size increases, the rock passes into gabbro; as it decreases, diabase passes into basalt. See also Basalt; Gabbro.

Diabase forms by relatively rapid crystallization of basaltic magma (rock melt). It is a common and extremely widespread rock type. It forms dikes, sills, sheets, and other small intrusive bodies. The Palisades of the Hudson, near New York City, are formed of a thick horizontal sheet of diabase. In the lower part of this sheet is a layer rich in the mineral olivine. This concentration is attributed by some investigators to settling of heavy olivine crystals through the molten diabase and by others to movement of early crystals away from the walls of the passageway along which the melt flowed upward from depth, before it spread horizontally to form the sill. See also Magma.

As defined, diabase is equivalent to the British term dolerite. The British term diabase is an altered diabase in the sense defined here. See also Basalt; Gabbro; Igneous rocks.