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Diabase

A dark-gray to black, fine-textured igneous rock composed mainly of feldspar and pyroxene and used for monuments and as crushed stone. A diabase is a basaltic rock with grain size more or less transitional between gabbro (coarse) and basalt (fine).

Diabase is a fine grained, dark colored igneous rock composed of lath shaped plagioclase (feldspar) crystals surrounded by smaller grains of pyroxene and olivine. It is equivalent to the coarser grained gabbros or diorites in chemical composition. It commonly occurs as tabular bodies (dikes and sills) intruded into surrounding rocks. Diabasic texture is best seen on a weathered surface where the feldspars stand out against the more altered mafic minerals (minerals rich in magnesium and iron). The Palisades of the Hudson River, near New York City, are an example. Diabase dikes are common in areas of crustal tension, such as the eastern coast of North America and mid ocean spreading centers.

Diabase is an intrusive igneous rock with the same mineral composition as basalt. It cools under basaltic volcanoes, like those at mid-ocean ridges. Diabase cools moderately quickly when magma moves up into fractures and weak zones below a volcano. There, it forms dikes (tabular igneous rock bodies that cut across pre-existing rock layers or bodies) or sills (tabular igneous rock bodies that form parallel to pre-existing rock layers). The moderate cooling rate allows small visible crystals to form in the rock.

Fine- to medium-grained, dark gray to black intrusive igneous rock. Diabase is one of the dark rocks known commercially as "black granite." It is extremely hard and tough and is commonly quarried for crushed stone, under the name "trap." Chemically and mineralogically, diabase closely resembles the volcanic rock basalt, but it is generally somewhat coarser grained.

Rock having the same composition as basalt, but with larger crystals which are just visible to the unaided eye; also called traprock.

Material Notes:
Igneous rock, with a grain size < 1 mm (extrusive). It is a basic rock, with a total silica content of 45-55%.

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