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Definition: diabase from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

Alternative name for dolerite (a form of basalt that contains very little silica), especially dolerite that has metamorphosed.


Summary Article: Diabase
From Guide to Minerals, Rocks and Fossils

Colour When fresh it is black, dark grey or green; may be mottled black and white. Grain size Medium. Texture Occasionally ophitic texture (see gabbro) can be distinguished in hand specimen. May be porphyritic. Structure Vesicles and amygdales occur. Sometimes has segregations of coarser rock enriched in feldspar. Mineralogy Phenocrysts comprise olivine (olivine diabase) and/or pyroxene or plagioclase. The groundmass comprises the same minerals with iron oxide, and sometimes with some quartz, hornblende or biotite. Field relations Dykes and sills. These may form swarms of hundreds or perhaps thousands of individual dykes or sills which often radiate from a single volcanic centre.

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