Diabase

• Webster's Revised Unabridged Dictionary
  • n Diabase (Min) A basic, dark-colored, holocrystalline, igneous rock, consisting essentially of a triclinic feldspar and pyroxene with magnetic iron; -- often limited to rocks pretertiary in age. It includes part of what was early called greenstone.
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• • n diabase The name originally given by A. Brongniart to a rock which Haüy later designated as diorite, which name Brongniart himself adopted in preference to that of diabase. Later (in 1842) Hausmann again introduced the word diabase, and by it designated a variety of pyroxenic rock, occurring in the Harz, and characterized by the presence of chlorite in considerable quantity. At the present time the name diabase is used to designate a crystalline-granular rock, consisting essentially of augite and a triclinic feldspar, with more or less magnetite or titaniferous iron, or both, and occasionally apatite or olivin, to which is added chloritic matter in varying amount. To this chloritic material the name viridite is frequently applied, this being the substance which gives the mass the greenish color which it frequently has. Diabase is one of the rocks included under the popular designation of greenstone, and also under that of trap. It is an altered form of basalt. “The main difference between diabase and basalt appears to be that the rocks included under the former name have undergone more internal alteration, in particular acquiring the diffused ‘viridite’ so characteristic of them” (Geikie, 1885). See greenstone, trap, diorite, and melaphyre.
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• • n Diabase dī′a-bās a compact igneous rock, an altered form of basalt—included under the popular names greenstone and trap
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