Cross polarized photomicrograph, Plagioclase Diabase or dolerite or microgabbro
Diabase or dolerite or microgabbro is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine grained to aphanitic chilled margins which may contain tachylite (dark mafic glass). Diabase is the preferred name in North America, dolerite is the preferred name in most of the rest of the world, where sometimes the name diabase is applied to altered dolerites, though many petrologists prefer the name microgabbro to avoid this confusion. Diabase normally has a fine, but visible texture of euhedral lath-shaped plagioclase crystals (62%) set in a finer matrix of clinopyroxene, typically augite (20–29%), with minor olivine (3% up to 12% in olivine diabase), magnetite (2%), and ilmenite (2%). Accessory and alteration minerals include hornblende, biotite, apatite, pyrrhotite, chalcopyrite, serpentine, chlorite, and calcite. The texture is termed diabasic and is typical of diabases. This diabasic texture is also termed interstitial. The feldspar is high in anorthite (as opposed to albite), the calcium endmember of the plagioclase anorthite-albite solid solution series, most commonly labradorite.
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Photo credit: © Scenics & Science / Alamy / Afripics
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Keywords: cross, diabase, dolerite, holocrystalline, mafic, microgabbro, photomicrograph, plagioclase, polarized, subvolcanic
