diabase - hand/display specimen from a sill in the Alexander Hills, Inyo County, California

Diabase is compositionally equivalent to basalt and gabbro, but lies between them in crystal size. If the magma had cooled more rapidly, it would have become basalt. It is primarily composed of plagioclase feldspar and pyroxine.

In basalt, rapid cooling results in crystals too small to be seen by the naked eye. In diabase, the crystals range up to 2 mm in size. In a gabbro, which cools more slowly deep below the surface, the crystals may be over a centimeter across, since the atoms have had time to move around in the melt and arrange themselves into crystals. In this diabase, feldspar laths are clearly visible to the eye, and glisten when the specimen is tilted back and forth.

In the Death Valley talc region, an extensive but locally discontinuous diabase sill intruded cherty dolomite of the Precambrian Crystal Spring Formation. Thin sections from the sill indicate that the feldspar is plagioclase, mostly calcic labradorite. It composes from 30 to 60% of the melt, by volume, with hypersthene and augite making up 30 to 60% and magnetite and ilmenite making up another 2 to 10%. Apatite is an abundant accessory, sphene is less common and quartz is uncommon.

Contact metamorphism of the Crystal Spring carbonate produced talc and tremolite. This diabase came from the Acme talc mine in the Alexander Hills, Inyo County, California.

Diabase was first described by French geologist, mineralogist, chemist and zoologist Alexandre Brongniart in 1807, using the term for rocks that were composed of feldspar and hornblende, rocks that are now called diorite, thanks to French crystallographer René-Just Haüy, who preferred to call a rock composed of feldspar and hornblende "diorite."  Diabase was preserved by German geologists in the late 1800s as the name for a rock composed of plagioclase feldspar and pyroxine, the rock we have here.

In Europe, this rock is often referred to as dolerite. Diabase usually occurs in smaller and shallow intrusive bodies such as sills, laccoliths and dikes associated with crustal extension.

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