Gabbros: Coarse-Grained Mafic Rocks

Gabbros are generally dark colored. They are dominated by Ca-rich plagioclase, which tends to be darker than the Na-rich variety, and by pyroxene. You may see olivine, but not in the samples shown here.

This gabbro is roughly half pyroxene and half plagioclase. Pyroxene is typically duller and more blocky than hornblende, but most important are the cleavages intersecting at 90°.

Click here to get a super-close up that shows the long, narrow rectangles (laths) typical of plagioclase. The striations are not visible in the photos.

This gabbro has a lot of coarse, dark pyroxene crystals in it. There is also a lot of plagioclase, but it is tough to spot because it is also quite dark. The shiny cleavage planes in the upper park of this sample are all plagioclase.

Click here to see a super close-up that shows striations on one of these cleavage surfaces.

This typical gabbro is made of pyroxene and plagioclase, but it is tough to see really good features of either mineral. Click here to see a super close-up. This sample illustrations how geologists may have to name a rock in the field without being sure of its minerals. Lab work will make accurate mineral identifications and a precise rock name.

The red dot in the center is barn paint.

This is a typical tombstone gabbro. The polished stone is the same rock as the rough stone at the base.

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