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.