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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 90Â°. 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.
Mafic. A term used to describe an igneous rock that has a large percentage of dark-colored minerals such as amphibole, pyroxene, and olivine. Also used in reference to the magmas from which these rocks crystallize. Mafic rocks are generally rich in magnesium and iron (think MAgnesium and FerrIC).aar. A maar is a small, shallow volcanic crater with steep sides that forms in an explosive volcanic eruption that is triggered when rapidly ascending magma contacts groundwater at a shallow depth and a steam explosion occurs.