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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.
The two best-known igneous rock types are basalt and granite, which have distinctly different compositions and textures. Basalt is the dark, fine-grained stuff of many lava flows and magma intrusions. Its dark minerals are rich in magnesium (Mg) and iron (Fe), hence basalt is called a mafic rock.So basalt is mafic and either extrusive or intrusive.Granite is the light, coarse-grained rock formed at depth and exposed after deep erosion.he main thing about igneous rocks is that they were once hot enough to melt. The following traits are all related to that. 1  Because their mineral grains grew together tightly as the melt cooled, they are relatively strong rocks. 2  They're made of primary minerals that are mostly black, white or gray.