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Gabbro

A usually coarse-grained igneous rock composed chiefly of calcic plagioclase and pyroxene. Also called norite.

Any of several medium- or coarse-grained rocks that consist primarily of plagioclase feldspar and pyroxene. Gabbros are found widely on the Earth and on the Moon. They are sometimes quarried for dimension stone ("black granite"), but the direct economic value of gabbro is minor. Far more important are the nickel, chromium, and platinum minerals that occur almost exclusively in association with gabbroic or related rocks. Magnetite (iron) and ilmenite (titanium) are also found in gabbroic complexes.

Gabbro is a dark-colored, coarse-grained intrusive igneous rock. Gabbro is very similar to basalt in its mineral make up. It is composed mostly of the mineral plagioclase feldspar with smaller amounts of pyroxene and olivine. It is the intrusive equivalent of a basalt. Gabbros are often used as facing stones on buildings, especially the attractive dark varieties, but it is too fragile to make a good construction material.

Gabbro is typically found on continents in dikes, sills, and laccoliths from slow cooling of mafic magma, mostly derived from hot spots. Gabbro is also part of the oceanic lithosphere (ophiolite suite) forming as a thick layer below the top-most pillow lavas across all the oceanic floors.

Gabbro and basalt (its extrusive equivalent) are said to be mafic rocks because of their dark tone, in contrast to the light tone of the felsic/sialic rocks granite and rhyolite, and intermediate tone of diorite/andesite. The coarse size of the dark mineral grains in gabbro can be discerned by rotating a fresh specimen in the light to look for flashes of light reflecting off of cleavage faces in the plagioclase feldspar and pyroxene grains. Basalt and gabbro form from the partial mentling of the upper mantle, and constitute the dominant rock types in the oceanic crust.

Material Notes:
Granite is an igneous rock with a coarse texture and grain size > 3 mm (plutonic). Gabbros are basic rocks, with a total silica content of 45-55%.Predominent minerals in this generally dark colored rock are the ferromagnesium minerals (horneblende, olivine, pyroxene) and plagioclase feldspar. Chemically and mineralogically, gabbro is equivalent to basalt. Gabbros may occur as border rocks around granitic and other plutons, or as small individual plutons or dikes. Their most common occurrence is in the lower parts of large, layered complexes. Most gabbros appear to intrude the rocks surrounding them and so are thought to be of igneous origin, although some may also be produced by metamorphic processes. It is lower in silica and darker in color than diorite, and chemically the equivalent of diabase.

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