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Gabbro
What Is Gabbro, What Minerals Are In Gabbro and What Is It Used For?
What is Gabbro?
Gabbro is a coarse-grained, dark-colored, intrusive igneous rock. It is usually black or dark green in color and composed mainly of
the minerals plagioclase and augite. It is the most abundant rock in the deep oceanic crust. Gabbro has a variety of uses in the
construcion industry. It is used for everything from crushed stone base materials at construction sites to polished stone counter
tops and floor tiles.
What Minerals are in Gabbro?
Gabbro is composed mainly of calcium-rich plagioclase feldspar (usually labradorite or bytownite) and clinopyroxene (augite).
Minor amounts of olivine and orthopyroxene might also be present in the rock. (see composition chart at right)
This mineral composition usually gives gabbro a black to very dark green color. A minor amount of light-colored mineral grains
may also be present. Unlike many other igneous rocks, gabbro usually contains very little quartz. (see a close-up view of gabbro at
right)
Gabbro and Basalt are Related
Gabbros are equivalent in composition to basalts. The difference between the two rock types is their grain size. Basalts are
extrusive igneous rocks that cool quickly and have fine-grained crystals. Gabbros are intrusive igneous rocks that cool slowly and
have coarse-grained crystals.
Gabbro in Oceanic Crust
It is often stated that Earth's oceanic crust is made-up of basalt. The word "basalt" is used because the rocks of the oceanic crust
have a "basaltic" composition. However; only a thin surface veneer of oceanic crust is basalt. The deeper rocks of the oceanic crust
are generally coarser-grained gabbro. Basalt occurs at the surface of the crust because the rocks there have cooled quickly. At
greater depth the cooling rate is slower and large crystals have time to develop. (see illustration at right).
Gabbro in Continental Crust
On the continents, gabbro can be found within thick lava flows of basaltic composition, where slow cooling allows large crystals to
form. Gabbro will also be present in the deep plutons that form when magma chambers that feed basaltic eruptions crystallize.
Large volumes of gabbro are present beneath extensive flood basalts such as the Columbia River flood basalts of Washington and
Oregon and the Deccan Traps of India
Uses of Gabbro
Gabbro can be polished to a brilliant black luster. Brightly polish gabbro is used to make cemetery markers, kitchen counter tops,
floor tiles, facing stone and other dimension stone products. It is a highly desirable rock that stands up to weathering and wear.
In the dimension stone industry gabbro is sold under the name "black granite". Gabbro is also used to make a number of rough-cut
products such as curbing, ashlars, paving stones and other products.
The most common use of gabbro is as a crushed stone or aggregate. Crushed gabbro is used as a base material in construction
projects, as a crushed stone for road construction, as railroad ballast and anywhere that a durable crushed stone is needed as fill.
Gabbro as an Ore
Gabbro sometimes contains economic amounts of some relatively rare metals. Gabbros containing significant amounts of the
mineral ilmenite are mined for their titanium content. Other gabbros are mined to yield nickel, chromium or platinum.
Gabbro is a dark-colored coarse-grained intrusive igneous
rock. The specimen shown above is about two inches (five
centimeters) across.
Magnified view of the gabbro shown in the photograph above.
The area shown in this image is about 1/2 inch across.
A chart that illustrates the generalized mineral composition of
igneous rocks. By studying this chart you can see that gabbros
and basalts are composed mainly of plagioclase feldspar,
micas, amphiboles and olivine.
In the oceanic crust basalt forms near the surface at a
divergent boundary but gabbro forms at depth from slow
crystallization. Learn about teaching plate tectonics.
A view of polished gabbro (labradorite). Polished gabbro is
sold under the name "black granite) and is used for cemetery
markers, floor tile, kitchen counter tops, facing stone and
other dimension stone uses.