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Gabbro
Gabbro is a coarse-grained, dark-colored, intrusive igneous rock that contains feldspar, pyroxene, and sometimes olivine. The specimen shown above is about two inches (five centimeters) across.
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Granite
Granite is a coarse-grained, light-colored, intrusive igneous rock that contains mainly quartz, feldspar, and mica minerals. The specimen above is about two inches (five centimeters) across.
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Diabase
What is Diabase? In the United States and Canada, the name “diabase” is used for a dark gray to black, fine-grained, intrusive igneous rock that has a composition similar to basalt and gabbro. The difference between basalt, diabase, and gabbro is in their grain size – which was determined by their cooling rates. Basalt: rapid cooling in a lava flow produced…
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Diorite and Andesite
Diorite and andesite are similar rocks. They have the same mineral composition and occur in the same geographic areas. The differences are in their grain sizes and their rates of cooling. Diorite crystallized slowly within the Earth. That slow cooling produced a coarse grain size. Andesite forms when a similar magma crystallizes quickly at Earth’s surface. That rapid cooling produces…
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What is Diorite?
Diorite is the name used for a group of coarse-grained igneous rocks with a composition between that of granite and basalt. It usually occurs as large intrusions, dikes, and sills within continental crust. These often form above a convergent plate boundary where an oceanic plate subducts beneath a continental plate. Partial melting of the oceanic plate produces a basaltic magma that…
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What Is Dacite?
Dacite is a fine-grained igneous rock that is normally light in color. It is often porphyritic. Dacite is found in lava flows, lava domes, dikes, sills, and pyroclastic debris. It is a rock type usually found on continental crust above subduction zones, where a relatively young oceanic plate has melted below. Composition of Dacite A generalized mineral composition for dacite is intermediate between rhyolite and andesite. It…
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What is Basalt?
Basalt is a dark-colored, fine-grained, igneous rock composed mainly of plagioclase and pyroxene minerals. It most commonly forms as an extrusive rock, such as a lava flow, but can also form in small intrusive bodies, such as an igneous dike or a thin sill. It has a composition similar to gabbro. The difference between basalt and gabbro is that basalt is a fine-grained rock while gabbro is…
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What are Igneous Rocks?
Igneous rocks are formed from the solidification of molten rock material. Some form below Earth’s surface. Some form on or above Earth’s surface. We describe these two basic types: Intrusive igneous rocks crystallize below Earth’s surface, and the slow cooling that occurs there allows large crystals to form. Examples of intrusive igneous rocks are: diabase, diorite, gabbro, granite, pegmatite, and peridotite. Extrusive igneous…
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Minerals: The Keys to Coloring Glass
The sources of the oxides, sulfides, and other metallic compounds used to color glass are minerals. These minerals are typically mined, processed to remove impurities, and used to manufacture coloring agents for glass. The keys to beauty often come right from the Earth.
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Widely Known Glass Colors
Some colors of glass are widely known. Perhaps the best example of this is “cobalt blue” that is produced by adding cobalt oxide to the glass melt. “Vaseline glass” is a fluorescent yellow-green glass that contains small amounts of uranium oxide. “Ruby gold” and “cranberry glass” are red glasses produced by the addition of gold.…