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Minerals
A mineral is composed of one or more elements, such as oxygen, silicon, hydrogen, calcium and iron. A few minerals are made of only one element and can be found that way in nature. These are called native elements. Examples of native elements include sulfur, gold and carbon. Minerals have other properties that can be used to identify them. Some commonly used properties in addition to crystal form are: color, luster, hardness, streak, heft or specific gravity, cleavage and fracture. |
Specimen/Gem and Mineral Hall
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Specimens/Gem and Mineral Hall
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Igneous rocks
Igneous rocks are created as molten magma inside the earth cools and hardens. Relatively few minerals are found in igneous rocks. Most are silicate compounds of various types, most often types of quartz, feldspar, amphibole and pyroxene. The minerals that make up igneous rocks (their composition), as well as the sizes and shapes of the different crystals (their texture), are used to classify them. Where they form is also important in classification and is used to divide igneous rocks into two groups, intrusive rocks and extrusive rocks. Intrusive rocks are formed deep inside the earth. They are also called plutonic rocks named for Pluto, the mythical god of the underworld. Intrusive rocks tend to be coarse-grained with larger crystals, because they cool slowly in the depths of the earth. A good example of a plutonic or intrusive rock is granit |
Sedimentary Rocks
Sedimentary rocks are the most abundant rocks found on the earth and are especially common in the former sea beds now known to us as much of Texas. A good example is Texas Limestone, often used as a material for building throughout the state. These rocks are formed by the accumulation of layers of deposited materials such as shells, small pieces of rock (called sediment), or plant and animal remains. Increasing pressure by subsequent layers compresses the rock. Sedimentary rocks are sometimes called stratified rock because the layers of sediment visible in them. Sedimentary rocks can be roughly grouped into two types by texture: clastic and nonclastic rocks. Most sedimentary rocks are a mixture of materials of different composition, origin and mode of deposition so it can be very difficult to classify them to a greater degree. The majority of sedimentary rocks are made up of only four ingredients: quartz, calcite, clay and rock fragments. |
Specimen/Gem and Mineral Hall
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Click Image to Enlarge |
Metamorphic Rocks
Eventually all rocks in the earth’s crust undergo a change in metamorphosis. Igneous and sedimentary rocks that have undergone these changes are called metamorphic rocks. These rocks are different from the source rocks from which they are made due to changes that are the result of stress from tremendous heat or pressure, or form chemical reactions.
Three types of metamorphism occur. Contact metamorphism occurs when rocks are heated by contact with molten magma or lave, typically resulting in nonfoliated (unbanded) rocks. Regional metamorphism takes place in large areas when rocks are buried deep beneath the surface of the earth and are subject to increases in temperature and pressure, usually resulting in foliated (banded) rocks. Kinetic metamorphism, also called dislocation, happens when the earth shifts so that rocks are crushed or dragged. |
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