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Andalusite: An Indicator Mineral
Stability fields: This chart shows the temperature / pressure ranges where andalusite, kyanite, and sillimanite are stable. Andalusite, kyanite, and sillimanite all share the chemical composition of Al2SiO5. However, they have different crystal structures. Their crystal structure differs because they form under extremely different conditions of temperature and pressure. The phase diagram at left summarizes the…
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How Does Andalusite Form?
Andalusite forms during the regional metamorphism of shale. It is found in schist and gneiss at some present and ancient convergent plate boundaries where the rocks have been exposed to the temperatures and pressures needed for its formation. In these rocks, andalusite is often associated with kyanite and sillimanite. Andalusite also forms during the contact metamorphism of shale. In this situation,…
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Physical Properties and Uses of Andalusite
Andalusite has a number of useful physical properties. It has the ability to withstand high temperatures without alteration. For that reason it is used to make high-temperature ceramics and refractories. The white porcelain of some spark plugs is made using andalusite. Andalusite is one of a small number of minerals that commonly forms prismatic crystals…
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What is Chiastolite?
Chiastolite is a variety of andalusite that contains black particles of graphite arranged in geometric patterns. The graphite is pushed aside by crystal growth within a rock that is being metamorphosed. As growth occurs, the particles become concentrated at crystal interfaces. The result can be a cross-shaped pattern within the mineral – similar to the “cross-stone” shown in the accompanying…
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What is Andalusite?
Andalusite is an aluminosilicate mineral with a chemical composition of Al2SiO5. It forms at low to medium metamorphic temperatures and pressures where argillaceous rocks (such as shale) are exposed to regional and contact metamorphism. Andalusite is a rock-forming mineral most commonly found in schist, gneiss, and hornfels. Minor amounts are also found in granite and granitic pegmatite. The most important use of andalusite is in making heat-resistant bricks and…
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Uses of Chalcopyrite
The only important use of chalcopyrite is as an ore of copper, but this single use should not be understated. Chalcopyrite has been the primary ore of copper since smelting began over five thousand years ago. Some chalcopyrite ores contain significant amounts of zinc substituting for iron. Others contain enough silver or gold that the precious metal content more than pays…
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Geologic Occurrence of Chalcopyrite
Chalcopyrite forms under a variety of conditions. Some is primary, crystallizing from melts as accessory minerals in igneous rocks. Some forms by magmatic segregation and is in the stratified rocks of a magma chamber. Some occurs in pegmatite dikes and contact metamorphic rocks. Some is disseminated through schist and gneiss. Many volcanogenic massive sulfide deposits containing chalcopyrite are known. The most significant chalcopyrite deposits…
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Physical Properties of Chalcopyrite
The most obvious physical properties of chalcopyrite are its brassy yellow color, metallic luster, and high specific gravity. These give it a similar appearance to pyrite and gold. Distinguishing these minerals is easy. Gold is soft, has a yellow streak and has a much higher specific gravity. Chalcopyrite is brittle and has a greenish gray streak. Pyrite is hard enough…
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What is Chalcopyrite?
Chalcopyrite is a brass-yellow mineral with a chemical composition of CuFeS2. It occurs in most sulfide mineral deposits throughout the world and has been the most important ore of copper for thousands of years. The surface of chalcopyrite loses its metallic luster and brass-yellow color upon weathering. It tarnishes to a dull, gray-green color, but in the presence of acids the tarnish…
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Other Uses for Streak Plates
In addition to their use in doing the streak test, streak plates can be used any time you need a small amount of powdered mineral. In doing the acid test to distinguish calcite from dolomite, dolomite might require being powdered to show effervescence with dilute hydrochloric acid. Simply use the streak plate to make some powder of your specimen and add acid…