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Kunzite
Pink to lilac specimens of gem-quality spodumene are highly prized and known as “kunzite”. The color of these specimens is attributed to the presence of manganese as a chromophore. Kunzite is the most commonly encountered spodumene gem. Many specimens of kunzite are strongly pleochroic, with the deepest color observed when the gem is viewed down…
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Spodumene as a Gemstone
Spodumene sometimes occurs in transparent crystals in pastel shades of pink, purple, green, and yellow. These have been cut into gemstones that are prized by collectors. However, their use in jewelry is limited to pieces that will be subject to limited abuse because of spodumene’s perfect cleavage.
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Spodumene as an Ore of Lithium
Spodumene once served as the most important ore of lithium metal. It was very costly to liberate lithium from the silicate mineral; however, lithium refined from spodumene was of very high purity. In the late 1900s, subsurface brines with high concentrations of lithium were developed in Argentina, Chile, China and other locations. These brines could be pumped to the…
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Enormous Crystals
Spodumene often occurs in extremely large crystals. One of the earliest accounts of large spodumene crystals is from the Etta Mines, Black Hills, Pennington County, South Dakota. The United States Geological Survey, Bulletin 610 reports: “The crystals are often of enormous size. In the Etta Mine, where they are best exposed both in the open…
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What is Spodumene?
Spodumene is a pyroxene mineral that is typically found in lithium-rich pegmatites. It is usually associated with other lithium minerals such as lepidolite, eucryptite, and petalite. Spodumene has a chemical composition of LiAlSi2O6 but small amounts of sodium sometimes substitute for lithium. Throughout most of the 20th century, spodumene was the most important ore of lithium metal. Lithium brines discovered in South America and…
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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…