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Andalusite and Chiastolite


faceted andalusite

Andalusite: A scatter of faceted andalusite. If you look closely at these gems, you can see that many of them appear to be composed of a mosaic of color. This is an expression of the strong pleochroism of andalusite. Image by cobalt123, used here under a Creative Commons license.

What is Andalusite?

Andalusite is a rock-forming mineral that is mined for use in high-temperature refractories. Gem-quality specimens are cut into faceted gems and cabochons.

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 argillaceous rocks. In this situation, it can form within the metamorphosed rock or in veins and cavities within the igneous rock. It can be associated with cordierite in hornfels, granite, and granitic pegmatite.



chiastolite

Chiastolite: A cabochon cut from a specimen of the chiastolite variety of andalusite. This specimen exhibits a sharp cross, formed from graphite particles that were pushed out of the way during crystal growth. The diagonal fiber of this specimen is a result of needle-like crystals (possibly rutile crystals) that have grown within the andalusite crystal.

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 photo here. People have known about these cross stones for centuries and have valued them for their perceived religious or spiritual meaning. Attractive specimens are often cut and polished for use as amulets, charms, and novelty gems.

andalusite crystals

Twinned andalusite crystals: Twinned crystals of andalusite (chiastolite) in a piece of black micaceous schist. Photo by Moha112100, used here under a Creative Commons license.




Physical Properties of Andalusite

Chemical Classification Silicate
Color Reddish brown, olive green, white to gray
Streak White
Luster Vitreous
Diaphaneity Transparent to nearly opaque
Cleavage Good
Mohs Hardness 6.5 to 7.5
Specific Gravity 3.17
Diagnostic Properties Crystal form, associated minerals, strongly pleochroic, symmetrical inclusions
Chemical Composition Al2SiO5
Crystal System Orthorhombic
Uses Used to manufacture high-temperature porcelain of spark plugs; used to make high-temperature ceramics used in furnaces, kilns, incinerators; high-quality crystals are often used as gemstones.

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 many spark plugs is made using andalusite.

Andalusite is one of a small number of minerals that commonly forms prismatic crystals with a square cross-section. This can be important information for identification in the field.

Transparent specimens of andalusite are often strongly pleochroic. This makes them have different apparent colors when viewed from different directions. This pleochroic effect allows andalusite to be cut into unique gemstones.

Although twinning is not common in andalusite, nicely crystallized specimens that possess twinning can be distinctive. Twinning can produce cross-shaped structures perpendicular to the crystallographic c-axis, similar to what is shown in the rock in the photo above.

andalusite crystals

Andalusite: Crystals of andalusite showing their prismatic habit and square cross-section. These crystals are from Lisens Valley, Austria. Specimen and photo by Arkenstone / www.iRocks.com.

Mineral collection

The best way to learn about minerals is to study with a collection of small specimens that you can handle, examine, and observe their properties. Inexpensive mineral collections are available in the Geology.com Store.

Andalusite as an Indicator Mineral

andalusite stability fields
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 conditions under which these minerals form.

Andalusite is the low-temperature mineral of the three. Sillimanite is the high-temperature mineral, and kyanite forms at high pressures and lower temperatures.

Information from a phase diagram can be useful during mineral exploration. If a geologist finds andalusite in the field, the phase diagram reveals the possible range of temperatures and pressures that the rocks were subjected to when the andalusite crystallized. If the mineral being sought has a dramatically different temperature and pressure of crystallization, then it might not be present in those rocks. If the pressure range of the target mineral is higher, then it is possible that it exists at depth. If the temperature range of the target mineral is higher, then exploration should move toward a heat source or toward greater depth. That is a simplified example of how the phase diagram can be used.

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