Andalusite and Chiastolite

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.
Andalusite gets its name from the Spanish province of Andalusia, where it was first discovered. Andalusite is an aluminium silicate that has been called the "poor man's alexandrite." Some andalusite crystals have inclusions arranged so that in cross-section, they form a dark cross. This form of andalusite is called chiastolite, which is a name that comes from the Greek word for cross. Chiastolite occurs in schists and can be found in Santiago de Compostela, a town in Northwest Spain. In the past, many amulets made of chiastolite were sold to pilgrims. Andalusite is heated to form mullite, a mineral that has many industrial uses. For example, mullite is used in the manufacturing of spark plugs. Andalusite is a polymorph with two other minerals, kyanite and sillimanite. It is a pleochroic stone; it can display shades of colour varying from light yellowish brown to green brown, light brownish pink, greyish green, or definite green, with pleochrism, making it hard to identify the main colour.

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.

Colour

Transparent green andalusite is the most valued form of andalusite. Unlike other pleochroic gemstones, such as iolite and zoisite, where gem cutters try to reduce the pleochrism and highlight the single best colour, cutters of andalusite actually attempt to get a good mix of colours in the stone. When cut in an emerald cut, andalusite may look mostly green, with bits of orange showing at the ends of the emerald shape. When cut in a round cut, the green body colour is visible, as are simultaneous flashes of other colours. The rare and sometimes expensive emerald green coloured variety may exhibit a bright yellow colour simultaneously, or when viewed from different angles. In contrast, the pink variety does not exhibit this kind of colour phenomenon.

Properties

Andalusite is a fairly durable stone, rating a 7 to 7.5 on the hardness scale. It is a vitreous and transparent to translucent stone. Andalusite occurs as crystals with poor luster, typically in thermally metamorphosed peltic rocks, and in muddy rocks that have been metamorphosed under low-pressure conditions. It also occurs, together with corundum, tourmaline, topaz and other minerals, in some pegmatites. Andalusite is found mainly as water worn pebbles and it is these pebbles that are usually cut 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 colours when viewed from different directions. This pleochroic effect allows andalusite to be cut into unique gemstones.
Although twinning is not common in andalusite, nicely crystallised specimens that possess twinning can be distinctive.
Chemical FormulaAl2SiO5
ColourWhite, Green, Yellow, Orange, Brown, Pink, Gray, Black, Multicolored
Hardness7 - 7.5
Crystal SystemOrthorhombic
Refractive Index1.62 - 1.65
SG3.1 - 3.2
TransparencyTransparent to opaque
Double Refraction-0.01
LusterVitreous
Cleavage2,1
Mineral ClassAndalusite

Occurrence

Beautiful dark green forms of adalusite come from alluvial gravels in Sri Lanka, while green andalusite gemstones are found in Brazil, principally in the states of Espirito Santo and Minas Gerais. Andalusite from Brazil is small, rare and very expensive. Brazil is actually the primary source for andalusite. Large columnar crystals of andalusite occur at Lisenz, Austria. Andalusite also occurs in the United States in Massachusetts, the White Mountains in California, Standish, Maine, and Delaware County, Pennsylvania. Other important sites for andalusite include Canada, Bimbowrie Australia, the Hunan Province in China, as well as various locations around Siberia, Russia, Brittany, France, and England. It has been said that the person who is attracted to this stone is a fighter. However, by wearing the stone, the wearer supposedly becomes wise, gives up fighting, and starts to embark on the path of love. With andalusite, the wearer perceives that with fighting, we live contrary to our own nature.

Andalusite as an Indicator Mineral

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 summaries 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.

Andalusite Healing Properties

Andalusite is sometimes called "The Seeing Stone" because it is used in metaphysical works to calmly see the various parts of one's character without bias. It is also reported to be helpful to see the different sides of a problem, and is used for scrying.
Andalusite is said to enhance memory and recall. It is also said to bring chivalry. Andalusite is said bring moderation and balance. It is also a stone that is said to be helpful for meditation and centring.
Physically andalusite is used in crystal healing and folk healing for AIDS, eye problems, deficiencies in calcium, oxygen, iodine, and water retention.
Note that healing crystal meanings are spiritual supports to healing and are not prescriptions or healthcare information.
Andalusite is associated with the solar plexus and heart chakras.