Andalusite
Chemical formula: Al₂SiO₅
Andalusite is a mineral from the silicate group, polymorphic with kyanite and sillimanite, characterized by variable color and often occurring as crystals with a distinctive cross pattern (chiastolite).
Description
## Characteristics Andalusite is a silicate mineral belonging to the neosilicate group. It typically occurs as prismatic, orthorhombic crystals, often with a square cross-section. A characteristic feature of some varieties of andalusite, especially chiastolite, is the presence of dark inclusions of graphite or other minerals, which are arranged in a cross shape visible in the crystal's cross-section. The surface of the crystals can be striated. Andalusite can also form massive aggregates. ## Physical Properties The hardness of andalusite on the Mohs scale ranges from 6.5 to 7.5, making it a relatively scratch-resistant mineral. It has a vitreous luster. It is transparent, translucent, or opaque. The specific gravity of andalusite is approximately 3.13 g/cm³. ## Colors and Varieties Andalusite occurs in a variety of colors, from pink to reddish-brown, as well as yellow, green, white, gray, and less commonly purple. The color change is often associated with the content of manganese or iron impurities. The most well-known variety is chiastolite, characterized by the aforementioned cross pattern. Another variety, viridine, has an intense green color. ## History and Name The name andalusite comes from Andalusia, a region in Spain where this mineral was first described. It was named in 1789 by Jean-Claude Delamétherie. Chiastolite derives its name from the Greek word "chiastos," meaning cross, which refers to its characteristic appearance. ## Applications Andalusite, due to its high-temperature resistance, is used in the ceramic and refractory industries for the production of refractory materials, such as bricks and crucibles. Transparent varieties of andalusite, especially those with attractive color and pleochroism, are used in jewelry as ornamental stones.
Diagnostic features
## Identification Andalusite can be identified by its prismatic habit, orthorhombic cross-section, and in the case of chiastolite, by its characteristic cross pattern. A hardness of 6.5-7.5 on the Mohs scale is also a diagnostic feature. It often exhibits pleochroism, which is a change in color depending on the viewing angle. ## Distinguishing from Similar Minerals Andalusite is polymorphic with kyanite and sillimanite, meaning they have the same chemical composition but different crystal structures. It differs from kyanite by its lower hardness and lack of perfect cleavage in two directions. It differs from sillimanite by its crystal habit and crystallographic system. It can be confused with tourmaline, but andalusite has a different crystallographic system and lacks a triangular cross-section. ## Crystal Forms Andalusite most commonly forms prismatic crystals, often with a square or orthorhombic cross-section. It also occurs as granular or massive aggregates. Chiastolite crystals are particularly prized for their unique pattern.
Geological environment
## Genesis Andalusite forms under conditions of regional and contact metamorphism, typically in aluminum-rich rocks such as schists, gneisses, and hornfels. It forms at high temperatures and moderate pressures. It is an index mineral for specific metamorphic conditions. ## Mineral Associations Andalusite often co-occurs with other metamorphic minerals such as muscovite, biotite, quartz, cordierite, garnet, staurolite, as well as its polymorphs – kyanite and sillimanite. ## Localities Important deposits of andalusite are found in Spain (Andalusia), Brazil (Minas Gerais), Sri Lanka, Australia, Russia (Urals), United States (California, Massachusetts), Canada (Yukon), and Tyrol (Austria). Chiastolite occurs, among other places, in Chile, France, Russia, and the United States.
Rarity
Very Common
Collector aspects
## Quality Criteria The most valued specimens of andalusite are well-formed, transparent crystals with intense and uniform color, especially those exhibiting strong pleochroism. For chiastolite, the clarity and symmetry of the cross pattern are key. Large sizes and lack of inclusions also increase collectible value. ## Popular Localities Andalusite specimens from Brazil are particularly sought after due to their color and transparency, as are chiastolites from Chile and France, prized for their perfectly formed cross patterns.
Care and storage
## Cleaning Andalusite can be cleaned using a soft brush and lukewarm water with mild soap. After washing, rinse thoroughly with clean water and dry with a soft cloth. ## What to Avoid Avoid contact of andalusite with strong acids and bases, which can damage its surface. This mineral is sensitive to sudden temperature changes, which can lead to cracks. Prolonged exposure to direct sunlight may, in some cases, affect the intensity of its color. ## Storage Andalusite is best stored in separate, soft pouches or boxes to prevent scratching by harder minerals. A dry and temperature-stable environment is ideal. Chiastolite specimens with a distinct cross are worth displaying in a way that emphasizes this feature.