Andalusite
Andalusite is an aluminum silicate, Al2SiO5, and one of three natural polymorphs sharing that exact formula along with kyanite and sillimanite. It crystallizes in the orthorhombic system as stout, nearly square prisms formed under low-pressure, moderate-temperature metamorphism. Its variety chiastolite, which displays a dark cross pattern in cross-section, makes it one of the more recognizable metamorphic minerals to casual collectors.
Andalusite properties at a glance
| Type | Minerals |
|---|---|
| Chemical formula | Al2SiO5 |
| Mohs hardness | 6.5–7.5 |
| Crystal system | Orthorhombic |
| Luster | Vitreous |
| Streak | White |
| Cleavage | Good prismatic, two directions at ~89° |
| Fracture | Uneven to subconchoidal |
| Specific gravity | 3.13–3.21 |
| Transparency | Transparent to translucent |
| Common colors | Reddish brown, Olive green, Gray, Pink |
| Rarity | common |
Geology & background
Andalusite grows during contact and low-pressure regional metamorphism of aluminous shales and mudstones, typically near granite intrusions where heat rather than deep burial drives the reaction. Because kyanite and sillimanite form the same chemical compound under higher pressure or higher temperature, geologists use whichever polymorph is present as a rough thermometer and barometer for the rock's metamorphic history, reading the mineral almost like a recorded snapshot of conditions deep in the crust. Notable sources include the Spanish province of Andalusia, for which the mineral is named, along with Brazil, Sri Lanka, and the state of California, where chiastolite-bearing schists near Mariposa County are locally famous among rockhounds.
Field collectors often find andalusite in metamorphosed shale outcrops as stubby, weathered prisms that stand out against a duller, finer-grained matrix once the surrounding rock has softened through exposure. Because the mineral can alter partly to fine-grained mica along its edges, freshly broken faces usually show its color and pleochroism far better than a weathered outer surface, so a rock hammer is worth carrying on any serious search for good specimens.
How to identify Andalusite
Andalusite typically appears as thick, squarish prisms with a nearly square cross-section, often pink-brown, gray, or olive-green, with a vitreous luster and a colorless to white streak. Hardness around 7.5 helps separate it from softer look-alikes, and it shows distinct pleochroism, appearing to change color slightly as a crystal is rotated in light. The chiastolite variety reveals a dark carbonaceous cross when a prism is cut and polished perpendicular to its length, a feature unique enough to prevent confusion with kyanite or sillimanite.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Andalusite is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Andalusite worth?
| Entry-level specimen | $2 |
|---|---|
| Typical collector specimen | $20 |
| Premium specimen | $150+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Collector specimens
- Education
- Field identification
- Industrial or lapidary use when suitable
Interesting facts
- Andalusite, kyanite, and sillimanite share the identical chemical formula but form under different pressure-temperature conditions.
- The chiastolite variety encloses carbon-rich inclusions arranged in a symmetrical cross visible when the crystal is sliced.
- Despite sharing a formula with the gem mineral kyanite, andalusite shows strong pleochroism that can shift its apparent color with viewing angle.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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