Andalusite

Andalusite (Al₂SiO₅) is a *nesosilicate* belonging to the group of aluminum silicates, together with kyanite and sillimanite, with which it shares the same chemical formula but different crystal structure (*polymorphism*). It forms predominantly in metamorphic environments at moderate pressure, especially in *metapelites* (metamorphosed shales and argillites) rich in aluminum. Its *pleochroism* is particularly pronounced: by observing a crystal from different angles, it is possible to appreciate colorations ranging from yellow-brown to red-violet to green, making it a gem sought after by collectors. The *chiastolite* variety (from the Greek *chiasma*, cross) contains inclusions of *carbon* and *hematite* arranged geometrically, creating a distinct black cross visible in cross-section. The principal gemological localities are Spain (Andalusia, from which the name derives), Brazil, Sri Lanka, and Myanmar. In the gemological market, transparent and well-colored specimens command moderate prices, lower than corundum but appreciated by enthusiasts for their rarity and character.

**Crystallographic data:** Orthorhombic system, space group *Pbnm*; cell parameters: a = 7.79 Å, b = 7.90 Å, c = 5.57 Å. **Optical properties:** Refractive index *nα* = 1.630–1.634, *nβ* = 1.633–1.638, *nγ* = 1.638–1.643; birefringence *δ* ≈ 0.009–0.013; strong pleochroism (X = yellow-brown, Y = red-violet, Z = olive-green). **Density:** 3.16–3.20 g/cm³. **Hardness:** 7–7.5 Mohs; imperfect cleavage along (110). **UV-Vis spectroscopy:** Characteristic absorption bands around 370–380 nm (Fe³⁺); weak fluorescence under long-wave UV. **Composition:** Pure Al₂SiO₅; traces of Fe²⁺/Fe³⁺ responsible for coloration. **Diagnostic inclusions:** In chiastolite, graphite and hematite inclusions oriented along crystallographic axes. **Distinction from polymorphs:** From kyanite by lower hardness and lower refractive indices; from sillimanite by higher density and crystal system. **Treatments:** Rarely treated; occasionally heated to intensify color.