Variscite

Variscite is a hydrated aluminum phosphate belonging to the secondary phosphate group, with formula AlPO₄·2H₂O. It crystallizes in the orthorhombic system, forming nodular, crusty aggregates or well-developed crystals with colors ranging from pale green to intense green-blue, sometimes with yellowish tones due to iron impurities. Its moderate hardness (3.5–4.5 on the Mohs scale) and relatively low density (2.40–2.57 g/cm³) distinguish it from turquoise, with which it is frequently confused in gemological markets.

It forms predominantly in superficial weathering environments, where phosphorus and aluminum-rich solutions percolate through sedimentary and metamorphic rocks. Typical mineralogical associations include quartz, altered feldspars, limonite, and other secondary phosphates such as wavellite. The most important deposits are found in Arkansas (USA), where variscite constitutes significant nodules in Cretaceous phosphatic deposits, as well as in Australia, Germany, and Brazil. Historically appreciated as an ornamental gem by pre-Columbian peoples, variscite maintains collector and gemological interest, although its fragility and tendency to dehydrate over time require careful conservation.

Variscite: AlPO₄·2H₂O. Orthorhombic crystal system, space group Pmn2₁ (no. 31). Cell parameters: a = 9.73 Å, b = 8.64 Å, c = 4.69 Å. Mohs hardness 3.5–4.5; density 2.40–2.57 g/cm³; ordinary refractive index (nα) approximately 1.563, extraordinary (nγ) approximately 1.594; birefringence δ ≈ 0.031. Imperfect cleavage along plane 010. Color pale green to intense green-blue, sometimes with yellow tints; vitreous luster; transparent to translucent. Soluble in dilute acids. Thermogravimetric analysis reveals loss of crystallization water at temperatures above 100 °C, with complete dehydration around 250 °C. Infrared spectroscopy shows characteristic absorption bands around 3300 cm⁻¹ (O-H), 1200–1000 cm⁻¹ (P-O stretching), and 600–400 cm⁻¹ (P-O bending). Fluorescence under UV radiation is generally absent. X-ray diffractometric analysis permits distinction from turquoise (tetragonal system, CuAl₆(PO₄)₄(OH)₈·4H₂O) and wavellite (Al₃(PO₄)(OH,F)₈·5H₂O, triclinic system). Primary deposits in Cretaceous phosphatic rocks; secondary associations include quartz, altered feldspar, limonite, millisite, wavellite, and turquoise.