Epidote

Epidote is a sorosilicate (containing both isolated SiO₄ tetrahedra and Si₂O₇ groups) that crystallizes in the monoclinic system. Its name derives from the Greek *epidosis* (growth), as it often occurs in well-developed crystals. It forms primarily in low to medium pressure metamorphic environments, where feldspar-rich rocks undergo hydrothermal alteration, and represents an index mineral of the greenschist facies. Color varies from yellowish-green to intense green depending on the isomorphic substitution Fe³⁺↔Al at the M3 position in the crystal structure. Typical mineral associations include quartz, feldspar, chlorite, and albite. Despite its relative abundance, epidote has little commercial gemological interest, although extremely rare transparent gem-quality crystals have been cut for collectors.

Crystal system: monoclinic, space group P21/m. Lattice parameters: a ≈ 8.89 Å, b ≈ 5.63 Å, c ≈ 10.19 Å, β ≈ 115.4°. Hardness: 6-7 Mohs. Density: 3.35-3.45 g/cm³ (increases with Fe content). Cleavage: perfect on ((100)), good on (01). Fracture: subconchoid. Luster: vitreous. Transparency: transparent to translucent. Refractive indices: nα ≈ 1.715, nβ ≈ 1.725, nγ ≈ 1.734; birefringence Δn ≈ 0.019 (positive). Axial angle 2V ≈ 64-90° (composition-dependent). Pleochroism: weak to moderate (yellow-green → yellow-brown → green). UV Fluorescence: weak or absent. Absorption: characteristic band in visible spectrum around 460 nm (Fe³⁺). Composition: solid solution series with pistacinite (Fe-epidote) and clinozoisite (Al-epidote); isomorphic substitution Fe³⁺ ↔ Al governs optical properties. Thermal stability: stable to ~500 °C, dehydrates above this temperature. Solubility: insoluble in dilute cold acids.