epidote (ep'-i-dote). (a) A yellowish-green, pistachio-green, or blackish-green mineral: Ca₂Al₂(Fe³⁺,Al)(SiO₄)(Si₂O₇)O(OH) . It commonly occurs associated with albite and chlorite as formless grains or masses or as monoclinic crystals in low-grade metamorphic rocks (derived from limestones), or as a rare accessory constituent in igneous rocks, where it represents alteration products of ferromagnesian minerals. Syn: pistacite; arendalite; delphinite; thallite. (b) A name for a mineral group with an analogous formula, but with Ca partially replaced by rare-earth elements, and Fe³⁺ replaced by Al, Mn³⁺, V³⁺, Fe²⁺ or Mg.