The Mineral & Gemstone Epidote
Chemistry: Ca2(Al, Fe)3(SiO4)3(OH)
Composition: Calcium Aluminum Iron Silicate Hydroxide
Crystal System: monoclinic
Fracture: uneven to conchoidal
Specific Gravity: 3.3-3.5
Refractive Index: 1.67-1.83
Streak: white to gray
Color: Green, Greenish yellow to brownish green to almost Black
Cleavage: good in lengthwise direction
Transparency: transparent to translucent.
Associated Minerals: actinolite, andradite garnet, biotite, calcite, hornblende
The chemical formula of epidote averages about 23.5% CaO, ".5% Fe,03, 25.0% Al,03,
38.0% SiOz, and just under 2% H,O).
Epidote is a structurally complex mineral having both single silicate tetrahedrons, SiO4, and double silicate tetrahedrons, Si2O7. Parallel chains make up the structure of epidote. So crystals tend to be prismatic.
Since the chains are arranged in parallel planes, a perfect cleavage is formed between these planes in the lengthwise direction. Its unique green color is often described as "pistachio"
The color and the general appearance of epidote are so characteristic that tests are rarely necessary. Epidote has two defiantly different colors as you look into a translucent crystal. Looking through it in one direction usually shows green and a brown color as you look into it in another direction.
Actinolite, the green amphibole, has two cleavages and does not show the pronounced color change as the crystal is rotated. Tourmaline shows no color change this way and has no cleavage.
Epidote forms in metamorphic rocks and metamorphosed limestones, altered igneous rocks, pegmatites, and in traprocks with zeolites. Epidote is often found on shrinkage seams in granite, formed from the last gases or solutions to escape.
Epidote is commonly crystallized, including long, somewhat prismatic or tabular crystals or in long, slender, grooved prisms, which are actually stretched out along a horizontal direction and give the impression that the side faces are slanting. The terminations are wedge shaped or tappered pyramids. Many clusters show grooved slender crystals or acicular sprays. Epidote may also be massive, fiberous or granular. It forms in very thin crusts, of small crystals, paler in color, and in greenish films of massive or fine grained "pistacite" (from the color).
Fuses with bubbling to, a dull black scoriaceous glass, usually magnetic. Since it is insoluble in dilute hydrochloric acid, can be exposed in calcite veins by an acid soaking of the specimen.
The Prince of Wales Island (Alaska) crystals are remarkable for their size, up to 3 inches,
and their short, prismatic, almost tablet, shape. Which are the variety of specimen we
presently have available. Slender prisms are found in the Mitchell County area (North Carolina) on pegmatite feldspar. Epidote and garnet are abundant at several localities in California, where they sometimes can form alternating layers, with the shape of the garnet crystal determining the outline.
Massive unakite, a granite containing green epidote and pink feldspar, occurs as large blocks suitable for producing ornamental objects and is found throughout the Blue Ridge Mountains in Virginia, North Carolina, and Georgia. The world's leading locality is Untersulzbachtal, in the Austrian Tyrol, where magnificent, dark, lustrous crystals up to a foot long and an inch or more across were found in a pocket in a chiorite actinolite schist with colorless apatite crystals. There are many other numerous locations, including several here in Idaho.
Mostly as a rare mineral specimen and occasionally as gemstones.
FACTS & HISTORY:
From the Greek epidosis - "addition."