PHYSICAL PROPERTIES:
Chemistry: Ca2(Al,
Fe)3(SiO4)3(OH)
Composition: Calcium Aluminum Iron
Silicate Hydroxide
Class: Silicates
Subclass: Sorosilicates
Group: Epidote
Crystal system: monoclinic
Color: Green, Greenish yellow to
brownish green to almost Black
Fracture: uneven to conchoidal
Hardness: 6-7
Specific gravity: 3.3-3.5
Refractive Index: 1.67-1.83
Pleochroism: yes
Luster: vitreous
Streak: white to gray
Cleavage: good in lengthwise direction
Transparency: transparent to translucent.
Associated Minerals: actinolite,
andradite garnet, biotite, calcite, hornblende
COMPOSITION:
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"
ENVIRONMENT:
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.
CRYSTAL DESCRIPTION: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).
TESTS:
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.
DISTINGUISHING CHARACTERISTICS:
The color and the general appearance of epidote
are so characteristic that tests are rarely necessary. Epidote has two
defiantly different colors as a translucent prism is rotated. It usually
shows green and dark brown. Actinolite,
the green amphibole, has two cleavages
and does not show the pronounced color change as the prism is rotated.
Tourmaline shows
no color change this way and has no cleavage.
LOCALITIES:
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.
USES:
Mostly as a rare mineral specimen and occasionally as gemstones.
FACTS & HISTORY:
From the Greek epidosis - "addition."
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