The Mineral epidote
Epidote is one of the most outstanding display minerals. The finest and most desirable form of this mineral is in delicate elongated crystals that are extremely lustrous and often interconnected. This form is highly treasured by collectors and creates one of the finest mineral showpieces. The color of Epidote is almost invariably green, in all different shades and tones, with a pistachio color habit being very typical.
Epidote forms a group of related minerals, with the mineral Epidote being the most prominent member of the group. Two uncommon members of the Epidote group are Epidote-(Pb) (which is more popularly known as Hancockite), and Epidote-(Sr). Though often regarded as varieties of Epidote, these two forms are classified by the IMA as individual mineral species.
Epidote is also almost identical in composition to Clinozoisite
but lacks significant iron in its structure. It forms a series
with Clinozoisite, and in many cases the actual distinction between these two
minerals cannot be made. In fact, some specimens from certain
localities are labeled as Clinozoisite-Epidote since they are
intermediary between these two minerals without an exact
determination, or may even contain both these minerals in different parts of a single crystal.
Chemical Formula
Ca2(Al,Fe)2(SiO4)3(OH)
Color
Light to dark-green, olive-green, brownish-green, yellowish-green, yellow, brown, black. Transparent forms can be strongly pleochroic with a greenish color on one angle and brownish color on the other angle.
Crystal System
Monoclinic
Properties
Streak
White |
Hardness
6 - 7 |
Transparency
Transparent to nearly opaque |
Specific Gravity
3.3 - 3.6 |
Luster
Vitreous |
Cleavage
1,1 |
Fracture
Uneven |
Tenacity
Brittle |
Crystal Habits
Usually in long slender prismatic crystals; also in thick tabular crystals. Crystals are sometimes striated and may have interesting wedge-shaped terminations. They also may have etchings or growth layers, and may contain late-growth small crystals layers growing upon a larger crystal. Also columnar reticulated, acicular, radiating, in fan-shaped and wheat sheaf crystal groups, and in long, slender fragile interconnected crystal groupings. May also form as a thin microcrystal crusting and may be massive.
Varieties
-
Rare strontium-rich member of Epidote with one of the calcium atoms replaced by strontium. Epidote-(Sr) is scientifically categorized as an individual mineral species.
-
Pink, lead-rich variety of Epidote. Hancockite is scientifically recognized as an individual mineral species with a chemical formula of CaPb2Al2Fe(SiO4)3(OH). Hancockite was named in honor of Elwood P. Hancock (1834-1916), a presitigous mineral collector specializing in New York and New Jersey minerals. His collection was acquired by The Mineralogical Museum of Harvard University in 1916 and is currently on loan to the New York State Museum in Albany. The IMA renamed this mineral in 2006 to Epidote-(Pb), though this name change has angered many in the mineral community and has generally not been accepted among collectors.
-
Pistachio-green variety of Epidote.
Uses
Good Epidote crystals, especially those outstanding forms from Austria, Alaska, and Pakistan, are highly valued among collectors. These are considered mineral classics and are well represented in many of the finest collections. Epidote is occasionally faceted as a collectors gemstone.
Noteworthy Localities
Perhaps the world's most classic Epidote locality is in the Austrian Alps, in Knappenwand in the Untersulzbach Valley, where large sharp and lustrous crystals are very highly regarded. Two other classic localities in the Alps are Wallis, Switzerland; and Le Bourg d'Oisans, Isère, France.
Some of the most outstanding Epidote crystals come from Pakistan, in the Tormiq valley, Skardu District, and in Alchuria and Hachupa in the Shigar Valley. A new and prolific occurrence is the Raskoh Mountains, Kharan, in Balochistan. This locality is producing unique, pseudohexagonal and pseudo-octahedral crystals with protruding secondary growths, often grainy in texture. Lustrous crystals from a limited find were described from Kuh-e Khorram, Zagros Mountains, Markazi Province, Iran. In China, excellent wedge-shaped crystals come from Handan, Hebei Province; and small sparkling grass-green crystals from the Hongquizhen Quarry, Sichuan Province.
Dark, chisel-shaped crystals associated with Prehnite are abundant in the Kayes Region, Mali; and large sharp crystals came from an undisclosed location in Northeastern Kenya. Small crystals and fan-shaped aggregates are from Imilchil, in the Atlas Mountains, Morocco; and sharp crystal clusters from Rehoboth, Hardap Region, Namibia. Beautiful tapering Epidote crystals come from Capelinha, in the Jequitinhonha valley, Minas Gerais, Brazil; and fine dark sprays from Rosario Mabel, Pampa Blanca, Peru. In Mexico, classic specimens came from San Quintin, Baja California Norte.
The most famous and productive Epidote locality in the U.S. is Green Monster Mountain, on Prince of Wales Island, Alaska, where dark lustrous Epidote crystals have been found in very large crystals. Large dark green to nearly black crystals come from Garnet Hill, Calaveras Co., California; and good crystals from several deposits in Death Valley, Inyo California. Other important localities include Hawthorne and the Julie Claim, Mineral Co., Nevada; the Calumet Iron Mine, Chaffee Co., Colorado; the Oxford Quarry, Warren County, New Jersey; and the Belvidere Mountain quarries, Lowell/Eden, Orleans & Lamoille Cos., Vermont. The variety Hancockite comes from the Franklin District, Sussex Co., New Jersey.
Common Mineral Associations
Quartz, Calcite, Actinolite, Hornblende, Prehnite, Biotite, Chlorite, Albite, Almandine, Andradite, Apatite
Distingushing Similar Minerals
Clinozoisite - Usually browner in color and slightly lower specific gravity; otherwise difficult to distinguish.
Actinolite - Has perfect cleavage in both directions whereas Epidote only has on one direction, usually less glassy in luster than Epidote.
Tourmaline - Different crystal habits, harder.
Vesuvianite - Different crystal habits.