THE MINERAL EPIDOTE
- Chemistry: Ca2(Al, Fe)3(SiO4)3(OH), Calcium Aluminum Iron Silicate Hydroxide.
- Class: Silicates
- Subclass: Sorosilicates
- Group: Epidote
- Uses: mineral specimens and rarely as a gemstone.
Epidote is a structurally complex mineral having both single silicate tetrahedrons, SiO4
, and double silicate tetrahedrons, Si2
The formula of epidote could be expressed in a such a way so as to reflect this organization; Ca2
The two aluminums represent the parallel chains of AlO6
octahedra that are the heart of the epidote structure.
The silicate groups and extra ions connect the chains together.
Since the chains are parallel, the crystals tend to be prismatic.
The chains are arranged in parallel planes and the perfect cleavage breaks the bonds between these planes.
Epidote, while not general known for exceptional crystals can produce some magnificent specimens.
Its unique green color which is often described as "pistachio", is quite striking in well colored specimens.
- Color is "pistachio" green to yellowish or brownish green, also brown to black.
- Luster is vitreous.
- Transparency crystals are transparent to translucent.
- Crystal System monoclinic; 2/m
- Crystal Habits include long, somewhat prismatic or tabular crystals with a typically dominant pinacoid that the crystal is often flattened against.
The terminations are wedge shaped or tappered pyramids.
Many clusters show grooved slender crystals or acicular sprays.
Also massive, fiberous or granular.
- Cleavage good in one direction lengthwise.
- Fracture is uneven to conchoidal.
- Hardness is 6-7
- Specific Gravity is 3.3-3.5
- Streak white to gray
- Associated Minerals calcite, biotite, hornblende, actinolite, andradite garnet and other metamorphic minerals.
- Other Characteristics: striated to the depth of grooves in some crystals.
- Notable Occurances Untersulzbachtal, Austria; Italy; Baja, Mexico and many localities in the USA.
- Best Field Indicators only one direction of cleavage, crystal habit, color and hardness.