Mohs hardness: 6-7
Specific gravity: 3.3-3.5
Streak- vitreous to resinous
Epidote is a name that is used in two different ways in mineralogy: 1) the “Epidote Group” is the name of a group of silicate minerals that share common structural and compositional characteristics; and, 2) “Epidote” is the name of the most common mineral in the Epidote Group.
Epidote- the mineral
Epidote is a silicate mineral that is commonly found in regionally metamorphosed rocks of low-to-moderate grade. In these rocks, epidote is often associated with amphiboles, feldspars, quartz, and chlorite. It occurs as replacements of mineral grains that have been altered by metamorphism. Frequently found in veins that cut granite. It occurs in monoclinic crystals in pegmatites. Also, it is found in massive form and as monoclinic crystals in marbles and schists.
Epidote usually ranges between yellowish green to pistachio green in color. Less often it is brownish green to black. In massive form it is usually translucent with a vitreous luster. Well-formed crystals from marble and pegmatite are often transparent.
Epidote has a chemical composition of Ca2(Al2,Fe)(SiO4)(Si2O7)O(OH). It is an end member of a solid solution series with clinozoisite. In that series, the iron of epidote is gradually replaced by aluminum to the end member clinozoisite composition of Ca2Al3(SiO4)(Si2O7)O(OH). Clinozoisite is usually lighter in color than epidote because iron is what produces epidote’s greenish to brownish color.
Epidote- The mineral group-
Members of the epidote mineral group have a crystal structure that consists of isolated and paired silica tetrahedrons. They share a generalized chemical composition of A2M3(Si2O7)(SiO4)O(OH). “A” is a pairing of calcium, manganese, strontium, lead, or sometimes a rare earth element. “M” is usually aluminum pairing with iron, magnesium, manganese, or vanadium. Some of the member minerals of the epidote group are listed in the table with their chemical compositions.
Epidote in Rocks-
Epidote is a rock forming mineral. Many regionally-metamorphosed rocks contain a small amount of epidote. Two types of rocks contain significant amounts of epidote are epidosite and unakite. The epidosite is composed of small amounts of quartz. It forms when basalts in sheeted dikes and ophiolites are transformed by hydrothermal activity or metasomatism. (Metasomatism is the process by which the chemical composition of a rock is changed through the introduction or extraction of chemicals dissolved in fluids that migrate through the rock’s pores.)
Unakite is a rock that forms from the metamorphism of granite. Less-resistant minerals in the granite are altered to epidote or replaced by epidote, with the orthoclase and quartz remaining. It is an interesting pink and green colored rock that was first discovered in the Unakas Mountains of North Carolina, from which its name was derived. Unakite is a favored stone to the lapidary arts in that is often cut into cabochons and beads of all shapes and sizes.
Epidote- Rockbridge County, Virginia
Epidote- Skardu District, Baltistan, Northern Areas, Pakistan