Epidote
Epidote is a calcium aluminum iron sorosilicate with the approximate formula Ca2Al2(Fe3+,Al)(SiO4)(Si2O7)O(OH), instantly recognizable to many collectors by its distinctive pistachio-green color. It crystallizes in the monoclinic system, typically as elongated, striated prismatic crystals. The color and composition vary with the ratio of iron to aluminum, grading toward the related mineral clinozoisite as iron decreases.
Epidote properties at a glance
| Type | Minerals |
|---|---|
| Chemical formula | Ca2Al2(Fe3+,Al)Si3O12(OH) |
| Mohs hardness | 6–7 |
| Crystal system | Monoclinic |
| Luster | Vitreous |
| Streak | White to grayish |
| Cleavage | Perfect, one direction (basal) |
| Fracture | Uneven to conchoidal |
| Specific gravity | 3.3–3.5 |
| Transparency | Transparent to opaque |
| Common colors | Pistachio green, Yellowish green, Dark green, Black |
| Rarity | common |
Geology & background
Epidote forms through hydrothermal alteration and metamorphism of calcium-rich rocks, especially basalt, gabbro, and diorite, as well as impure limestone, and it is a hallmark mineral of greenschist- to amphibolite-facies metamorphic rocks. It commonly fills veins and fractures in altered volcanic terrain and forms fine crystals in Alpine-type fissures in the Alps, as well as in Baja California, Norway, and Pakistan. The rock unakite, a pink-and-green granite in which epidote has partly replaced the original feldspar, is a well-known and widely collected epidote-bearing rock. Field identifiers look first for epidote's saturated yellow-green to olive-green color, often described as pistachio, along with its glassy luster and elongated, striated crystal faces. It is pleochroic, showing different shades of green, yellow, and brown depending on viewing direction, a trait that can be checked with a simple loupe under strong light.
How to identify Epidote
Epidote typically appears pistachio to olive green, occasionally brown or black, with a vitreous luster and one perfect cleavage direction. Crystals are elongated and prismatic with fine striations running parallel to their length. Hardness is 6 to 7, streak is colorless to gray, and pleochroism is often visible as a shift between green, yellow, and brown. It can resemble actinolite or tourmaline, but its distinctive green tone and single good cleavage, rather than the amphibole's two, usually settle the identification.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Epidote is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Epidote worth?
| Entry-level specimen | $2 |
|---|---|
| Typical collector specimen | $20 |
| Premium specimen | $150+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Collector specimens
- Education
- Field identification
- Industrial or lapidary use when suitable
Interesting facts
- Epidote's saturated yellow-green color is so recognizable that geologists nicknamed it 'pistachio green' long before formal color charts existed.
- The popular ornamental rock unakite gets its pink-and-green look from epidote that has partially replaced feldspar in an altered granite.
- Epidote and the mineral clinozoisite form a continuous series, with the exact color and name shifting as iron content changes.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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