Epidote: Pistachio-Green Prisms and the Rocks They Color

Short answer: epidote is a calcium aluminum iron silicate, Ca2(Al,Fe)3(SiO4)3(OH), that grows as glassy, lengthwise-striated prisms in shades from pistachio and olive green to near black. It rates 6 to 7 on the Mohs scale with a specific gravity of about 3.3 to 3.6, so it feels heavy for a green silicate. It forms where rocks are altered by heat and hot water, and the same mineral supplies the green in unakite.

What gives epidote its color and weight?

Iron. Epidote's structure has metal sites that can hold either aluminum or ferric iron, and the more iron it takes in, the darker, greener and denser it becomes. Geology.com describes epidote as one end of a solid solution series with clinozoisite, in which iron is gradually replaced by aluminum, and notes that clinozoisite is usually lighter in color because iron is what produces epidote's greenish to brownish color. Webmineral's epidote data records the old synonym "pistacite", for the pistachio shade that gave many collectors their first name for the mineral.

The name has nothing to do with color. Webmineral traces it to the Greek epidosis, "addition", and dates the species to 1801.

How does epidote form?

Epidote is a mineral of change. Geology.com describes it as common in regionally metamorphosed rocks of low to moderate grade, often with amphiboles, feldspars, quartz and chlorite, and frequently as replacements of grains altered during metamorphism. It also grows in veins cutting granite, as crystals in pegmatites, and in marbles and schists altered by contact metamorphism. Minerals.net adds hornfels and skarns at intrusive contacts, and occasional occurrences in basalt and diabase.

A useful way to picture it: take a rock with calcium-bearing feldspar and iron-bearing dark minerals, heat it moderately and push water through it. The feldspar and the dark minerals break down, and epidote grows from their calcium, aluminum and iron. Two rocks show this process at large scale:

• Unakite. Geology.com's unakite article explains that unakite forms when granite is altered by hydrothermal fluids and the plagioclase in the granite is transformed into epidote, leaving pink orthoclase and quartz behind. The rock is named after the Unaka mountain range of western North Carolina and eastern Tennessee. More in the unakite guide.
• Epidosite. A rock made mainly of epidote with a little quartz, which forms when basalts in sheeted dikes and ophiolites are transformed by hydrothermal activity.

Free-standing crystals need open space: Alpine fissures, skarn pockets and cavities where the mineral could grow into fluid. That is where the specimen-grade prisms come from. For the broader picture of minerals that grow under heat and pressure, see metamorphic minerals.

Where do the best epidote crystals come from?

The table follows the localities Minerals.net lists. It ranks Knappenwand as perhaps the world's most classic epidote locality and singles out fine crystals from Austria, Alaska and Pakistan as mineral classics that are well represented in major collections. Malian epidote is better known to most buyers as the dark needles inside prehnite.

How do you recognize epidote?

Start with the crystal. Minerals.net describes long, slender prismatic crystals, sometimes thick and tabular, often striated and sometimes with wedge-shaped terminations, plus radiating, fan-shaped and wheat-sheaf groups. Then check four things:

1. Striations along the length. Fine parallel grooves running the long way are typical.
2. Color shift. Minerals.net notes that transparent epidote can be strongly pleochroic, green from one angle and brown from another. Turn a thin crystal against a light and watch it change.
3. Weight. At 3.3 to 3.6, an epidote cluster feels noticeably heavier than quartz or calcite of the same size.
4. One good cleavage. Broken crystals show one bright, flat cleavage face rather than two.

Epidote vs its look-alikes

The clinozoisite problem is real. Minerals.net explains that the two form a series, that some specimens are labeled clinozoisite-epidote because they sit in between, and that a single crystal can even contain both minerals in different parts. Without analysis, the name on a pale, brownish green crystal is a best guess.

What is the epidote group?

Epidote also names a family. Geology.com describes the epidote group as minerals sharing a structure of isolated and paired silica tetrahedra, with a generalized formula A2M3(Si2O7)(SiO4)O(OH). Swap the metals and the color changes completely:

• Piemontite, manganese-bearing and red to purplish, named for Piedmont, Italy, according to Webmineral's piemontite data. Geology.com shows tumbled epidote and piemontite sold as "Dragon Blood" or "Dragon Stone".
• Clinozoisite, the iron-poor end member of the series with epidote.
• Zoisite, a close relative with the same formula as clinozoisite in a different crystal system. Minerals.net on zoisite covers its varieties: blue tanzanite, discovered in Tanzania in 1967; pink, manganese-rich thulite, first found in Norway; and the green massive zoisite with red ruby from Longido, near Mount Kilimanjaro, sold as ruby zoisite.
• Allanite, a rare-earth-bearing member.

So a buyer who owns tanzanite, thulite and unakite already owns three relatives of epidote.

Is epidote used as a gemstone?

Rarely, and mostly for collectors. Geology.com notes that epidote has no significant industrial use and only minor use as a gem: high-quality transparent crystals are sometimes faceted, but the colors have never attracted much interest in the commercial jewelry market, and most faceted stones go to gem and mineral collectors. In jewelry you are far more likely to meet epidote inside unakite cabochons and beads.

Care is simple. At 6 to 7 the surface resists everyday scratching, but the perfect cleavage means a sharp knock can split a crystal, and thin prisms snap off a cluster easily. Dust clusters with a soft brush or blower; rinse in plain water if needed and let it dry fully.

Epidote in crystal traditions

In crystal traditions, epidote is associated with growth and with amplifying intentions, often in combination with other stones. These are cultural associations, not mineral properties.

Shopping for epidote and its relatives

For a crystal specimen, the questions are luster, termination and damage. Look for bright, glassy faces rather than a dull or etched skin, at least some terminated tips, and clean prism edges. Inspect where the crystals meet the rock: a natural contact looks irregular and grainy, and a stack of loose prisms sitting oddly flat on matrix deserves a closer look. Darker color usually means more iron, not lower quality; transparent, lighter green crystals with visible color shift are the prized exception.

• Epidote Crystal Clusters on Matrix: a 5 by 2.5 by 2 inch, 273 g cluster. A size large enough to show the striated prisms and their attachment to the rock without a loupe.
• Unakite: tumbles, a 40 to 41 mm sphere and a small tower. The green areas are epidote, so this is the inexpensive way to own the mineral in polished form.
• Ruby Zoisite: tumbles and towers of green zoisite with red ruby, an epidote-group relative with a very different look.
• Thulite: pink manganese zoisite as a 2.5 inch palm stone, tumbles or a 35 mm sphere, showing how manganese changes the color of the group.

Frequently asked questions

What is epidote made of?

Epidote is a calcium aluminum iron silicate with hydroxyl, Ca2(Al,Fe)3(SiO4)3(OH). Iron gives it the green to brownish green color; when iron is replaced by aluminum, the mineral grades into paler clinozoisite.

How hard is epidote?

Epidote rates 6 to 7 on the Mohs scale, close to quartz, so everyday dust will not scratch it easily. It has one perfect cleavage, though, so a hard knock can split a crystal, and slender prisms break off clusters if handled roughly.

Is epidote the green in unakite?

Yes. Unakite is altered granite in which the plagioclase feldspar has been replaced by epidote. The green areas are epidote, the pink areas are orthoclase feldspar, and the clear to milky grains are quartz.

How can you tell epidote from tourmaline?

Look at the cross section and cleavage. Tourmaline prisms have a rounded triangular cross section and no good cleavage. Epidote prisms are more blade-like, show one perfect cleavage, and are often strongly pleochroic, shifting between green and brown as the crystal turns.

Can epidote go in water?

Yes. Epidote is insoluble and hard enough for a rinse in plain water. Dry clusters thoroughly so water does not sit in cracks or under the matrix, and avoid scrubbing, which can snap thin prisms.

Sources

• Geology.com, Epidote: https://geology.com/minerals/epidote.shtml
• Webmineral, Epidote Mineral Data: https://www.webmineral.com/data/Epidote.shtml
• Minerals.net, The mineral epidote: https://www.minerals.net/mineral/epidote.aspx
• Geology.com, Unakite: https://geology.com/gemstones/unakite/
• Minerals.net, Zoisite: https://www.minerals.net/mineral/zoisite.aspx
• Webmineral, Piemontite Mineral Data: https://www.webmineral.com/data/Piemontite.shtml