Epidote
Epidote is hardness 6 to 7, density 3.38 to 3.49, in striated prisms to 35 cm. It forms a series with clinozoisite, and the two density ranges meet exactly.
Epidote is a calcium aluminium iron sorosilicate, hardness 6 to 7 and measured density 3.38 to 3.49, forming prismatic crystals to 35 cm elongated and striated parallel to [010], with one perfect cleavage on {001}. It forms a continuous series with clinozoisite, whose density range ends at exactly 3.38 — so density alone cannot place a specimen in the middle of that series.
In short
- Crystals reach 35 cm, deeply striated along their length and terminated obliquely. The diagnostic colour is pistachio-green; the Handbook of Mineralogy also records yellow, greenish yellow and greenish black.
- Hardness 6 to 7 and density 3.38 to 3.49. One perfect cleavage on {001} and an imperfect one on {100}, which is what long prisms part along when a specimen is dropped.
- Epidote and clinozoisite are one series, and iron is the only difference. Clinozoisite is the aluminium end at density 3.21 to 3.38; epidote is the iron end at 3.38 to 3.49. The ranges meet at a single point and do not overlap.
- Zoisite has the same formula as clinozoisite and a different crystal system. Orthorhombic against monoclinic — the two are dimorphs. Three familiar names, two compositions, one series.
- Twinning on {100} is lamellar and common, which for a species this hard is unusual: 69 of the 108 Handbook sheets covering this site's species record twinning at all, and only 27 call it common or better.
| Property | Epidote | Clinozoisite | Zoisite | Does it separate them? |
|---|---|---|---|---|
| Formula | Ca2Al2(Fe3+,Al)(SiO4)(Si2O7)O(OH) | Ca2Al3(SiO4)(Si2O7)O(OH) | Ca2Al3(SiO4)(Si2O7)O(OH) | Zoisite and clinozoisite are identical |
| Crystal system | Monoclinic | Monoclinic | Orthorhombic | Yes — zoisite is the dimorph |
| Hardness | 6 to 7 | 6.5 | 6 to 7 | No |
| Density | 3.38 to 3.49 | 3.21 to 3.38 | 3.15 to 3.36 | Only at the ends of the range |
| Cleavage | {001} perfect | {001} perfect | {010} perfect | Yes, against zoisite |
| Colour | Pistachio-green | Colourless to pale | White, grey, pink, blue | Broadly — iron is what makes it green |
| First described | 1801, grandfathered | 2006 s.p., Austria | 1805, Austria | — |
Why the density of an epidote is really an iron assay
Epidote and clinozoisite are the same structure with a single substitution: ferric iron for aluminium at one site. Iron is the heavier atom, so the more iron a crystal carries, the denser and the greener it is. That makes density an approximate compositional measurement rather than just an identification aid.
The Handbook of Mineralogy gives clinozoisite a measured density of 3.21 to 3.38 and epidote 3.38 to 3.49. Those two ranges touch at exactly 3.38 and nowhere overlap. A specimen weighed at 3.45 is epidote; one at 3.25 is clinozoisite; one at 3.38 is precisely the point where the reference stops answering.
The practical rule follows from the arithmetic rather than from convention: density places a specimen at the ends of the series and abandons you in the middle, which is exactly where most collection material sits. Colour helps in the same direction and with the same weakness — deep pistachio-green means iron, pale or colourless means aluminium, and the middle is ambiguous in both tests at once.
If you want to run the weighing yourself, the method and its error limits are in our note on measuring specific gravity at home. For epidote the honest reading is that you need a hundredth of a gram of resolution to make the test say anything at all.
Three names, two compositions, one series
The nomenclature here trips up more old labels than the chemistry does, and it is worth stating plainly.
Clinozoisite and zoisite have the same formula. Both are Ca2Al3(SiO4)(Si2O7)O(OH). Clinozoisite is monoclinic, zoisite orthorhombic; the Handbook records them as dimorphs. Zoisite is the older name, published in 1805 from Austria and named for Siegmund Zois; clinozoisite carries a 2006 special-procedure entry on the IMA list, also Austria.
Epidote is the iron-bearing end of the series clinozoisite sits at. It was described in 1801 and is grandfathered on the IMA list. The list also carries epidote-(Sr), approved in 2006 from Japan, so epidote is a group name as well as a species name — the same pattern this site has now recorded for apophyllite, tetrahedrite, stilbite and lepidolite.
For a collector the consequence is modest and specific: an old label reading simply “epidote” from a pale, colourless or grey specimen is more likely to be clinozoisite, and that is a determination to a series rather than a mistake. The type-locality note covers what a name on a label can and cannot be taken to mean.
The localities that produce crystals rather than granular masses
Epidote is a common rock-forming mineral — it is characteristic of greenschist-facies metamorphism and of contact zones, and it forms when plagioclase alters. Almost all of it is granular and worthless as a specimen. The crystallised localities are a short list.
The Handbook names the Knappenwand in the Untersulzbachtal, Salzburg, Austria for exceptional crystals — the classic occurrence, an alpine cleft worked specifically for epidote. Then Bourg d'Oisans in Isère, France; Arendal in Norway; Traversella in Piedmont; Sulzer on Prince of Wales Island, Alaska; the Calumet mine in Colorado; Garnet Hill in California; San Quentin in Baja California; Naukluft Farm near Rehoboth in Namibia; and Tormiq in Pakistan, which supplies most of the fine modern material.
Habit is what distinguishes them. Knappenwand crystals are lustrous, deeply striated and often doubly terminated; Pakistani material tends to sharper, slimmer prisms, frequently on or with quartz and orthoclase. A striated pistachio-green prism with a lustrous oblique termination is epidote at a glance, and no other species in a British or worldwide collection looks quite like it.
We keep no catalogue. If you hold Knappenwand or Cornish epidote with its original label, the wanted list says what we are looking for and what documentation matters. Our other species notes cover the silicates epidote is most often found alongside.
How it breaks, and what that means in a cabinet
Hardness 6 to 7 is high enough that epidote will not be scratched by anything ordinary in a display cabinet. The risk is not abrasion, it is cleavage.
Epidote has a perfect cleavage on {001}, which runs across the prism rather than along it, and an imperfect one on {100}. Long striated prisms are therefore easy to snap into segments and hard to break lengthways. A 35 cm crystal is a fragile object despite its hardness, and the common damage is a clean transverse break near the base — often repaired, and repairable so neatly that it is easy to miss.
Check any long epidote prism across its length under raking light before you commit to it. A repaired transverse break shows as a discontinuity in the striations rather than as a glue line, because the striations are the one feature no repair can realign. Our note on spotting repairs covers the general case; the striation test is specific to species like this one and is more reliable here than anywhere else.