Is epidote a pyroxene?
No. Epidote crystals are built from paired and single SiO4 tetrahedra; pyroxene species are single-chain silicates. Epidote has one perfect cleavage, not two.
No. Epidote, Ca2(Al2Fe3+)(Si2O7)(SiO4)O(OH), hardness 6 to 7, belongs to the epidote group, built from paired and single SiO4 tetrahedra (2006 IMA report); pyroxenes are single-chain silicates, M2M1T2O6 (1988 IMA report). In the hand, epidote has one perfect cleavage and is optically negative; diopside and augite have two cleavages at about 87° and are positive.
In short
- Two groups, two IMA reports. The 2006 IMA epidote-group report (Armbruster et al., European Journal of Mineralogy 18, 551) defines the epidote group as monoclinic minerals with the general formula A2M3[T2O7][TO4](O,F)(OH,O), built of Si2O7 and SiO4 units. The 1988 IMA pyroxene report (Morimoto et al., Mineralogical Magazine 52, 535) says pyroxenes contain single SiO3 chains of linked SiO4 tetrahedra, general formula M2M1T2O6. Epidote is in the first; diopside and augite are in the second.
- The tetrahedra test. Read the silicon in the September 2026 IMA-CNMNC Master List formula: (Si2O7)(SiO4) points to the epidote group, Si2O6 to a pyroxene. By script, 60 rows contain (Si2O7)(SiO4) and 31 end in Si2O6; no row has both. Worked, epidote’s silicate units carry 3.67 oxygens per silicon, a pyroxene’s 3.00.
- Where the formula test fails. It is a filter, not proof. Zoisite carries clinozoisite’s exact formula and the 2006 report excludes it from the group; pumpellyite-(Al) has the same silicate units with water added. Esseneite, CaFe3+AlSiO6, is an accepted pyroxene in the 1988 report without Si2O6; wollastonite’s Ca2Si2O6 composition is a pyroxenoid, the report says.
- In the hand: cleavage and optic sign. The epidote sheet gives one perfect cleavage, {001}, and an imperfect {100}; the diopside and augite sheets give two {110} cleavages meeting at about 87°. Epidote is biaxial (−); both pyroxenes are biaxial (+). Hardness overlaps: 6 to 7 against 5.5 to 6.5 and 5.5 to 6.
- Documented failure: the balance. Worked as 20 − 20/D, a 20.00 g epidote reads 14.08 to 14.27 g in water, wholly inside augite’s 13.73 to 14.38 g. Diopside, 13.79 to 14.08 g, meets epidote at exactly 14.08 g, a density of 3.38 on both sheets. Weight cannot answer the question.
| Criterion | Epidote | Diopside | Augite | Does it choose? |
|---|---|---|---|---|
| IMA formula | Ca2(Al2Fe3+)(Si2O7)(SiO4)O(OH) | CaMgSi2O6 | (Ca,Mg,Fe)2Si2O6 | Yes, on paper |
| Group (report) | Epidote group: A2M3[T2O7][TO4](O,F)(OH,O) | Pyroxene: M2M1T2O6 | Pyroxene: M2M1T2O6 | Yes |
| Silicate units | Si2O7 pairs and single SiO4 | Single SiO3 chains | Single SiO3 chains | Yes: the tetrahedra test |
| Oxygens per silicon, worked | 3.67 | 3.00 | 3.00 | Yes, on paper |
| Crystal system | Monoclinic | Monoclinic | Monoclinic | No |
| Habit | Prismatic, to 35 cm, elongated and striated ∥ [010] | Prismatic, nearly square cross sections, to 50 cm | Stubby prismatic, square or octagonal in section, to 10 cm | Partly |
| Cleavage | Perfect {001}; imperfect {100} | Distinct {110}, about 87° | Good {110}, about 87° | Yes, on a broken edge |
| Optic sign | Biaxial (−), 2V 90°–116° | Biaxial (+), 2V 59° | Biaxial (+), 2V 25°–61° | Yes, with a microscope |
| Pleochroism | Strong: X colourless, pale yellow, pale green; Y greenish yellow; Z yellowish green | Not printed | Printed: pale greens, browns and violet; absorption weak | Partly |
| Hardness | 6–7 | 5.5–6.5 | 5.5–6 | No: overlaps |
| Density, measured | 3.38–3.49 | 3.22–3.38 | 3.19–3.56 | No |
| 20.00 g piece in water | 14.08–14.27 g | 13.79–14.08 g | 13.73–14.38 g | No: inside augite |
| Colour | Pistachio-green to pale green, yellow, greenish black | Colourless, white, yellow, pale to dark green, black | Black, brown, greenish, violet-brown | No |
| Streak | Not printed | White, grey, grey-green | Grey-green | No |
| Separator best range (USGS) | 0.40–0.70 amp | 0.40–0.60 amp | 0.40–0.90 amp, depends on chemistry | No: overlaps |
Is epidote a pyroxene? Two families of silicate
No. Epidote is the mineral the epidote group is named after; pyroxenes are a separate group defined by single chains of silicon tetrahedra. The September 2026 IMA-CNMNC Master List gives epidote Ca2(Al2Fe3+)(Si2O7)(SiO4)O(OH), status G, 1801, and the Handbook sheet files it in the epidote group. The 2006 report says the structure is composed of Si2O7 and SiO4 units linked to chains of edge-sharing octahedra, and that structural studies showed epidote to contain both single SiO4 tetrahedra and double-tetrahedral Si2O7 groups.
The pyroxene side. The 1988 IMA pyroxene report says pyroxenes are silicates that, in their simplest form, contain single SiO3 chains of linked SiO4 tetrahedra, with a repeat of two tetrahedra along the chain, about 0.52 nm, and a general formula M2M1T2O6. It accepts 20 mineral names. Diopside, CaMgSi2O6, and augite, (Ca,Mg,Fe)2Si2O6, both A, 1988 s.p. on the IMA list, are among them, and both sheets say pyroxene group.
Why the question comes up. The epidote sheet names pyroxenes and omphacite, a pyroxene, among epidote’s associates in metamorphic rocks, so the two occur in the same rocks. The chain trap: the 2006 report also describes chains in epidote, but they are chains of octahedra, parallel to the b axis, not chains of tetrahedra.
How to read the formula: the tetrahedra test
The tetrahedra test: in an IMA formula, (Si2O7)(SiO4) points to the epidote group and Si2O6 to a pyroxene, because one is paired and single tetrahedra and the other a chain. A script over the September 2026 list, checked by a line scan, finds 60 rows containing (Si2O7)(SiO4), 41 of them with the epidote-group anion tail O(OH), F(OH) or O2, and 31 rows ending in Si2O6. No row carries both.
Worked. In a pyroxene, Si2O6 gives 3.00 oxygens per silicon. In epidote, Si2O7 plus SiO4 is Si3O11, 3.67 oxygens per silicon; the Si2O7 pair alone is 3.50 and the single SiO4 is 4.00.
Where the test fails. The tail is necessary on paper, not sufficient. zoisite carries clinozoisite’s formula exactly, and the 2006 report leaves it out of the epidote group because it is orthorhombic. Pumpellyite-(Al), Ca2Al3(Si2O7)(SiO4)(OH,O)2(H2O), has clinozoisite’s silicate units but water and an anion tail outside the report’s general formula. On the pyroxene side, the 1988 report accepts esseneite, CaFe3+AlSiO6, which has no Si2O6, and says Ca2Si2O6 exists in nature as wollastonite, a pyroxenoid. One formula twin among the pyroxenes is worked on is spodumene a pyroxene, and is not repeated here.
Epidote or diopside in the hand: cleavage and optic sign
Cleavage decides on a broken edge. The epidote sheet gives one perfect cleavage, on {001}, and an imperfect one on {100}. The diopside sheet gives a distinct cleavage on {110} with (110) to (1-10) at about 87°, and the augite sheet a good one at the same angle. On these three sheets, two cleavages at nearly a right angle mean pyroxene, and one perfect cleavage means epidote. The method is on cleavage or fracture: how to tell.
Optic sign decides under a microscope. Epidote is biaxial (−) with 2V 90° to 116°; diopside biaxial (+) with 2V 59°; augite biaxial (+) with 2V 25° to 61°. Pale, iron-poor members of epidote’s own series are positive too: that limit is on clinozoisite.
Hardness and colour do not. Epidote is 6 to 7, diopside 5.5 to 6.5 and augite 5.5 to 6, overlapping. Epidote’s pistachio-green sits beside diopside’s pale to dark green. The method is on how to test mineral hardness properly, and the diopside numbers agree with the diopside species page.
When the tests fail: the balance and the separator
Documented failure: a balance cannot tell epidote from a pyroxene. Worked from the sheets’ measured densities, a 20.00 g piece reads 14.08 to 14.27 g in water as epidote, 13.79 to 14.08 g as diopside and 13.73 to 14.38 g as augite. Epidote’s window lies wholly inside augite’s, and meets diopside’s at 14.08 g, where both sheets print a density of 3.38. Hedenbergite, the iron calcium pyroxene, reads 14.38 g at its single measured density of 3.56. The method is on how to measure specific gravity at home.
The magnet does not either. Table 1 of USGS Open-File Report 99-529 gives best extraction ranges of 0.40 to 0.70 amp for epidote across 10 samples, 0.40 to 0.60 for diopside across 4, and 0.40 to 0.90 for augite across 4, depending on chemistry. All three overlap. The report says iron-rich members of the pyroxene and epidote families are more magnetic than members with less iron.
Which name fits? Who each label is wrong for
Call it epidote when the specimen is green to greenish black, prismatic and striated, with one perfect cleavage and negative optics; it belongs with clinozoisite and piemontite in the epidote group, not with the pyroxenes; zoisite, its orthorhombic relative, is outside both. Call it a pyroxene, and then name the species, when it has two cleavages at about 87° and positive optics. The species within the pyroxenes are named by the 1988 report’s rules; the jadeite page and is jade a mineral show one case.
Neither name fits a green prismatic crystal with only poor cleavage: vesuvianite, for instance, is tetragonal or monoclinic on its sheet, with {110} poor. Amphiboles such as actinolite cleave at 56° and 124°. Other identification guides are in the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.