Clinozoisite
Clinozoisite is Ca2Al3(Si2O7)(SiO4)O(OH): monoclinic, hardness 6.5, density 3.21 to 3.38. Once ferric iron passes 0.50 per formula unit, the name is epidote.
Clinozoisite is the aluminium end of the epidote group, Ca2Al3(Si2O7)(SiO4)O(OH) on the September 2026 IMA list: monoclinic, hardness 6.5, density 3.21 to 3.38, one perfect {001} cleavage, optically positive. Under the 2006 IMA epidote-group rules it becomes epidote once ferric iron passes 0.50 per formula unit, 8.52% Fe2O3 on the ideal formula.
In short
- Same formula as zoisite, different crystal. The September 2026 IMA-CNMNC Master List gives clinozoisite Ca2Al3(Si2O7)(SiO4)O(OH), status A, 2006 s.p., Austria: the formula zoisite also carries. The Handbook of Mineralogy clinozoisite sheet says monoclinic, epidote group, dimorphous with zoisite. The two-row count and the weight overlap with zoisite are on the zoisite page and are not repeated here.
- The half-iron rule. The 2006 IMA epidote-group report (Armbruster et al., European Journal of Mineralogy 18, 551) names a clinozoisite-subgroup mineral by the dominant trivalent cation on the M3 site, and says ferric iron strongly prefers M3. So the name changes from clinozoisite to epidote when Fe3+ passes 0.50 per formula unit: 8.52% Fe2O3 on the ideal formula, against 16.52% in end-member epidote.
- The sheets’ own analyses, read by the rule. Kälviä, Finland, 0.04 Fe3+ and Camaderry Mountain, Ireland, 0.27, both on the clinozoisite sheet; Westfield, Massachusetts, 1.04 on the epidote sheet. Recomputed from the printed oxides on 12.5 oxygens: 0.035, 0.270 and 1.048. Two clinozoisites and one epidote, as each sheet files them.
- Documented failure: the sign gap. Clinozoisite was traditionally the optically positive member and epidote the negative one. The 2006 report puts the optical changeover at about 40% epidote component, about 0.40 Fe3+, while the name changes at 0.50. Between them, roughly 6.86 to 8.52% Fe2O3, a grain reads negative and is still clinozoisite. The report says optical criteria alone should not be used.
- Where. The sheet calls it widespread and names the Goslarwand near Prägraten, Tirol; Vernirovice, Czech Republic; Kälviä, Finland; Arendal, Norway; Camaderry Mountain, Co. Wicklow, Ireland; Amborompotsy, Madagascar; Sittampundi, India; the Belvidere Mountain quarries, Vermont; and Alamos, Sonora, among others.
| Composition | Source | Fe3+ pfu | Fe2O3 by weight | Name under the 2006 rule | Side of the optical changeover (about 0.40) |
|---|---|---|---|---|---|
| Ideal clinozoisite | IMA list | 0 | 0% | Clinozoisite | Positive side |
| Kälviä, Finland | Clinozoisite sheet, analysis 1 | 0.04 (0.035) | 0.61% printed | Clinozoisite | Positive side |
| Camaderry Mountain, Ireland | Clinozoisite sheet, analysis 2 | 0.27 (0.270) | 4.68% printed | Clinozoisite | Positive side |
| Optical changeover | 2006 report: about 40% epidote component | About 0.40 | 6.86% worked | Clinozoisite | Sign changes here |
| Older optical limit | Deer et al. 1986, quoted in the 2006 report: about 15 mol% Ca2Fe3Si3O12(OH) | About 0.45 | 7.69% worked | Clinozoisite | A second estimate of the same change |
| The half-iron line | 2006 report: dominant trivalent cation on M3 | 0.50 | 8.52% worked | Boundary | Negative side |
| Westfield, Massachusetts | Epidote sheet, analysis 1 | 1.04 (1.048) | 17.22% printed | Epidote | Negative side |
| Ideal epidote | IMA list | 1.00 | 16.52% worked | Epidote | Negative side |
What is clinozoisite? The row, the sheet and the name
The row. The September 2026 IMA-CNMNC Master List gives Ca2Al3(Si2O7)(SiO4)O(OH), status A, 2006 s.p., Austria. The country column records where a mineral was first described; it is not a type-locality field, and the sheet gives no type material.
On the sheet. Monoclinic, point group 2/m. Prismatic crystals, typically elongated and striated parallel to [010]; commonly coarse to fine granular; also fibrous. Lamellar twinning on {100}, uncommon. Cleavage perfect on {001}; fracture uneven; brittle. Hardness 6.5; density 3.21 to 3.38 measured, 3.364 calculated. Colourless, pale yellow, pink, red, grey or green; colourless in thin section. Biaxial (+), α = 1.670 to 1.718, β = 1.670 to 1.725, γ = 1.690 to 1.734, 2V 14° to 90°. Worked on standard atomic weights, the ideal formula is SiO2 39.67%, Al2O3 33.66%, CaO 24.68% and H2O 1.98%.
The name. The sheet derives it from the monoclinic crystallography and the relation to zoisite. The 2006 IMA epidote-group report says Weinschenk gave the name in 1896 to iron-poor epidote from Prägratten, Tyrol, Austria, and that Lacroix had probably described the mineral first, in 1889, as fouquéite. A script finds no fouquéite row on the September 2026 list.
When does clinozoisite become epidote? The half-iron rule
The half-iron rule: clinozoisite becomes epidote when ferric iron, not aluminium, is the dominant trivalent cation on the M3 site, which on the ideal formula is above 0.50 Fe3+ per formula unit, or 8.52% Fe2O3 by weight. The 2006 IMA epidote-group report sets the rule: in the clinozoisite subgroup the dominant trivalent cation on M3 determines the root name. It adds that structure refinements and spectroscopy consistently show a strong preference of Fe3+ for M3, which is why epidote is kept as a name at all; its Table 2 lists clinozoisite as Al on M3 and epidote as Fe3+ on M3.
The M3 ladder on the IMA list. A script finds 6 rows written as calcium-two, aluminium-two and one more M3 cation with the O(OH) tail: clinozoisite (Al), epidote (Fe3+), piemontite (Mn3+, A, 1962 s.p., Italy), mukhinite (V3+, A, 1968-035, Russia), heflikite (Sc, A, 2022-139, Poland), and zoisite (Al), which the 2006 report excludes from the group because it is orthorhombic. It is the principle Nickel (1992), Solid solutions in mineral nomenclature sets out for solid solutions: different names for different elements dominant in specified structural sites.
The rule applied to the sheets. The clinozoisite sheet prints Fe3+ 0.04 for its Kälviä, Finland analysis and 0.27 for Camaderry Mountain, Ireland; the epidote sheet prints 1.04 for Westfield, Massachusetts. Recalculated from the printed oxides to 12.5 anhydrous oxygens they come out at 0.035, 0.270 and 1.048. The epidote sheet says the two minerals form a series; the line between them falls at 0.50.
Why optics cannot settle clinozoisite against epidote
The sign gap: about 40% epidote component flips the optic sign, but the name does not change until 50%, so an optically negative grain can still be clinozoisite. The 2006 report says the traditional distinction was optical, clinozoisite positive and epidote negative, with the changeover at about 40% epidote component. Deer et al. (1986), quoted in the same report, put the positive range at up to about 15 mol% Ca2Fe3Si3O12(OH), which is 0.45 Fe per formula unit. Both optical limits sit below the 0.50 naming line.
What the sheets show. The clinozoisite sheet gives biaxial (+) with 2V 14° to 90°; the epidote sheet gives biaxial (−) with 2V 90° to 116°. The two ranges meet at 90°, where the sign flips. The report recommends that optical criteria alone not be used to distinguish epidote-group minerals, and that electron-microprobe data decide the name of a solid-solution member.
What that means on a label. A pale, optically positive epidote-group grain is very likely clinozoisite; a negative one is epidote or iron-rich clinozoisite, and only an analysis says which. Deer et al., as the report quotes them, add that around that composition the optical distinction is difficult and, without a chemical analysis, generally based on colour and pleochroism.
Clinozoisite, epidote or zoisite in the hand
Cleavage and crystal system. Clinozoisite and epidote are both monoclinic with a perfect {001} cleavage on their sheets; the epidote sheet adds an imperfect {100}. zoisite is orthorhombic with a perfect {010} cleavage. The method is on cleavage or fracture: how to tell.
Weight. Worked as 20 − 20/D, a 20.00 g piece reads 13.77 to 14.08 g in water as clinozoisite and 14.08 to 14.27 g as epidote: the windows touch at 14.08 g, a density of 3.38 on both sheets. A balance therefore separates only the ends of the series. The zoisite overlap is worked on the zoisite page, and the method is on how to measure specific gravity at home.
Colour and the separator. The epidote sheet prints pistachio-green to pale green with strong pleochroism; the clinozoisite sheet prints colourless, pale yellow, pink, red, grey or green, and no pleochroism. On a laboratory magnetic separator, Table 1 of USGS Open-File Report 99-529 pulled clinozoisite at 0.30 to 1.30 amp (best 0.50 to 1.00, depending on chemistry) and epidote at 0.30 to 1.00 (best 0.40 to 0.70): overlapping, so not a test. The same report says the iron-rich end members of epidotes are more magnetic than members with less iron. Tanzanite, a zoisite, is on is zoisite the same as tanzanite.
Where clinozoisite is found
The sheet’s Distribution field. A widespread mineral. Typical occurrences for good crystals: the Goslarwand, near Prägraten, Tirol, Austria; Vernirovice, Czech Republic; Kälviä, Finland; Arendal, Norway; Camaderry Mountain, Co. Wicklow, Ireland; Amborompotsy, Madagascar; Sittampundi, Madras, India. In the USA, the Belvidere Mountain quarries, Lowell, Vermont; northern Spade Spring Canyon, Los Angeles Co., California; the Nightingale district, Pershing Co., Nevada. In Mexico, the Juarez district, Baja California, and Alamos, Sonora. In Canada, Timmins, Ontario.
Setting. The Occurrence field: typically in low- to medium-grade regionally metamorphosed igneous and sedimentary rocks and in contact-metamorphosed calcium-rich sediments; also an alteration product of plagioclase feldspars (saussuritization). Associates: amphiboles, plagioclase, quartz. Whether epidote is related to the pyroxenes is on is epidote a pyroxene; the species pages are in the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.