Epidote
Ca₂(Al,Fe)₃(SiO₄)₃(OH) · Monoclinic
Epidote forms when other minerals are altered by hot fluids, and its yellow-green colour is one of the more memorable in mineralogy. It is a common vein and skarn mineral and a favourite of collectors when it forms sprays of sharp crystals.
Properties
- Mohs hardness
- 6–7
- Streak
- white, gray
- Lustre
- vitreous
- Colour
- A very particular yellow-green often described as pistachio green. Once you have seen it, it is hard to mistake.
- Cleavage
- One perfect cleavage along the length of the crystal.
- Fracture
- Uneven
- Specific gravity
- 3.3–3.6 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Not fluorescent
Tests that confirm epidote
- Pistachio-green colour, deeply striated elongated crystals.
- Hardness 6–7; scratches glass.
- Commonly intergrown with quartz and pink feldspar in altered rock.
What epidote gets confused with
Each of these is settled by one test. Run that test first.
Olivine is a purer bottle green with no cleavage; epidote is yellower and has one good cleavage.
Tourmaline has no cleavage and a triangular cross-section.
Where it is found
Metamorphosed limestone, altered volcanic rock, and alpine veins.
Is it worth anything?
Low, except for well-formed crystal clusters from classic alpine localities.