Marcasite
FeS₂ · Orthorhombic · also called white iron pyrite
Marcasite has the same chemistry as pyrite but a different crystal structure, and it is the less stable of the two. Collectors know it mainly for its tendency to fall apart in storage. Note that 'marcasite jewellery' is almost always pyrite, not marcasite.
Properties
- Mohs hardness
- 6–6.5
- Streak
- green black, black
- Lustre
- metallic
- Colour
- Pale brass to tin-white when fresh, tarnishing quickly to a dull yellow-brown or greenish grey.
- Cleavage
- Poor cleavage.
- Fracture
- Uneven, brittle
- Specific gravity
- 4.8–4.9 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Not fluorescent
Tests that confirm marcasite
- Forms blades, spearheads, and 'cockscomb' aggregates rather than cubes.
- Tarnishes and crumbles far faster than pyrite — old specimens often self-destruct into powder and sulfuric acid.
- Paler than pyrite on a freshly broken surface.
What marcasite gets confused with
Each of these is settled by one test. Run that test first.
Crystal habit. Pyrite makes cubes and pyritohedra; marcasite makes blades and cockscombs.
Where it is found
Chalk, clay, and shale, often as nodules; also in low-temperature hydrothermal veins.
Is it worth anything?
Low value, and specimens degrade over time. Store in a dry, sealed container away from other minerals.