Marcasite
Marcasite is an attractive metallic mineral. The name Marcasite is derived from the Arabic word for Pyrite. Marcasite is a polymorph of Pyrite which means that it has the chemistry, FeS2, as Marcasite; but a different structure and therefore different symmetry and crystal shapes. Marcasite and Pyrite are often difficult to distinguish from one another due to their similar characteristics.
Marcasite was used by the ancient Greeks and was also polished in large slabs by the Incas of Central America. Marcasite was popular in jewelry in the eighteenth century and the Victorian era but is not very popular currently. It is quite brittle and easily broken. Much of the “Marcasite” in antique jewelry is actually Pyrite.
Marcasite is slightly unstable, and over a period of years of exposure to air, will oxidize, freeing sulfur which forms sulfuric acid. Over a period of decades, most Marcasite specimens will be disintegrated into dust. A sulfur smell released during this reaction is an easy characteristic distinguishing Marcasite from Pyrite.
Marcasite is available from many localities worldwide.
| Category: | Sulfide mineral |
| Formula: | FeS2 |
| Iron sulphide | |
| Crystallography: | Orthorhombic – Dipyramidal |
| Crystal Habit: | Crystals typically tabular on [010], also pyramidal, prismatic, and, rarely, capillary; curved faces common. Stalactitic, reniform, fine-granular massive; cockscomb and spearhead shapes due to twinning on [101]. |
| Twinning: | Common and repeated on [101]; less common on [011]. Intense twin lamellae development observed in polished section. |
| Cleavage: | [101] Distinct, [110] Indistinct (in traces) |
| Fracture: | Irregular/Uneven |
| Tenacity: | Brittle |
| Hardness (Mohs): | 6.0 – 6.5 |
| Density: | 4.887 (g/cm3) |
| Luminescence: | None |
| Radioactivity: | Not Radioactive |
| Other: | Magnetic after heating |
| Color: | Bronze, light Brass Yellow, Tin White |
| Transparency: | Opaque |
| Luster: | Metallic |
| Refractive Index: | None (opaque) |
| Birefringence: | None (opaque) |
| Dispersion: | None (opaque) |
| Pleochroism: | Strong; [100] creamy white; [010] light yellowish white; [001] white with rose-brown tint. |
| Anisotrophism: | Very Strong; yellow through light green to dark green |
| Health Warning: | Marcasite is unstable to metastable and decrepitates, altering to melanterite, which contains sulfuric acid. Always wash hands after handling. Avoid inhaling dust when handling or breaking. Never lick or ingest. |
