Arsenopyrite vs Pyrite
Arsenopyrite and Pyrite are commonly confused, and 4 measured properties separate them — crystal system most clearly.
Arsenopyrite vs Pyrite
Arsenopyrite
FeAsS
Iron arsenic sulfide — the commonest arsenic mineral, and the one that makes gold ore refractory and gold mining an arsenic problem.
Pyrite
FeS2
Iron sulfide — fool's gold — a mineral more valuable for the sulfur it yields and the acid it produces than for the iron in it.
| Property | Arsenopyrite | Pyrite |
|---|---|---|
| Cleavage Only one of the two has a figure we can stand behind, so there is nothing to compare. | Not known | indistinct; fractures conchoidally |
| Crystal system Categorical values can be listed side by side but not ordered or subtracted. | monoclinic | cubic |
| Density | 5.9–6.2 g/cm³ higher | 4.95–5.1 g/cm³ |
| Lustre Categorical values can be listed side by side but not ordered or subtracted. | metallic | metallic |
| Mohs hardness Mohs hardness is an ordinal scale: values can be ranked, but differences and ratios between them are not meaningful. | 5.5–6 | 6–6.5 higher |
| Streak Categorical values can be listed side by side but not ordered or subtracted. | dark grey to black | greenish-black to brownish-black |
Why these get confused
both brassy metallic sulfides of iron; arsenopyrite is paler and harder, and smells of garlic when struck
The confusion is a real and documented one, not a resemblance we have invented.