Arsenopyrite
Arsenopyrite is FeAsS: silver-white to steel-grey, hardness 5.5 to 6, density 6.07. The 17-gram line separates it from löllingite; colour from pyrite.
Arsenopyrite is iron arsenic sulphide, FeAsS: monoclinic but pseudo-orthorhombic, hardness 5.5 to 6, measured density 6.07, silver-white to steel-grey with a metallic lustre and striated, often twinned crystals to 30 cm. The Handbook calls it the most abundant and widespread arsenic mineral. A 20.00 g piece weighs 16.71 g in water.
In short
- FeAsS, on both lists. The September 2026 IMA-CNMNC Master List gives FeAsS, status A, 1962 s.p., country unknown; the Handbook of Mineralogy sheet prints the same formula and an ideal 34.30% iron, 46.01% arsenic and 19.69% sulphur, which our arithmetic reproduces. The name is a contraction of arsenical pyrites.
- Arsenopyrite vs pyrite is a colour question. Pyrite's sheet gives pale brass-yellow, arsenopyrite's silver-white to steel-grey. The scale agrees: a 20.00 g piece reads 16.01 g in water as pyrite and 16.71 g as arsenopyrite, 0.69 g apart.
- The 17-gram line, for silver-white grains. Arsenopyrite reads 16.62 to 16.79 g across the sheet's stated density uncertainty; löllingite, FeAs2, reads 17.30 g. Above 17.00 g the piece is not arsenopyrite. Cobaltite, at 16.84 g, cannot be separated this way; its perfect cleavage can.
- Is arsenopyrite magnetic? The sheet records nothing. USGS Open-File Report 99-529 pulled its 3 samples only from 1.00 amp up, with the best range above 1.70 amp, the separator’s maximum, where the report’s definitions place diamagnetic minerals.
- Gold-quartz and tin veins. The sheet's Occurrence field puts it in pegmatites, high-temperature gold-quartz and tin veins and contact metamorphic sulphide deposits, with gold, cassiterite and scheelite among its associates. In England it names a number of mines in Cornwall and Tavistock in Devon.
| Species | Formula | Crystal system | Hardness | Density (measured) | 20.00 g piece in water | Colour on the sheet | Streak on the sheet | Cleavage on the sheet | Side of the 17.00 g line |
|---|---|---|---|---|---|---|---|---|---|
| Arsenopyrite | FeAsS | Monoclinic, pseudo-orthorhombic | 5.5 to 6 | 6.07(15) | 16.71 g (16.62 to 16.79 g) | Silver-white to steel-grey | Not printed | Distinct on {101}; {010} in traces | Below |
| Löllingite | FeAs2 | Orthorhombic | 5 to 5.5 | 7.40 (synthetic) | 17.30 g | Steel-grey to silver-white | Greyish black | Rarely distinct on {010}, {101} | Above |
| Cobaltite | CoAsS | Orthorhombic, pseudocubic | 5.5 | 6.33 | 16.84 g | Silver-white | Greyish black | Perfect on {001} | Below |
| Pyrite | FeS2 | Cubic | 6 to 6.5 | 5.018 | 16.01 g | Pale brass-yellow, tarnishes darker and iridescent | Greenish black to brownish black | Indistinct on {001} | Below |
| Marcasite | FeS2 | Orthorhombic | 6 to 6.5 | 4.887 | 15.91 g | Tin-white on fresh surface, pale bronze-yellow, darkening on exposure | Greyish or brownish black | {101} rather distinct | Below |
What is arsenopyrite? An iron arsenic sulphide
The names. The September 2026 IMA-CNMNC Master List lists arsenopyrite as FeAsS, status A, 1962 s.p., with the country of first description unknown. The Handbook of Mineralogy arsenopyrite sheet gives the same formula and files it in the arsenopyrite group. Its Distribution field opens with the sheet’s own superlative: the most abundant and widespread arsenic mineral.
What it looks like. Crystals to 30 cm, flat tabular to blocky to prismatic, striated parallel to [001]; also compact, granular and columnar. Twinning is common on {100} and {001}, as contact or penetration twins on {101}, and on {012} it makes star-shaped trillings or cruciform twins. Colour is silver-white to steel-grey, lustre metallic, cleavage distinct on {101}. The sheet prints no streak, and we add none. Twin laws in general are on mineral twinning explained.
Where it forms. The Occurrence field: of hydrothermal origin, typically one of the earliest minerals to form; in pegmatites, high-temperature gold-quartz and tin veins and contact metamorphic sulphide deposits; less commonly low-temperature; also in gneisses and schists. Associates: pyrrhotite, pyrite, chalcopyrite, galena, gold, scheelite, cassiterite. The arsenic arithmetic and what it means for a collection are on is arsenopyrite dangerous and toxic minerals in a collection; this page gives no handling advice and nothing here is health advice.
Arsenopyrite or löllingite? The 17-gram line
Why it is needed. Löllingite, FeAs2, is steel-grey to silver-white and metallic, hardness 5 to 5.5 against arsenopyrite’s 5.5 to 6. Colour and hardness overlap. Density does not: 7.40 (measured on synthetic material) against 6.07.
The rule. Weigh a clean 20.00 g piece in air, then suspended in water. Arsenopyrite reads 16.71 g; taking the sheet’s stated uncertainty, 6.07 plus or minus 0.15, it reads 16.62 to 16.79 g. Löllingite reads 17.30 g. The line sits at 17.00 g, a specific gravity of about 6.67: 0.21 g above arsenopyrite’s heaviest reading and 0.30 g below löllingite. Above it, the piece is not arsenopyrite. The method, with its error sources, is on how to measure specific gravity at home.
Documented failure: cobaltite. Cobaltite, CoAsS, silver-white, hardness 5.5, reads 16.84 g, less than 0.14 g from arsenopyrite and inside the scatter of a home balance. The scale cannot separate them. The sheets can: cobaltite’s cleavage is perfect on {001} and its crystals are pseudocubic or pseudopyritohedral, striated as with pyrite; arsenopyrite’s cleavage is only distinct and its crystals tabular to prismatic. Cobaltite has its own page, cobaltite. A crust on matrix weighs the matrix, and the line then says nothing.
Arsenopyrite vs pyrite and marcasite
Colour first. All three are hard, brittle and metallic, and hardness barely helps: 5.5 to 6 for arsenopyrite, 6 to 6.5 for both iron disulphides. The sheets separate them by colour. Pyrite is pale brass-yellow; marcasite is tin-white on a fresh surface but pale bronze-yellow and darkening on exposure; arsenopyrite is silver-white to steel-grey. A tarnished piece needs a fresh surface.
The scale backs the colour. A 20.00 g piece reads 16.71 g in water as arsenopyrite, 16.01 g as pyrite and 15.91 g as marcasite: 0.69 g and 0.80 g lighter. Habit helps too: pyrite cubic and pyritohedral, marcasite tabular with cockscomb and spearhead twins, arsenopyrite striated prisms and cruciform twins. The two iron disulphides are compared on is marcasite the same as pyrite, and the brassy look-alikes on what is fool’s gold.
On a laboratory separator. USGS Open-File Report 99-529, Table 1 gives arsenopyrite a total range of 1.00 to above 1.70 amp and a best range above 1.70, over 3 samples; löllingite and cobaltite were not pulled at 1.70. The report reads ranges that start at 0.50 or 1.00 amp and run past 1.70 as probably involving some contamination. Neither source tests a hand magnet; the wider question is on which minerals are magnetic.
Where arsenopyrite is found: Cornwall, Devon, Panasqueira and Freiberg
The Distribution field says only a few localities for large and fine crystals can be mentioned, and names: Altenberg, Ehrenfriedersdorf and Freiberg, Saxony, Germany; the Stari Trg mine, Trepča, Serbia; Panasqueira, Portugal; Sala, Tunaberg, Stollberg, Boliden and Nordmark, Sweden; Stratonik, Greece; Franconia, New Hampshire, and Franklin, New Jersey (Franklin and Sterling Hill); Hidalgo del Parral and Santa Eulalia, Chihuahua, Mexico; the Cobalt district, Ontario; and, for large crystals, the Obira mine, Oita Prefecture, and the Ashio mine, Tochigi Prefecture, Japan.
In England the sheet says a number of mines in Cornwall, without naming one, and Tavistock in Devonshire. We name no Cornish mine for it. Its two analyses come from the O’Brien mine, Cobalt, Canada, and Franklin, New Jersey; the Franklin one carries 1.16% cobalt.
The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.