Proustite
Proustite is Ag3AsS3: scarlet-vermilion, vermilion streak, hardness 2 to 2.5, density 5.57; it darkens in light. How to tell it from pyrargyrite and cinnabar.
Proustite is silver arsenic sulphide, Ag3AsS3: hexagonal, scarlet-vermilion with a vermilion streak and adamantine lustre, hardness 2 to 2.5, measured density 5.57, and its Handbook sheet says it darkens with exposure to light. It is 65.41% silver by mass. Pyrargyrite is its antimony counterpart; cinnabar, also vermilion-streaked, is far denser.
In short
- Named for a chemist. The September 2026 IMA-CNMNC Master List gives Ag3AsS3, status G, 1832, country unknown. The Handbook of Mineralogy sheet prints the same formula, names it for Joseph Louis Proust (1754 to 1826), and makes it dimorphous with xanthoconite, which the IMA list also gives as Ag3AsS3.
- It darkens in light, on the sheet. Its optical field opens translucent, darkens with exposure to light. Of the seven red minerals in the table, a script finds that wording on 2 sheets, proustite and pyrargyrite; realgar’s says it disintegrates on long exposure. The sheet does not say whether the darkening reverses, and neither do we.
- The cinnabar gap. Cinnabar shares the hardness, 2 to 2.5, and a red streak. It does not share the weight: a 20.00 g piece reads 17.55 g in water, against 16.41 g for proustite. Every other mineral in the table reads 16.74 g or less, a gap of 0.81 g. Above about 17.15 g, a red, red-streaked crystal from this table is cinnabar.
- Documented negative: proustite against pyrargyrite by weight. 16.41 g against 16.56 g, 0.15 g apart. The streak does the work: vermilion against purplish red. In the sheet analyses the arsenic share of the arsenic-plus-antimony site is 99.69% and 100% for proustite, 0.59% and 3.76% for pyrargyrite.
- Silver, worked. On standard atomic weights proustite is 65.41% silver, pyrargyrite 59.76%: arsenic is lighter than antimony. The sheet’s analyses give 64.12% from Cobalt, Ontario, and 64.65% from Veta Rica, Mexico. Arsenic is part of the formula; toxicity is routed to toxic minerals in a collection, and none of this is advice.
| Species | IMA formula | System (Handbook) | Colour | Streak | Hardness | Density (measured) | 20.00 g piece in water | Light, on the sheet |
|---|---|---|---|---|---|---|---|---|
| Proustite | Ag3AsS3 | Hexagonal | Scarlet-vermilion | Vermilion | 2 to 2.5 | 5.57 | 16.41 g | Darkens with exposure to light |
| Pyrargyrite | Ag3SbS3 | Hexagonal | Deep red | Purplish red | 2.5 | 5.82 | 16.56 g | Darkens with exposure to light |
| Xanthoconite | Ag3AsS3 | Monoclinic | Dark cochineal-red to dull orange to clove-brown | Orange-yellow | 2 to 3 | 5.54(14) | 16.39 g (16.30 to 16.48 g) | Nothing printed |
| Pyrostilpnite | Ag3SbS3 | Monoclinic | Hyacinth-red | Yellow-orange | 2 | 5.94 | 16.63 g | Nothing printed |
| Cuprite | Cu2O | Cubic | Cochineal-red, ruby-red, purplish red to nearly black | Brownish red, shining | 3.5 to 4 | 6.14 | 16.74 g | Nothing printed |
| Cinnabar | HgS | Hexagonal | Cochineal-red, towards brownish red and lead-grey | Scarlet | 2 to 2.5 | 8.176 | 17.55 g | Nothing printed |
| Realgar | AsS | Monoclinic | Red to yellow-orange | Red-orange to red | 1.5 to 2 | 3.56 | 14.38 g | Disintegrates on long exposure to light |
What is proustite? Silver arsenic sulphide, Ag3AsS3
The names. The September 2026 IMA-CNMNC Master List lists proustite as Ag3AsS3, status G, 1832, with the country of first description unknown. The Handbook sheet gives the same formula and the name to honor Joseph Louis Proust (1754–1826), celebrated French chemist. It is dimorphous with xanthoconite: the same formula, a different, monoclinic structure, with a dark cochineal-red to clove-brown colour and an orange-yellow streak.
What it looks like. Hexagonal, in prismatic crystals to 8 cm, commonly rhombohedral and also scalenohedral; massive and compact too. Twinning produces trillings. The cleavage is distinct, the fracture conchoidal to uneven, the tenacity brittle. Colour scarlet-vermilion, streak vermilion, lustre adamantine. Its refractive indices are omega 3.0877 and epsilon 2.7924; the high-index point is made on the pyrargyrite page and not repeated here.
Composition, worked on standard atomic weights (Ag 107.8682, As 74.9216, S 32.06): formula mass 494.71, of which 65.41% is silver, 15.14% arsenic and 19.44% sulphur. The sheet’s ideal column prints 65.42% silver; the hundredth is atomic-weight rounding. Its analysis from Cobalt, Ontario, carries 0.25% iron and 0.12% cobalt plus nickel as a remainder.
Does proustite darken in light? What the sheet says
The sheet’s words. The optical field of the Handbook proustite sheet opens: translucent, darkens with exposure to light. Pyrargyrite’s sheet uses the same words. Across the seven sheets in the table, a case-insensitive script search for darken, exposure to light and disintegrat finds those two, plus realgar’s disintegrates on long exposure to light to a powder composed of pararealgar or arsenolite and orpiment. Xanthoconite, pyrostilpnite, cuprite and cinnabar print nothing about light.
What this page does not say. The sheet gives no rate, no wavelength and no statement that the change reverses. The NatSCA leaflet Care and Conservation of Geological Specimens says light will irreversibly damage some mineral species and names the arsenic sulphide group as the most susceptible; it does not name proustite, and we do not extend its wording to it. The site’s general page is which minerals fade in light. We give no storage recipe as tested fact.
Not magnetic either. The sheet records no magnetic property, and USGS Open-File Report 99-529, Table 1 gives proustite a total and best range above 1.70 amp from one sample, diamagnetic by that report’s definitions; pyrargyrite, two samples, the same. The magnet question in general is on which minerals are magnetic.
Proustite or cinnabar? The cinnabar gap
Why the two are confused. Cinnabar is red and adamantine with a scarlet streak, and its hardness, 2 to 2.5, is proustite’s exactly. Colour words on the sheets overlap: cinnabar cochineal-red, proustite scarlet-vermilion, with cochineal-red to blood-red pleochroism. A scratch test cannot separate them.
The rule. Weigh a clean 20.00 g piece in air and suspended in water. Cinnabar, density 8.176, reads 17.55 g. Proustite reads 16.41 g, pyrargyrite 16.56 g, pyrostilpnite 16.63 g, xanthoconite 16.30 to 16.48 g across its stated uncertainty, and cuprite, the heaviest of the rest, 16.74 g. The gap between cuprite and cinnabar is 0.81 g; a reading above about 17.15 g puts a red, red-streaked crystal from this table on the cinnabar side. The method is on how to measure specific gravity at home. Mercury and its questions are on is cinnabar toxic.
Where the gap fails. It needs a piece of about 20 g, clean of matrix; a crystal on matrix weighs the matrix. Below the line the scale cannot sort the silver sulphosalts: proustite and xanthoconite overlap, and proustite and pyrargyrite are 0.15 g apart. There the streak decides: vermilion, purplish red, orange-yellow, yellow-orange, on a streak plate. Cuprite is separated by hardness, 3.5 to 4, and its brownish red streak.
Proustite or pyrargyrite? Arsenic against antimony
Two compositions, measured. Proustite is the arsenic compound and pyrargyrite the antimony one. In the sheets’ own analyses, worked as atom fractions, arsenic fills 99.69% of the semimetal site in the Cobalt proustite and 100% in the Veta Rica one, where antimony is only a trace; in pyrargyrite it is 0.59% at Săcărâmb and 3.76% at Freiberg. The sheets cite a 1963 study of proustite–pyrargyrite solid solutions; we report the four analyses and no more.
What separates them in hand. Colour: scarlet-vermilion against deep red. Streak: vermilion against purplish red. Weight, not usefully. The pyrargyrite page compares the two with its own table; this page agrees with its figures, hardness 2 to 2.5 and density 5.57 for proustite.
Where it is found, per the sheet: many localities, but rarely in fine crystals or as an important ore mineral. Exceptional crystals from Chañarcillo, Atacama, Chile; the Himmelsfürst mine near Freiberg and Niederschlema, Saxony; Săcărâmb, Romania; Jáchymov and Příbram; Sainte-Marie-aux-Mines, France; Sarrabus, Sardinia; Red Mountain and Georgetown, Colorado; large masses at the Poorman mine, Idaho; the Keeley mine, Ontario; Batopilas and Sombrerete, Mexico. Associates include silver, xanthoconite, acanthite and tetrahedrite. 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.