Realgar
Realgar is AsS, 70.03% arsenic: red to orange, hardness 1.5 to 2, density 3.56, crystals to 12 cm. Its sheet says it disintegrates on long exposure to light.
Realgar is arsenic sulphide, AsS, 70.03% arsenic by mass: monoclinic, red to yellow-orange with a red-orange streak, hardness 1.5 to 2, density 3.56, in prismatic crystals to 12 cm. The Handbook records that it disintegrates on long exposure to light, to a powder of pararealgar or arsenolite and orpiment. The IMA list dates it to 1747.
In short
- Realgar is the only sheet of the 176 that uses the word disintegrates. The Handbook of Mineralogy sentence reads: Disintegrates on long exposure to light to a powder composed of pararealgar or arsenolite and orpiment. Five other sheets mention exposure to light: four describe a colour change, and the fifth is pararealgar, the product.
- It is light in the hand for a red mineral: the 4.5 line. Of the 125 sheets in this site's reference set that record a streak, 12 give a red or orange one. Realgar, at 3.56, is the second-lightest of the 12, after erythrite at 3.06. Every other one that is as soft as hardness 3 is 5.57 or denser.
- Pararealgar has the same composition and a different structure. Its sheet calls it an alteration product of realgar, typically a result of exposure to light: bright yellow when powdery, hardness 1 to 1.5.
- It is 70.03% arsenic, calculated from AsS, the same figure as on is realgar safe to handle. That page, not this one, covers the hazard and the storage question.
- The name is Arabic: rahj al ghar, powder of the mine, according to the Handbook. The IMA list records realgar as grandfathered, 1747.
| Species | Formula | Hardness | Density | Streak | What the sheet says about light | 20.00 g piece, weighed in water |
|---|---|---|---|---|---|---|
| Realgar | AsS | 1.5 to 2 | 3.56 | Red-orange to red | Disintegrates on long exposure to a powder | 14.38 g |
| Pararealgar | AsS (IMA: As4S4) | 1 to 1.5 | 3.52 | Bright yellow | Forms from realgar, typically by exposure to light | 14.32 g |
| Orpiment | As2S3 | 1.5 to 2 | 3.49 | Pale lemon-yellow | Nothing recorded | 14.27 g |
| Erythrite | Co3(AsO4)2·8H2O | 1.5 to 2.5 | 3.06 | Pale red to pink | Nothing recorded | 13.46 g |
| Proustite | Ag3AsS3 | 2 to 2.5 | 5.57 | Vermilion | Darkens with exposure to light | 16.41 g |
| Pyrargyrite | Ag3SbS3 | 2.5 | 5.82 | Purplish red | Darkens with exposure to light | 16.56 g |
| Crocoite | PbCrO4 | 2.5 to 3 | 6.0 to 6.1 | Yellow-orange | Nothing recorded | 16.67 to 16.72 g |
| Cinnabar | HgS | 2 to 2.5 | 8.176 | Scarlet | Nothing recorded | 17.55 g |
What happens to realgar in light? What the sheets say
The Handbook's realgar sheet states it in its physical-properties block, directly after the density: Disintegrates on long exposure to light to a powder composed of pararealgar or arsenolite and orpiment. The sheet also describes realgar as transparent when fresh.
Method. The 176 sheets in this site's reference set were searched in their pdftotext -layout text. Neither search word contains a ligature. Disintegrat appears on one sheet, realgar's. Exposure to light appears on six: baryte and chlorargyrite (a colour change), proustite and pyrargyrite (darkens), pararealgar, and realgar. That is the same six found on the older 158-sheet set used by is realgar safe to handle. Adding 18 sheets changed nothing.
This page states only what the sheets state. They do not give a rate, a light level or a wavelength, and neither does this page. What the powder means for handling, and what the conservation literature recommends, is set out on is realgar safe to handle and which minerals fade in light. Nothing here is handling advice.
Is realgar the same as pararealgar?
Same composition, different mineral. The Handbook gives both as AsS and ideal composition 70.03% arsenic, 29.97% sulphur. It calls realgar trimorphous with alacránite and pararealgar, and it names pararealgar in allusion to its chemical identity with realgar. The September 2026 IMA-CNMNC Master List writes realgar AsS and pararealgar As4S4, the same ratio. It writes alacránite as As8S9, which is not the same ratio. That is the IMA's formula, and this page does not reconcile it with the Handbook's word trimorphous.
What pararealgar looks like. Its sheet describes it as a fine powder, rarely as granular aggregates under 0.02 mm, bright yellow when powdery, yellow-orange to orange-brown when granular, with a bright yellow streak. It is hardness 1 to 1.5 and density 3.52. Its occurrence line reads: an alteration product of realgar in stibnite-bearing quartz veins, the alteration typically a result of exposure to light. Its type locality, marked [TL] on the sheet, is Mount Washington on Vancouver Island, and the IMA list gives it approval number 1980-034, Canada.
So a yellow crust on a red realgar is usually the realgar changing, not a second mineral arriving. Pararealgar's streak is bright yellow, where realgar's is red-orange to red. Orpiment's is pale lemon-yellow, and each of the two sheets names the other as an associate. Orpiment is As2S3, 60.91% arsenic.
How to identify realgar: the 4.5 line
Method. Of the 176 sheets in this site's reference set, 125 have a Streak field. A case-insensitive search of that field for red, orange, vermilion, scarlet or cherry finds 12. By measured density they run: erythrite 3.06, realgar 3.56, lepidocrocite 4.09, goethite 4.28, hematite 5.26, proustite 5.57, pyrargyrite 5.82, pyrostilpnite 5.94, crocoite 6.0 to 6.1, cuprite 6.14, descloizite about 6.2 and vanadinite 6.88. Cinnabar, at 8.176 with a scarlet streak, is not in the set and is added from its own sheet.
The rule we call the 4.5 line: a soft mineral, hardness 3 or under, with a red or orange streak and a density below 4.5 is realgar or erythrite. Every other soft red-streaked sheet, proustite, pyrargyrite, pyrostilpnite, crocoite and vanadinite, is 5.57 or denser. Weighed in water, a 20.00 g realgar reads 14.38 g, against 15.56 g at the line and 16.41 g for proustite. Cinnabar reads 17.55 g.
Realgar against erythrite: erythrite is crimson to peach-red and pink, with a pale red to pink streak. Realgar is red to yellow-orange, with a red-orange to red streak and a resinous to greasy lustre. The methods are on how to use a streak plate and how to measure specific gravity at home. The ruby silvers are compared on pyrargyrite.
Where realgar is found
The sheet says realgar occurs most commonly as a low-temperature hydrothermal vein mineral associated with As–Sb minerals. It also forms as volcanic sublimations, in hot spring deposits, and in carbonate and clay sedimentary rocks. Its associates are orpiment, arsenolite, other arsenic minerals, calcite and baryte. Realgar is named in the Association field of four other sheets in the set: arsenolite, orpiment, pararealgar and stibnite.
The Handbook gives only localities for finely crystallised material. In Germany, Schneeberg in Saxony and St Andreasberg in the Harz. The Lengenbach quarry, Binntal, Switzerland, is the source of its analysed specimen, 69.54% arsenic. In Romania, Baia Sprie, Cavnic and Săcărîmb, and in the Czech Republic, Jáchymov. Further east, Alšar near Rošden, Macedonia, the source of its X-ray powder pattern; Men-Kyule in Sakha, Russia; and the Shimen mine in Hunan, China. In the USA it names fine large crystals at the Getchell mine, Humboldt County, Nevada, then Mercur in Utah and, in Washington, the Monte Cristo district and the Royal Reward mine. No British locality is recorded.
The rest of the sulphides are indexed at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.