Muséum national d'histoire naturelle

GENERAL DESCRIPTION

Realgar is an arsenic sulphide, but richer in arsenic than orpiment. It is a hydrothermal mineral, formed at relatively low temperature. It is associated with other mineral containing arsenic and antimony like orpiment and stibnite (etc.).

The crystals of realgar are prismatic, more or less lengthened, showing scratches in the elongation direction. The crystals are of red to yellow-orange colour, transparent with a greasy lustre. Crystals up to 12 cm were found.

The deposits where this mineral can be found are numerous, but mostly as incrustations or massive. Few deposits produce well formed crystals, as in Lengenbach in Switzerland, Jachimov in the Czech Republic, Baia Sprie in Romania and the very many Chinese mines (Yunnan, Guangxi, Guangdong, Guizhou, Hebei, Mandchouria, Hubei, Shaanxi, Outer Mongolia, Sichuan, Xizang...) and, more particularly, the Shimen mine in Hunan.

In France, realgar is present in Corsica in the deposit of Matra, the coal spoil heaps of La Ricamarie (Saint-Etienne, Loire) and of Decazeville (Aveyron), at the Giftgrube and Gabe Gottes mines in Sainte Marie aux Mines (Haut-Rhin), the Val d'Ajol (Vosges), the mines of Duranum and Luceram (Alpes-Maritimes), Jas Roux (Hautes-Alpes) and in Bains-de-Bretagne (Ile-et-Vilaine).

Under light and in particular of the ultraviolets, realgar is transformed into pararealgar. Realgar and pararealgar have the same chemical composition (As4S4) and the same crystalline system (monoclinical). But there are light differences in their crystalline structure, which was revealed only recently. This make that these two minerals are absolutely distinct mineral species despite closely related.
However, those slight differences are enough to explain why these two very close minerals have nevertheless different colors: red for realgar and yellow-orange-brown for the pararealgar. It is necessary to preserve this mineral outside natural light. At the Muséum national d’Histoire naturelle in Paris, we expose such minerals under a red light, free of UV light. It is important to realize that the realgar to pararealgar transformation is slow at the beginning but quickly speeds-up with time after a given illumination dose. Above this limit, the transformation into pararelagar is irreversible.

The type of this species and its place of conservation are not definable because this species is well known since Antiquity.

Did you know? Realgar was used as a rat poison because of its toxicity. It was also employed in pyrotechny to prepare white fireworks or, with orpiment, for the manufacture of red and yellow pigments.

Realgar

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Mineralogy