REALGAR  , a

mineral species consisting of arsenic mono-sulphide (AsS) and occurring as monoclinic crystals of a bright red colour . There is a perfect cleavage parallel to the plane of symmetry (r in fig.) . The lustre is resinous, and the streak has the same colour as • the crystals, namely, orange-red to aurora-red . The hardness is I2–2 and the specific gravity 3'55 . On exposure to light the crystals crumble to a yellow powder . The name realgar is of Arabic origin, and was used by the alchemists; the substance was known to Theophrastus under the name EavbaplKrt, and to Pliny as Sandaracha . The mineral usually occurs in association with the yellow arsenic sulphide, orpiment . Good crystals are found with ores of silver and lead in the mineral veins of Felsobanya, near Nagy-Banya, Kapnik-Banya and Nagyag, near Deva, in Hungary; with blende in the white crystalline dolomite of the Binnenthal in Switzerland; and in a bed of sandy clay at Mercur in Utah . It is deposited by the solfataras near Naples and by the hot springs of the Yellowstone National Park . Realgar has been used as a pigment and in pyrotechny for producing a brilliant white fire; but it is now replaced by the artificially prepared compound . The other native arsenic sulphide, As2S3, known as orpiment (Lat. auripigmentum, meaning " golden paint " ), occurs as foliated masses of a lemon-yellow colour, the foliation being parallel to a direction of perfect cleavage . It is sectile and soft (H.=12 2), and has a specific gravity 3.4 .

Distinctly

developed crystals are rare; they have usually been considered to be orthorhombic and isomorphous with stibnite (Sb2S3), but it is probable that they are really monoclinic . Orpiment is extensively mined near Julamerk in Asiatic Turkey . (L . J .