When in crystals, they are usually short prisms of the orthorhombic system, either end being terminated with a low roof. Though usually described as silver-white in color, there is always a brassy cast to the color. Its appearance is much like cobaltite and smaltite, but it can be easily distinguished from both these by putting a piece in nitric acid. The arsenopyrite will not materially change the color of the fluid, but the other two turn it rose-red, and all give off the smell of sulphur. It looks sometimes like marcasite, but that is yellower, and has the fibrous structure, not found in arsenopyrite.
It is found in veins or in metamorphic rocks, associated with argentite, galena, sphalerite, chalcopyrite and pyrite. It is distinctly a mineral formed by deposition from hot vapors or hot water rising from either lavas, or in the course of metamorphism.
It is found in New Hampshire, Vermont, Massachusetts, Connecticut, New York, New Jersey, California, etc.
[Realgar]
AsS
[Pl. 24]
Occurs in incrustations or scattered grains; hardness 1.5 to 2; specific gravity 3.5; color orange; streak orange; luster resinous; opaque on thin edges.
Crystals are very rare, but when found are short monoclinic prisms. The color is aurora-red, changing to orange as soon as it is exposed to the air. This and the streak are entirely characteristic. It is a mineral associated with hot vapors or hot waters, and is found about volcanoes, as deposits from the hot water of the geysers in Norris Basin, Yellowstone Park, and in veins, associated with barite, stibnite, quartz, etc., as in Massachusetts, Utah, California, etc.
[Orpiment]
As₂S₃
Occurs as incrustations or powdery masses; hardness 1 to 2; specific gravity 3.5; color lemon yellow; streak yellow; luster resinous.
This mineral is very like realgar in its physical properties, and likely to occur with it. It gives the lemon yellow color to the basins about hot springs, as in the Yellowstone Park, and about volcanoes. It also comes in veins with realgar.