THE MINERAL REALGAR

Chemistry: AsS, Arsenic Sulfide
Class: Sulfides and Sulfosalts
Uses: A major ore of arsenic, formerly used for pigments, firework coloring agent and as mineral specimens.
Specimens

Realgar is an oddball among the sulfides. It is one of only a few sulfides that are not metallic or opaque or blandly colored. Its structure is analogous to that of sulfur and resembles sulfur in most respects except for color (the name "ruby sulfur" has been applied to realgar). Sulfur has a structure composed of 8 sulfur atoms linked in a ring. Realgar's structure alternates between sulfur atoms and arsenic atoms producing rings of As₄S₄. The arsenic atoms affect the structure altering it from sulfur's orthorhombic symmetry to realgar's monoclinic symmetry.

Realgar occurs in hydrothermal veins with valuable metal sulfide ores and its bright red color can be an aid to prospectors. It also can be found in hot spring deposits and as a volcanic sublimate product (crystallizing from vapors). Realgar gets its name from the Arabic words for "powder of the mine" (rahj al ghar). Realgar is famous for some wonderfully beautiful specimens. Some specimens can have a deep ruby red color with an amazing clarity and a high luster. The color of realgar is truly something to appreciate and cherish. But realgar's beauty is sometimes fleeting.

It is an unstable mineral and will alter to a different mineral, pararealgar and eventually to a powder. This process takes time and is accelerated by exposure to light. Specimens should be stored in dark, enclosed containers, and only exposed to light for the brief enjoyment of its owner and friends. This sounds extreme, but wonderfully beautiful realgar specimens are worth preserving for as long as possible. If you are wondering how quickly the deterioration occurs, the answer is immediately, but fortunately very slowly. Ancient Chinese carvings of realgar are still in existence, but badly affected by the deterioration. The deterioration of realgar was thought to produce the closely related yellow orpiment, but this was recently proven to be false and the deterioration product is in fact yellow-orange pararealgar. In old paintings and manuscripts, realgar was a common pigment for paints and dyes. Many of these paintings now have a yellow or orange hue where once the color must have been an original red.

PHYSICAL CHARACTERISTICS:

Color is orange to red.
Luster is resinous, adamantine to sub-metallic.
Transparency: Crystals are translucent to transparent.
Crystal System: Monoclinic; 2/m.
Crystal Habits: include prismatic striated crystals with a rounded diamond-like cross-section. They are terminated by a wedge-like dome. Also found as grains, crusts and earthy masses.
Cleavage is good in one direction.
Fracture is subconchoidal.
Hardness is 1.5 - 2
Specific Gravity is 3.5 - 3.6
Streak is orange to orange-yellow.
Other Characteristics: Realgar is unstable in light; specimens should be stored in complete darkness, rarely some specimens fluoresce under UV light and crystals are pleochroic between dark red and orange red.
Associated Minerals almost always include orpiment, also calcite, stibnite and other metal sulfide ores.
Notable Occurrences include most importantly Hunan Province, China; but also Switzerland; Japan; Macedonia; Mercur, Utah, USA; Romania and many other localities.
Best Field Indicators are of course color as well as crystal habit, association with orpiment, softness and luster.