Realgar — the red arsenic sulfide mineral

Physical and chemical properties

Realgar is an arsenic sulfide that melts at around 320°C and produces characteristic fumes when burned, containing arsenic and sulfur species. Crystals are generally monoclinic and may appear prismatic or granular; specimens are easily abraded and can darken or powder with handling and light exposure. The conversion to pararealgar is a photochemical decomposition: the red crystalline realgar becomes a friable yellow powder on prolonged illumination, which is often found on museum specimens and historic artworks.

Occurrence and formation

Realgar is commonly found in low-temperature hydrothermal veins, hot-spring deposits and in association with other sulfide minerals. It often occurs alongside minerals such as orpiment and stibnite. Because it contains arsenic, realgar is frequently targeted in deposits where arsenic-bearing minerals accumulate.

Uses, history and cultural roles

Historically, realgar has been valued for several applications: as a red pigment in painting and decoration, as an ingredient in certain pyrotechnic compositions and in traditional medicinal practices. It was used in older firework recipes to produce red or crackling effects before many modern powdered metals and formulations replaced it; for example, it appeared in some early compositions alongside powdered metals and other ingredients like powdered metals. Realgar has a documented history as a dyed pigment and an item of trade (pigment, trade). It has also been used in traditional remedies, notably in parts of East Asia, where it appears in some formulations described as medicine in China.

Health, safety and regulation

Because it contains arsenic, realgar is toxic and must be handled with caution. Inhalation, ingestion or prolonged skin contact can present serious health risks. Its use in consumer products and medicines is limited or regulated in many jurisdictions owing to toxicity concerns. When heated or burned, realgar releases hazardous fumes; therefore, it should not be exposed to open flames or high temperatures outside controlled, ventilated conditions.

Related minerals and distinctions

Realgar is chemically and historically linked to orpiment (arsenic trisulfide, As2S3), which is yellow and more stable to light. Both minerals share arsenic and sulfur and often occur together, but they differ in stoichiometry, color and stability. Collectors and conservators often note the ease with which realgar alters to pararealgar and take steps to limit light exposure when preserving specimens or artworks.

• Common names and terms: realgar, pararealgar (the yellow alteration product).
• Typical hazards: arsenic toxicity, photochemical alteration, hazardous fumes on heating.
• Further information: consult mineralogical references or safety datasheets via mineral databases and health resources.

For additional background or specimen data, see mineralogical resources and collections references at ore-focused pages or general mineral guides (arsenic and sulfur topics).