As4S4

As4S4 is an inorganic compound consisting of arsenic and sulfur with the formula As4S4. The compound occurs naturally as the orange‑red mineral realgar, one of the principal arsenic sulfide minerals along with orpiment (As2S3). In solid form it consists of discrete As4S4 molecular units that assemble into a crystalline lattice.

Occurrence and production

Realgar forms in hydrothermal vein deposits and is commonly associated with other sulfide minerals such as cinnabar and orpiment. It has been found in many parts of the world, including China, Spain, Peru, and Morocco. The mineral can be found as crystals or weathered masses, and synthetic samples of As4S4 can be prepared in the laboratory by controlled reactions of arsenic and sulfur.

Properties

As4S4 appears as an orange‑red solid with a resinous to dull luster. It is relatively soft and brittle, and it is moderately dense. The material is photosensitive: exposure to light can cause slow darkening and degradation, a trait that historically limited its use as a pigment. It decomposes upon heating rather than melting cleanly in common conditions and can release sulfur and arsenic-containing species when subjected to heat or strong irradiation.

Safety

As4S4 and realgar are highly toxic due to arsenic content. Handling should minimize dust generation and exposure, with appropriate precautions to avoid inhalation or ingestion. Long‑term exposure poses health risks; contact with heat or light can enhance degradation and release of arsenic compounds.

Uses

Historically, realgar has been used as a pigment in paints and decorative arts, though its toxicity has largely limited modern use. It remains of interest for mineralogical study and for research in arsenic–sulfur chemistry and related materials science.