Realgar

Realgar is a mineral. Its chemical formula is As₄S₄. It is a soft (Mohs hardness 1.5 to 2) red mineral. It is an ore of arsenic. It changes into a yellow powder when exposed to light. This yellow powder is called pararealgar. It melts at 320°C and burns with a blue flame, making arsenic and sulfur fumes. It was used to make a white color in fireworks before powdered metals were used. It is still used in some crackling firecrackers. It was used as a red pigment and an item of trade. It is used as a medicine in China. Its uses are similar to orpiment, a similar mineral.

Realgar Media

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The unit cell of realgar, showing clearly the As
4S
4 molecules it contains
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Cluster of realgar crystals from Getchell Mine, Adam Peak, Potosi District, Humboldt County, Nevada, United States
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Cherry-red realgar crystals atop a matrix, and a sharp acicular spray of the rare species picropharmacolite (white needles) below
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Crystals of realgar, quartz, chalcopyrite and galena, from Quiruvilca Mine, La Libertad, Peru