Pararealgar
| Pararealgar | ||
|---|---|---|
Specific gravity 3.52 | | |
| Optical properties | Biaxial (?) | |
| Birefringence | 2.02 | |
| Pleochroism | High: x = orange yellow, y = bright yellow, z = orange red | |
| References | [2][3][4][5] | |
Pararealgar is an
It is one of the sulfides of arsenic and is one of two isomers of As4S4. It forms upon exposure of the symmetrical isomer to light. Its name derives from the fact that its elemental composition is identical to realgar, As4S4.
Structure
realgar (left) and pararealgar.
Both isomers of As4S4 are molecular, in contrast to the other main sulfide of arsenic,
Occurrence
Pararealgar occurs as an alteration product of realgar in
It was first described in 1980 for an occurrence in the Grey Rock Mine, Truax Creek, Bridge River area, Lillooet Mining Division, British Columbia, Canada.[3] It has since been reported from a variety of locations worldwide.
Formation
Pararealgar is an alteration product of realgar resulting from exposure to light. The process of alteration is dependent on the wavelength of light, with alteration only occurring at wavelengths longer than approximately 500 nm, in the visible light spectrum.
In realgar's crystal structure, each arsenic atom is bonded to two sulphur atoms and one other arsenic atom. The As-As bonds are 30% weaker than the As-S bonds and certain wavelengths of lights interact with the crystal structure of realgar, breaking the weaker bonds between arsenic atoms. The free As formed as a result of this process destabilises the realgar structure, causing the realgar to become powdery pararealgar without changing overall chemical composition.[8]
References
- S2CID 235729616.
- ^ mineralienatlas
- ^ a b c Mindat.org
- ^ Webmineral.com Mineralogy Database: Pararealgar
- ^ a b c Handbook of Mineralogy
- ^ Paola Bonazzi, Silvio Menchetti, Giovanni Pratesi "The crystal structure of pararealgar, As4S4" American Mineralogist, 1995, vol.80 400.
- .
- ^ Douglass, D. L.; Shing, Chichang; Wang, Ge (1992). "The light-induced alteration of realgar to pararealgar" (PDF). American Mineralogist. 77: 1266–1274. Retrieved 11 August 2014.