Pararealgar

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Pararealgar
Pararealgar-unit-cell-3D-balls.png
General
Category Sulfide mineral
Formula
(repeating unit)
As4S4
Strunz classification 2.FA.15b
Crystal symmetry Monoclinic H–M Symbol 2/m – prismatic
Unit cell a = 9.909 Å, b = 9.655 Å, c = 8.502 Å; β = 97.20°; Z = 16
Identification
Color Bright yellow when powdery, to yellow-orange and orange-brown when granular
Crystal habit Fine powder to granular
Crystal system Monoclinic
Fracture Uneven
Tenacity Brittle
Mohs scale hardness 1 – 1.5
Luster Vitreous to resinous
Streak Bright yellow
Diaphaneity Translucent
Specific gravity 3.52
Optical properties Biaxial (?)
Birefringence 2.02
Pleochroism High: x = orange yellow, y = bright yellow, z = orange red
References [1][2][3]

Pararealgar is an arsenic sulfide mineral with the chemical formula As4S4[1] also represented as AsS.[3] It forms gradually from realgar under exposure to light. Its name derives from the fact that its elemental composition is identical to realgar, As4S4. It is soft with a Mohs hardness of 1 - 1.5, is yellow orange in colour, and its monoclinic prismatic crystals are very brittle, easily crumbling to powder.

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[edit]

The two isomers of As4S4:
realgar (left) and pararealgar.

Both isomers of As4S4 are molecular, in contrast to the other main sulfide of arsenic, orpiment (As2S3), which is polymeric. In pararealgar, there are three kinds of As centres (and three kinds of S centres). The molecule has Cs symmetry. In realgar, the four As (and four S) centres are equivalent and the molecule has D2d symmetry.[4] An analogous pair of isomers is also recognized for the corresponding phosphorus sulfides P4S4.[5]

Occurrence[edit]

Pararealgar occurs as an alteration product of realgar in stibnite-bearing quartz veins typically as a result of exposure to light. It occurs associated with realgar, stibnite, tetrahedrite, arsenopyrite, duranusite, native arsenic, arsenolite, native sulfur, lepidocrocite and pyrite.[3]

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.[1] It has since been reported from a variety of locations worldwide.

References[edit]

  1. ^ a b c Mindat.org
  2. ^ Webmineral.com Mineralogy Database: Pararealgar
  3. ^ a b c Handbook of Mineralogy
  4. ^ Paola Bonazzi, Silvio Menchetti, Giovanni Pratesi "The crystal structure of pararealgar, As4S4" American Mineralogist, 1995, vol.80 400.
  5. ^ Jason, M. E.; Ngo, T.; Rahman, S. (1997). "Products and Mechanisms in the Oxidation of Phosphorus by Sulfur at Low Temperature". Inorg. Chem. 36: 2633–2640. doi:10.1021/ic9614879.