 An analogous pair of isomers is also recognized for the corresponding phosphorus sulfides P4S4.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.