Pharmacolite

(repeating unit)CaHAsO₄·2(H₂O)IMA symbolPmc^[1]Strunz classification8.CJ.50Dana classification39.1.1.2Crystal systemMonoclinicCrystal classDomatic (m)
(same H-M symbol)Space groupIaUnit cella = 5.959 Å,
b = 15.313 Å,
c = 6.357 Å;
β = 114.67°; Z = 4IdentificationColorColorless, white, pale grayCrystal habitCommonly acicular, silky fibrous, botryoidal to stalactitic; rare as elongated flattened crystalsCleavagePerfect on {010}FractureUnevenTenacityFlexibleMohs scale hardness2 – 2.5LusterVitreous, pearly on cleavagesDiaphaneityTransparent to translucentSpecific gravity2.53 – 2.725Optical propertiesBiaxial (−)Refractive indexnα = 1.580 – 1.583 nβ = 1.589 – 1.590 nγ = 1.590 – 1.594Birefringenceδ = 0.010 – 0.011PleochroismNot pleochroic2V angleMeasured: 77°References^[2][3][4]

Pharmacolite is an uncommon calcium arsenate mineral with formula CaHAsO₄·2(H₂O). It occurs as soft, white clusters of fibrous crystals and encrustations which crystallize in the monoclinic system. It is the arsenate analogue of the sulfate gypsum and the phosphate brushite.

Discovery and occurrence

Pharmacolite was first described in 1800 for an occurrence in the Sophia Mine in the Böckelsbach Valley of Wittichen, Schenkenzell, Black Forest, Baden-Württemberg, Germany. The name is from the Greek φάρμακον ("pharmakon"), alluding to its poisonous arsenic content.^[2]

It forms by secondary (oxidizing) processes from primary arsenic minerals. It is associated with picropharmacolite, hornesite, haidingerite and rosslerite.^[2][3]

References