Pharmacolite

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.

Pharmacolite from Sainte-Marie-aux-Mines
Pharmacolite
Pharmacolite on slag from Villanière, Aude, Languedoc-Roussillon, France (View 7.8 cm)
General
CategoryArsenate mineral
Formula
(repeating unit)
CaHAsO4·2(H2O)
Strunz classification8.CJ.50
Dana classification39.1.1.2
Crystal systemMonoclinic
Crystal classDomatic (m)
(same H-M symbol)
Space groupIa
Unit cella = 5.959 Å,
b = 15.313 Å,
c = 6.357 Å;
β = 114.67°; Z = 4
Identification
ColorColorless, white, pale gray
Crystal habitCommonly acicular, silky fibrous, botryoidal to stalactitic; rare as elongated flattened crystals
CleavagePerfect on {010}
FractureUneven
TenacityFlexible
Mohs scale hardness2 - 2.5
LusterVitreous, pearly on cleavages
DiaphaneityTransparent to translucent
Specific gravity2.53 – 2.725
Optical propertiesBiaxial (-)
Refractive indexnα = 1.580 - 1.583 nβ = 1.589 - 1.590 nγ = 1.590 - 1.594
Birefringenceδ = 0.010 - 0.011
PleochroismNot pleochroic
2V angleMeasured: 77°
References[1][2][3]

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.[1]

It forms by secondary (oxidizing) processes from primary arsenic minerals. It is associated with picropharmacolite, hornesite, haidingerite and rosslerite.[1][2]

References

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