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Pharmacolite on slag from Villanière, Aude, Languedoc-Roussillon, France (View 7.8 cm)
Category Arsenate mineral
(repeating unit)
Strunz classification 8.CJ.50
Dana classification
Crystal system Monoclinic
Crystal class Domatic (m)
(same H-M symbol)
Space group Ia
Unit cell a = 5.959 Å,
b = 15.313 Å,
c = 6.357 Å;
β = 114.67°; Z = 4
Color Colorless, white, pale gray
Crystal habit Commonly acicular, silky fibrous, botryoidal to stalactitic; rare as elongated flattened crystals
Cleavage Perfect on {010}
Fracture Uneven
Tenacity Flexible
Mohs scale hardness 2 - 2.5
Luster Vitreous, pearly on cleavages
Diaphaneity Transparent to translucent
Specific gravity 2.53 – 2.725
Optical properties Biaxial (-)
Refractive index nα = 1.580 - 1.583 nβ = 1.589 - 1.590 nγ = 1.590 - 1.594
Birefringence δ = 0.010 - 0.011
Pleochroism Not pleochroic
2V angle Measured: 77°
References [1][2][3]

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

Discovery and occurrence[edit]

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]