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
Category	Arsenate mineral
Formula	CaHAsO4·2(H2O)
IMA symbol	Pmc[1]
Strunz classification	8.CJ.50
Dana classification	39.1.1.2
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
Identification
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	[2][3][4]

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]