Picropharmacolite

Picropharmacolite
Picropharmacolite with guerinite and erythrite from Bauhaus, Hesse, Germany. Specimen size 5.5 cm
General
Category	Arsenate mineral
Formula (repeating unit)	Ca4Mg(AsO3OH)2(AsO4)2·11H2O
IMA symbol	Ppm[1]
Strunz classification	8.CH.15
Dana classification	39.2.4.1
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	P1
Unit cell	a = 13.547 Å, b = 13.5 Å, c = 6.71 Å; α = 99.85°, β = 96.41°, γ = 91.6°; Z = 2
Identification
Formula mass	940.48 g/mol
Color	White or colorless
Crystal habit	As prismatic crystals more typically in radial aggregates, globular crusts, and fibrous concretions
Twinning	None reported
Cleavage	Perfect on {100} and {010}
Fracture	Micaceous
Tenacity	Fragile
Mohs scale hardness	1 to 2
Luster	Silky, slightly pearly
Streak	White
Diaphaneity	Translucent to opaque
Specific gravity	2.58 - 2.60
Optical properties	Biaxial (+)
Refractive index	nα = 1.631, nβ = 1.632, nγ = 1.640[2] or nα = 1.557, nβ = 1.566 – 1.571, nγ = 1.577 – 1.579[3]
Birefringence	δ = 0.009
2V angle	40° – 50°
Dispersion	r < v strong
Other characteristics	Fluorescent blue-white under long wave and short wave ultraviolet light
References	[2][3][4][5]

Picropharmacolite, Ca₄Mg(AsO₃OH)₂(AsO₄)₂·11H₂O, is a rare arsenate mineral. It was named in 1819 from the Greek for bitter, in allusion to its magnesium content, and its chemical similarity to pharmacolite. The mineral irhtemite, Ca₄Mg(AsO₃OH)2(AsO₄)₂·4H₂O, has the same composition as picropharmacolite, except that it has only four water molecules per formula unit, instead of eleven. It may be formed by the dehydration of picropharmacolite.