Picropharmacolite

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.

Picropharmacolite
Picropharmacolite with guerinite and erythrite from Bauhaus, Hesse, Germany. Specimen size 5.5 cm
General
CategoryArsenate mineral
Formula
(repeating unit)		Ca4Mg(AsO3OH)2(AsO4)2·11H2O
Strunz classification8.CH.15
Dana classification39.2.4.1
Crystal systemTriclinic
Crystal classPinacoidal (1)
(same H-M symbol)
Space groupP1
Unit cella = 13.547 Å, b = 13.5 Å,
c = 6.71 Å; α = 99.85°,
β = 96.41°, γ = 91.6°; Z = 2
Identification
Formula mass940.48 g/mol
ColorWhite or colorless
Crystal habitAs prismatic crystals more typically in radial aggregates, globular crusts, and fibrous concretions
TwinningNone reported
CleavagePerfect on {100} and {010}
FractureMicaceous
TenacityFragile
Mohs scale hardness1 to 2
LusterSilky, slightly pearly
StreakWhite
DiaphaneityTranslucent to opaque
Specific gravity2.58 - 2.60
Optical propertiesBiaxial (+)
Refractive indexnα = 1.631, nβ = 1.632, nγ = 1.640[1] or nα = 1.557, nβ = 1.566 – 1.571, nγ = 1.577 – 1.579[2]
Birefringenceδ = 0.009
2V angle40° - 50°
Dispersionr < v strong
Other characteristicsFluorescent blue-white under long wave and short wave ultraviolet light
References[1][2][3][4]

Structure

Infrared spectra[5] show that picropharmacolite contains water molecules H2O, hydroxyl groups (OH)− co-ordinated with Mg2+ cations, and acid arsenate radicals (HAsO4)2−. There are strong structural similarities with guerinite, Ca5(AsO3OH)2(AsO4)2.9H2O which indicates a similar formula for the two minerals.[6]
X-ray diffraction methods indicate that As, Ca and Mg cations are positioned in corrugated layers parallel to the c axis, the layers being linked by hydrogen bonding only. Four independent water molecules are sandwiched between adjacent layers, and build up hydrogen-bonded chains which are also parallel to the c axis. The ratio of four Ca to one Mg remains fairly steady, and no significant Ca/Mg substitution occurs in any cation site. Hence if the formula of picropharmacolite is written as Ca4Mg(H2O)7(AsO3OH)2(AsO4)2.4H2O,[7] it is a better representation of the structure than the more usual formula Ca4Mg(AsO3OH)2(AsO4)2.11H2O.

Morphology

Picropharmacolite is usually found as small to microscopic pearly white botryoidal aggregates with a radiating foliated structure internally. Less commonly it occurs as silky fibrous aggregates or minute needle-like crystals, that are rectangular prisms elongated along the c axis.

Environment

Formed as an oxidation product of arsenic-bearing sulfides in reaction with surrounding calcium-bearing rocks, and as a recent efflorescence in mine workings. Erythrite and pharmacolite are common associated minerals.[2]

Type locality

It was first described for samples from ore dumps of the Richelsdorf Smelter, Süss, Richelsdorf District, Hesse, Germany.[3] The type material is stored at the National School of Mines, Paris, France, 95.497.[2]

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References
  1. Gaines et al (1997) Dana’s New Mineralogy Eighth Edition, Wiley
  2. http://rruff.geo.arizona.edu/doclib/hom/picropharmacolite.pdf Handbook of Mineralogy
  3. http://www.mindat.org/min-3207.html
  4. http://www.webmineral.com/data/Picropharmacolite.shtml
  5. American Mineralogist (1974) 59:807
  6. American Mineralogist (1976) 61:326
  7. American MIneralogist (1981) 66:385
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