picropharmacolite

{{Infobox mineral | name = Picropharmacolite | category = Arsenate mineral | image = picropharmacolite.jpg | caption = Picropharmacolite with guerinite and erythrite from Bauhaus, Hesse, Germany. Specimen size 5.5 cm | formula = Ca4Mg(AsO3OH)2(AsO4)2·11H2O | IMAsymbol = Ppm | molweight = 940.48 g/mol | strunz = 8.CH.15 | dana = 39.2.4.1 | system = Triclinic | class = Pinacoidal () (same H-M symbol) | symmetry = P | 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 | color = White or colorless | 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 = 1 to 2 | luster = Silky, slightly pearly | refractive = nα = 1.631, nβ = 1.632, nγ = 1.640 or nα = 1.557, nβ = 1.566 – 1.571, nγ = 1.577 – 1.579 | opticalprop = Biaxial (+) | 2V = 40° – 50° | dispersion = r 4Mg(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.

== Structure == Infrared spectra 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. 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, it is a better representation of the structure than the more usual formula Ca4Mg(AsO3OH)2(AsO4)2.11H2O.