Picropharmacolite

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Picropharmacolite
Picropharmacolite.jpg
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
IMA symbolPpm[1]
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[2] or nα = 1.557, nβ = 1.566 – 1.571, nγ = 1.577 – 1.579[3]
Birefringenceδ = 0.009
2V angle40° - 50°
Dispersionr < v strong
Other characteristicsFluorescent 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 , 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[6] 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 , Ca5(AsO3OH)2(AsO4)2.9H2O which indicates a similar formula for the two minerals.[7]
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,[8] 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.[3]

Type locality[]

It was first described for samples from ore dumps of the Richelsdorf Smelter, Süss, Richelsdorf District, Hesse, Germany.[4] The type material is stored at the National School of Mines, Paris, France, 95.497.[3]

References[]

  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85: 291–320.
  2. ^ a b Gaines et al (1997) Dana's New Mineralogy Eighth Edition, Wiley
  3. ^ a b c d http://rruff.geo.arizona.edu/doclib/hom/picropharmacolite.pdf Handbook of Mineralogy
  4. ^ a b "Picropharmacolite".
  5. ^ "Picropharmacolite Mineral Data".
  6. ^ American Mineralogist (1974) 59:807
  7. ^ American Mineralogist (1976) 61:326
  8. ^ American Mineralogist (1981) 66:385

External links[]

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