Pseudomalachite

Pseudomalachite is a phosphate of copper with hydroxyl, named from the Greek for “false” and “malachite”, because of its similarity in appearance to the carbonate mineral malachite, Cu2(CO3)(OH)2. Both are green coloured secondary minerals found in oxidised zones of copper deposits, often associated with each other. Pseudomalachite is polymorphous with reichenbachite and ludjibaite. It was discovered in 1813. Prior to 1950 it was thought that dihydrite, lunnite, ehlite, tagilite and prasin were separate mineral species, but Berry analysed specimens labelled with these names from several museums, and found that they were in fact pseudomalachite. The old names are no longer recognised by the IMA.[5]

Pseudomalachite
General
CategoryPhosphate minerals
Formula
(repeating unit)		Cu5(PO4)2(OH)4
Strunz classification8.BD.05
Dana classification41.04.03.01
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupP21/c
Unit cella = 4.47 Å, b = 5.75 Å,
c = 17.05 Å; β = 91.06°; Z = 2
Identification
Formula mass575.7 g/mol
ColorDark emerald green to blackish green
Crystal habitCrystals, which are rare, are prismatic, usually with uneven faces. Pseudomalachite is commonly compact, reniform or botryoidal, or it may be fibrous or in crusts and films.
TwinningOn {100}
CleavagePerfect on {100}, distinct on {010}
FractureSplintery or conchoidal
Mohs scale hardness4.5-5
LusterVitreous
StreakBlue green, paler than the mineral
DiaphaneityTranslucent to subtranslucent
Specific gravity(Measured) 4.15 to 4.35
Optical propertiesBiaxial (-)
Refractive indexnα = 1.791 nβ = 1.856 nγ = 1.867
Birefringenceδ = 0.076
PleochroismWeak; X = bluish green to pale green; Y = yellowish green; Z = deep bluish green to blue-green
2V angle48°
DispersionStrong r<v. Also biaxial (+) r>v
SolubilitySoluble in acids but without effervescence (in contrast to malachite which effervesces with warm HCl)
Other characteristicsNot fluorescent, not radioactive
References[1][2][3][4]

Type locality

The type locality is the Virneberg Mine, Rheinbreitbach, Westerwald, Rhineland-Palatinate, Germany. This is an area of ancient copper mining dating back to Roman times, and worked intermittently up until 1872.[1] The type material is held at the Mining Academy, Freiberg, Germany.[3]

Structure

The copper ions are co-ordinated by six oxygen ions to form distorted octahedra.[6] These octahedra are linked by sharing edges to form two distinct types of infinite chains, parallel to b. The chains are linked alternately, again by sharing octahedral edges, to form sheets parallel to the bc plane. Distorted phosphate tetrahedra link the sheets, and there is some doubt about the exact position of the hydrogen ions in the structure.[7]

Environment

It is a secondary mineral found in the oxidised zones of copper ore deposits. Associated with libethenite at several localities in New South Wales, Australia,[8] and at the Chino Mine, New Mexico, US.[9] Other associated minerals are apatite, azurite, chalcedony, chrysocolla, cornetite, cuprite, malachite, pyromorphite, tenorite, and iron oxyhydroxides.

Distribution

Pseudomalachite has been reported from Argentina, Australia, Austria, Belgium, Brazil, Canada, Chile, Czech Republic, Democratic Republic of Congo, France, Germany, Ireland, Israel, Italy, Japan, Kazakhstan, Madagascar, Mexico, Namibia, Norway, Poland, Portugal, Republic of Congo, Romania, Russia, Slovakia, South Africa, Spain, UK, US and Zambia.

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References
  1. http://www.mindat.org/min-3299.html Mindat.org
  2. http://www.webmineral.com/data/Pseudomalachite.shtml
  3. http://rruff.geo.arizona.edu/doclib/hom/pseudomalachite.pdf Handbook of Mineralogy
  4. Gaines et al (1997). Dana’s New Mineralogy Eighth Edition. Wiley
  5. Berry L G (1950). American Mineralogist 35: 365 to 385
  6. Ghose, Subrata (1963) The Crystal Structure of Pseudomalachite. Acta Crystallographica 16:124-128
  7. Shoemaker, G L, Anderson, J B and Kostiner, E (1977). American Mineralogist 62: 1042 to 1048
  8. The Australian Journal of Mineralogy 3:50, 10:55, 10:79, 11:97 and 11:117 to 118
  9. Rocks & Minerals (2009) 84:6 page 498
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