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.

Pseudomalachite
General
Category	Phosphate minerals
Formula (repeating unit)	Cu5(PO4)2(OH)4
IMA symbol	Pmlc
Strunz classification	8.BD.05
Dana classification	41.04.03.01
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/c
Unit cell	a = 4.47 Å, b = 5.75 Å, c = 17.05 Å; β = 91.06°; Z = 2
Identification
Formula mass	575.7 g/mol
Color	Dark emerald green to blackish green
Crystal habit	Crystals, 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.
Twinning	On {100}
Cleavage	Perfect on {100}, distinct on {010}
Fracture	Splintery or conchoidal
Mohs scale hardness	4.5–5
Luster	Vitreous
Streak	Blue green, paler than the mineral
Diaphaneity	Translucent to subtranslucent
Specific gravity	(Measured) 4.15 to 4.35
Optical properties	Biaxial (−)
Refractive index	nα = 1.791 nβ = 1.856 nγ = 1.867
Birefringence	δ = 0.076
Pleochroism	Weak; X = bluish green to pale green; Y = yellowish green; Z = deep bluish green to blue-green
2V angle	48°
Dispersion	Strong r<v. Also biaxial (+) r>v
Solubility	Soluble in acids but without effervescence (in contrast to malachite which effervesces with warm HCl)
Other characteristics	Not fluorescent, not radioactive
References

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. The type material is held at the Mining Academy, Freiberg, Germany.

Structure

The copper ions are co-ordinated by six oxygen ions to form distorted octahedra. 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.

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, and at the Chino Mine, New Mexico, US. 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.