Mixite

Mixite
Turquoise-blue mixite, Laurium, Greece. Size: 6.8 x 5.0 x 3.6 cm.
General
Category	Arsenate mineral
Formula (repeating unit)	BiCu6(AsO4)3(OH)6·3(H2O)
Strunz classification	8.DL.15
Crystal system	Hexagonal
Crystal class	Dipyramidal (6/m) (same H-M symbol)
Space group	P63/m
Unit cell	a = 13.646(2) Å, c = 5.920(1) Å; Z = 2
Identification
Color	Blue to emerald-green, pale green, white
Crystal habit	Acicular crystals often in radial clusters
Fracture	Uneven
Mohs scale hardness	3-4
Luster	Vitreous
Streak	Pale green
Diaphaneity	Transparent to translucent
Specific gravity	3.79-3.83
Optical properties	Uniaxial (+)
Refractive index	nω = 1.743 - 1.749 nε = 1.810 - 1.830
Birefringence	δ = 0.067
Pleochroism	O = colorless, E = bright green
References

Mixite is a rare copper bismuth arsenate mineral with formula: BiCu₆(AsO₄)₃(OH)₆·3(H₂O). It crystallizes in the hexagonal crystal system typically occurring as radiating acicular prisms and massive encrustations. The color varies from white to various shades of green and blue. It has a Mohs hardness of 3.5 to 4 and a specific gravity of 3.8. It has an uneven fracture and a brilliant to adamantine luster.

It occurs as a secondary mineral in the oxidized zones of copper deposits. Associated minerals include: bismutite, smaltite, native bismuth, atelestite, erythrite, malachite and barite.

It was discovered in 1879 near J´achymov, Czech Republic by mine engineer Anton Mixa. Mixite has also been found in Argentina,Australia,Austria,France,Germany,Greece,Hungary,Italy,Japan,Mexico,Namibia,Poland,Spain,Switzerland, the United Kingdom, and the United States.

Mixite is the namesake member of the mixite mineral group, which has the general chemical formula Cu²⁺₆A(TO₄)₃(OH)₆·3H₂O, where A is a REE, Al, Ca, Pb, or Bi, and T is P or As. In addition to mixite, this mineral group contains the isostructural minerals agardite-(Y),agardite-(Ce),agardite-(Nd),agardite-(La),calciopetersite,goudeyite,petersite-(Ce),petersite-(Y),plumboagardite, and zálesíite.