Aurichalcite

For the fabulous metal, mentioned in the legend of the lost Atlantis, see Orichalcum.

Aurichalcite is a carbonate mineral, usually found as a secondary mineral in copper and zinc deposits. Its chemical formula is (Zn,Cu)₅(CO₃)₂(OH)₆. The zinc to copper ratio is about 5:4.^[3] Copper (Cu²⁺) gives aurichalcite its green-blue colors.^[5]

Aurichalcite
General
Category	Carbonate mineral
Formula	(Zn,Cu)5[(OH)3|CO3]2
IMA symbol	Ach[1]
Strunz classification	5.BA.15
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/m
Unit cell	a = 13.82, b = 6.419 c = 5.29 [Å] β = 101.04°; Z = 2
Identification
Color	Pale green, greenish blue, light blue; colorless to pale blue, pale green in transmitted light
Crystal habit	Typically in tufted divergent sprays or spherical aggregates, may be in thick crusts; rarely columnar, laminated or granular
Twinning	Observed in X-ray patterns
Cleavage	{010} and {100} Perfect
Fracture	Uneven
Mohs scale hardness	2
Luster	Pearly, silky
Streak	Light blue
Diaphaneity	Transparent
Specific gravity	3.96
Optical properties	Biaxial (−)
Refractive index	nα = 1.655 nβ = 1.740 nγ = 1.744
Birefringence	0.0890
Pleochroism	Weak colorless to pale green
2V angle	Measured: 1° to 4°, Calculated: 22°
References	[2][3][4]

Occurrence

Aurichalcite typically occurs in the oxidized zone of copper and zinc deposits. Associated minerals include: rosasite, smithsonite, hemimorphite, hydrozincite, malachite and azurite.^[2]

It was first described in 1839 by Bottger who named the mineral for its zinc and copper content after the Greek όρειχαλκος, for "mountain brass" or "mountain copper", the name of orichalcum, a fabulous metal, mentioned in the legend of the mythic lost continent Atlantis. The type locality is the Loktevskoye Mine, Upper Loktevka River, Rudny Altai [ru], Altai Krai, Western Siberia, Russia.^[3]

Crystallography

Aurichalcite displays prismatic crystals often in the form of encrustations and sometimes columnar structures.^[6] The crystal system is monoclinic.