Rosasite is a carbonate mineral with minor potential for use as a zinc and copper ore. Chemically, it is a copper zinc carbonate hydroxide with a copper to zinc ratio of 3:2, occurring in the secondary oxidation zone of copper-zinc deposits. It was originally discovered in 1908 in the Rosas mine in Sardinia, Italy, and is named after the location. Fibrous blue-green rosasite crystals are usually found in globular aggregates, often associated with red limonite and other colorful minerals. It is very similar to aurichalcite, but can be distinguished by its superior hardness.

Rosasite
Blue velvety mass lining a cavity
General
Category	Carbonate mineral
Formula (repeating unit)	(Cu,Zn)2(CO3)(OH)2
IMA symbol	Rss[1]
Strunz classification	5.BA.10
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/a
Unit cell	a = 12.873(3) Å, b = 9.354(3) Å c = 3.156(2) Å; β = 110.36(3)°; Z = 4
Identification
Color	Blue, bluish green, green
Crystal habit	Acicular crystals as radiating fibrous clusters; botryoidal; mammillary; encrustations
Twinning	On {100}
Cleavage	Perfect on {100} and {010}
Fracture	Splintery, fibrous
Tenacity	Brittle
Mohs scale hardness	4
Luster	Silky, vitreous to dull
Streak	Light blue or green
Specific gravity	4–4.2
Optical properties	Biaxial (−)
Refractive index	nα = 1.672 – 1.688 nβ = 1.796 – 1.830 nγ = 1.811 – 1.831
Birefringence	δ = 0.139 – 0.143
Pleochroism	Strong: X = pale emerald green or colourless; Y = dark emerald green or pale blue; Z = dark emerald green or pale blue
2V angle	Measured: 33°
Solubility	Effervesces in cold, dilute hydrochloric acid
References	[2][3][4]
Major varieties
Nickeloan rosasite	Dark green