Chalcanthite
General
Category	Sulfate mineral
Formula (repeating unit)	CuSO4·5H2O
IMA symbol	Cct[1]
Strunz classification	7.CB.20
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	P1
Unit cell	a = 6.11 Å, b = 10.673 Å, c = 5.95 Å; α = 97.58°, β = 107.17°, γ = 77.55°; Z = 2
Identification
Color	Berlin blue to sky blue, greenish blue
Crystal habit	Typically stalactitic, encrusted, reniform or massive. Natural crystals are rare, but are short prismatic or tabular
Twinning	Rare as cruciform twins
Cleavage	Perfect on {110}; interrupted on {110}
Fracture	Conchoidal
Mohs scale hardness	2.5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.12–2.3
Optical properties	Biaxial (−)
Refractive index	nα = 1.514 nβ = 1.537 nγ = 1.543
Birefringence	δ = 0.029
2V angle	Measured: 56°
Solubility	Soluble in water, turning it blue
Other characteristics	Poisonous
References	[2][3][4]

Chalcanthite (from Ancient Greek χάλκανθον (khálkanthon), from χαλκός (khalkós) 'copper', and ἄνθος (ánthos) 'flower, bloom') is a richly colored blue-green water-soluble sulfate mineral CuSO₄·5H₂O. It is commonly found in the late-stage oxidation zones of copper deposits. Due to its ready solubility, chalcanthite is more common in arid regions.

Chalcanthite is a pentahydrate and the most common member of a group of similar hydrated sulfates, the chalcanthite group. These other sulfates are identical in chemical composition to chalcanthite, with the exception of replacement of the copper ion by either manganese as jokokuite, iron as melanterite, or magnesium as pentahydrite.^[5]

Other names include blue stone, blue vitriol, and copper vitriol.^[3]

Uses of chalcanthite

As chalcanthite is a copper mineral, it can be used as an ore of copper. However, its ready solubility in water means that it tends to crystallize, dissolve, and recrystallize as crusts over any mine surface in more humid regions. Therefore, chalcanthite is only found in the most arid regions in sufficiently large quantities for use as an ore.

Secondarily, chalcanthite, due to its rich color and beautiful crystals, is a sought after collector's mineral. However, as with its viability as an ore, the solubility of the mineral causes significant problems. First, the mineral readily absorbs and releases its water content, which, over time, leads to a disintegration of the crystal structure, destroying even the finest specimens. It is critical to store specimens properly to limit exposure to humidity. Second, higher quality crystals can be easily grown synthetically, and, as such, there is a concern that disreputable mineral dealers would present a sample as natural when it is not.

Associated minerals

Given that chalcanthite is found in oxidized copper deposits, it is frequently found in association with other copper minerals. Frequently associated minerals include:

• Calcite and its polymorph, aragonite, both CaCO₃
• Brochantite, Cu₄(SO₄)(OH)₆
• Chalcopyrite, CuFeS₂
• Malachite, Cu₂(CO₃)(OH)₂
• Melanterite, FeSO₄ · 7 H₂O

Notes for identification

Chalcanthite's blue color is one of its most notable features, but it is insufficient in identification. Other useful tests include associated minerals, crystal habit, solubility and subsequent coloring of the water blue. Chalcanthite can also dye materials blue when dissolved in water, and has a peculiarly sweet and metallic taste, although consuming it can induce dangerous copper poisoning.