Chalcophyllite

Chalcophyllite
Chalcophyllite from the Ting Tang Mine, Carharrack, Cornwall, England. Photo by Rob Lavinsky
General
Category	Arsenate minerals
Formula (repeating unit)	Cu18Al2(AsO4)4(SO4)3(OH)24·36(H2O) or Cu18Al2(AsO4)3(SO4)3(OH)27·33(H2O)
Strunz classification	8.DF.30
Dana classification	43.5.14.1
Crystal system	Trigonal
Crystal class	Rhomboedral (3) H-M symbol: (3)
Space group	R3
Identification
Color	Blue-green to emerald-green
Crystal habit	Crystals platy and six-sided, also as rosettes, drusy, foliated or massive.
Twinning	On {1010} as twin plane.
Cleavage	Perfect on {0001}
Fracture	Irregular
Mohs scale hardness	2
Luster	Vitreous to subadamantine, pearly on {0001}
Streak	Pale green
Diaphaneity	Transparent to translucent
Specific gravity	2.67 to 2.69 or 2.4 to 2.66
Optical properties	Uniaxial (-)
Refractive index	nω = 1.618 to 1.632, nε = 1.552 to 1.575
Pleochroism	O blue-green, E almost colorless
Solubility	Soluble in acids and in ammonia.
Other characteristics	Can be partially dehydrated. Alters to chrysocolla. Nonfluorescent, not radioactive
References

Cu₁₈Al₂(AsO₄)₄(SO₄)₃(OH)₂₄·36(H₂O) or

Chalcophyllite is a rare secondary copper arsenate mineral occurring in the oxidized zones of some arsenic-bearing copper deposits. It was first described from material collected in Germany. At one time chalcophyllite from Wheal Tamar in Cornwall, England, was called tamarite, but this name is now discredited (not to be confused with the amphibole mineral taramite, which is quite different). At Wheal Gorland a specimen exhibiting partial replacement of liriconite, Cu₂Al(AsO₄)(OH)₄·4(H₂O), by chalcophyllite has been found. The mineral is named from the Greek, chalco "copper" and fyllon, "leaf", in allusion to its composition and platy structure. It is a classic Cornish mineral that can be confused with tabular spangolite.

Two different formulae are quoted in the literature for chalcophyllite, Cu₁₈Al₂(AsO₄)₄(SO₄)₃(OH)₂₄·36(H₂O) (molar mass 3098 g) and Cu₁₈Al₂(AsO₄)₃(SO₄)₃(OH)₂₇·33(H₂O) (molar mass 2956 g). The difference reflects the fact that the water content varies at room temperature based on relative humidity.

Chalcophyllite crystallizes in the trigonal crystal class, 3 2/m with space group R 3m or 3 with space group R 3. Some authors choose a unit cell with three formula units per cell (Z=3), and some a smaller unit cell with only 1.5 formula units per cell (Z=1.5). For the larger unit cell a = 10.77 Å, c = 57.5 Å, Z=3. For the smaller unit cell c is only half as long, a = 10.756 Å, c = 28.678 Å, Z = 1.5.

Crystals are platy, six-sided and flattened perpendicular to the c crystal axis, and may be striated triangularly on these flattened faces. It may form rosettes, or be drusy, foliated or massive.