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Chalcophyllite
Chalcophyllite
	Chalcophyllite from the Ting Tang Mine, Carharrack, Cornwall, England. Photo by Rob Lavinsky
General
Category	Arsenate minerals
Formula; (repeating unit)	Cu; 18Al; 2(AsO; 4); 4(SO; 4); 3(OH); 24·36(H; 2O) or; Cu; 18Al; 2(AsO; 4); 3(SO; 4); 3(OH); 27·33(H; 2O)
IMA symbol	Chp
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

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^[2] (not to be confused with the amphibole mineral taramite, which is quite different). At Wheal Gorland a specimen exhibiting partial replacement of liriconite, Cu
₂Al(AsO
₄)(OH)
₄·(4H
₂O), by chalcophyllite has been found.^[2] 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.

Formula

Two different formulae are quoted in the literature for chalcophyllite, Cu
₁₈Al
₂(AsO
₄)
₄(SO
₄)
₃(OH)
₂₄·36(H
₂O)^[1]^[2]^[3] (molar mass 3098 g) and Cu
₁₈Al
₂(AsO
₄)
₃(SO
₄)
₃(OH)
₂₇·33(H
₂O)^[4]^[5]^[6] (molar mass 2956 g). The difference reflects the fact that the water content varies at room temperature based on relative humidity.^[4]

Unit cell

Chalcophyllite crystallizes in the trigonal crystal class, 3 2/m with space group R 3m^[5] or 3 with space group R 3.^[2]^[3]^[4]^[6] 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.^[5]^[6]^[9] For the smaller unit cell c is only half as long, a = 10.756 Å, c = 28.678 Å, Z = 1.5.^[3]^[4]

Crystal habit

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.^[4]

Physical properties

Cleavage is perfect perpendicular to the c crystal axis, and twinning occurs with {1010} as the twin plane. Fracture is irregular. The mineral is soft, with hardness only 2, the same as gypsum. Specific gravity is generally given in the range 2.67 to 2.69, but Webmineral has 2.4 to 2.66. Chalcophyllite is soluble in acids and in ammonia. It is not fluorescent, nor radioactive. The water content varies at room temperature based on relative humidity. Chalcophyllite alters to chrysocolla,^[4] which is a copper-aluminium silicate with the formula (Cu,Al)
₂H
₂Si
₂O
₅(OH)
₄·n(H
₂O).

Optical properties

Many copper minerals are blue or green in color; chalcophyllite is blue-green to emerald-green, with a pale green streak and vitreous to subadamantine luster, pearly on {0001}. Crystals are transparent to translucent. It is uniaxial (−) with refractive indices n_ω = 1.618 to 1.632 and n_ε = 1.552 to 1.575. Indices of refraction vary markedly depending on the relative humidity since the water content varies at ambient temperature.^[4] It is pleochroic with O blue-green and E almost colorless.