Malachite

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Malachite
Malachite from the Democratic Republic of the Congo
General
Category	Carbonate mineral
Formula (repeating unit)	Cu2CO3(OH)2
IMA symbol	Mlc[1]
Strunz classification	5.BA.10
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/a
Identification
Formula mass	221.1 g/mol
Color	Bright green, dark green, blackish green, with crystals deeper shades of green, even very dark to nearly black commonly banded in masses; green to yellowish green in transmitted light
Crystal habit	Massive, botryoidal, stalactitic, crystals are acicular to tabular prismatic
Twinning	Common as contact or penetration twins on {100} and {201}. Polysynthetic twinning also present.
Cleavage	Perfect on {201} fair on {010}
Fracture	Subconchoidal to uneven
Mohs scale hardness	3.5–4
Luster	Adamantine to vitreous; silky if fibrous; dull to earthy if massive
Streak	light green
Diaphaneity	Translucent to opaque
Specific gravity	3.6–4
Optical properties	Biaxial (–)
Refractive index	nα = 1.655 nβ = 1.875 nγ = 1.909
Birefringence	δ = 0.254
References	[2][3][4][5]

Malachite is a copper carbonate hydroxide mineral, with the formula Cu₂CO₃(OH)₂. This opaque, green-banded mineral crystallizes in the monoclinic crystal system, and most often forms botryoidal, fibrous, or stalagmitic masses, in fractures and deep, underground spaces, where the water table and hydrothermal fluids provide the means for chemical precipitation. Individual crystals are rare, but occur as slender to acicular prisms. Pseudomorphs after more tabular or blocky azurite crystals also occur.^[5]