Olivenite

Olivenite is a copper arsenate mineral, formula Cu₂AsO₄OH. It crystallizes in the monoclinic system (pseudo-orthorhombic),[1] and is sometimes found in small brilliant crystals of simple prismatic habit terminated by domal faces. More commonly, it occurs as globular aggregates of acicular crystals, these fibrous forms often having a velvety luster; sometimes it is lamellar in structure, or soft and earthy.

Olivenite
Olivenite, Špania Dolina, Slovakia
General
Category	Arsenate minerals
Formula (repeating unit)	Cu2AsO4OH
Strunz classification	8.BB.30
Dana classification	41.06.06.01
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/n
Identification
Color	Olive green to yellow or brown, gray-green, grayish white
Crystal habit	Fibrous, globular and reniform; granular, earthy, massive
Twinning	On {010}
Cleavage	{101}, {110}, indistinct
Fracture	Conchoidal to irregular
Mohs scale hardness	3
Luster	Adamantine to vitreous, pearly to silky if fibrous
Streak	Olive-green to brown
Diaphaneity	Translucent to opaque
Specific gravity	4.46
Optical properties	Biaxial (+/-)
Refractive index	nα = 1.747 - 1.780 nβ = 1.788 - 1.820 nγ = 1.829 - 1.865
Birefringence	δ = 0.082 - 0.085
Pleochroism	Weak green and yellow
2V angle	Measured: 80° to 90°, Calculated: 46° to 84°
Dispersion	strong r > v or r < v
References	[1][2][3]

A characteristic feature, and one to which the name alludes (German, Olivenerz, of A. G. Werner, 1789), is the olive-green color, which varies in shade from blackish-green in the crystals to almost white in the finely fibrous variety known as woodcopper. The hardness is 3, and the specific gravity is 4.3. The mineral was formerly found in some abundance, associated with limonite and quartz, in the upper workings in the copper mines of the St Day district in Cornwall; also near Redruth, and in the Tintic Mining District in Utah. It is a mineral of secondary origin, a result of the oxidation of copper ores and arsenopyrite.

Olivenite from Mammoth Mine, Tintic, Utah

The arsenic of olivenite is sometimes partly replaced by a small amount of phosphorus, and in the species libethenite we have the corresponding copper phosphate Cu₂PO₄OH. This is found as small dark green crystals resembling olivenite at Ľubietová in the Slovak Republic, and in small amount also in Cornwall. Other members of this isomorphous group of minerals are adamite, Zn₂AsO₄OH, and eveite, Mn₂AsO₄OH.