Greenockite

Greenockite is a rare cadmium bearing metal sulfide mineral consisting of cadmium sulfide (CdS) in crystalline form. Greenockite crystallizes in the hexagonal system. It occurs as massive encrustations and as hemimorphic six-sided pyramidal crystals which vary in color from a honey yellow through shades of red to brown. The Mohs hardness is 3 to 3.5 and the specific gravity is 4.8 to 4.9.

Greenockite
Greenockite crystals from Tsumeb Mine, Namibia (Picture width 1 mm)
General
Category	Sulfide mineral
Formula (repeating unit)	CdS
Strunz classification	2.CB.45
Crystal system	Hexagonal
Crystal class	Dihexagonal pyramidal (6mm) (same H-M symbol)
Space group	P63mc
Unit cell	a = 4.136 c = 6.713 [Å]; Z = 2
Structure
Jmol (3D)	Interactive image
Identification
Formula mass	144.48
Color	Honey yellow, citron yellow, orange yellow
Crystal habit	Colloform - forming from a gel or colloidal mass; encrustations - forms crust-like aggregates on matrix; radial - crystals radiate from a center without producing stellar forms (e.g. stibnite)
Twinning	Rare on {1122} forming trillings
Cleavage	Distinct on {1122}, imperfect on {0001}
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	3.0-3.5
Luster	Adamantine to resinous
Streak	Yellow orange to brick red
Diaphaneity	Nearly opaque to translucent
Specific gravity	4.8 - 4.9
Optical properties	Uniaxial (-)
Refractive index	nω = 2.529 nε = 2.506
Birefringence	δ = 0.023
Pleochroism	Weak
Ultraviolet fluorescence	yellow-orange fluorescence under UV when rich in zinc
Other characteristics	Toxic
References	[1][2][3]

Greenockite belongs to the wurtzite group and is isostructural with it at high temperatures. It is also isostructural with sphalerite at low temperatures. It occurs with other sulfide minerals such as sphalerite and galena, and is the only ore mineral of cadmium. Most cadmium is recovered as a byproduct of copper, zinc, and lead mining. It is also known from the lead-zinc districts of the central United States.

It was first recognized in 1840 in Bishopton, Scotland, during the cutting of a tunnel for the Glasgow, Paisley and Greenock Railway. The mineral was named after the land owner Lord Greenock (1783–1859).[2][4]