greenockite

[After Charles Murray Cathcart, Second Earl Greenock (1783-1859), British soldier.]

A mineral having composition CdS (cadmium sulfide). Greenockite usually occurs as earthy coatings with resinous luster and yellow-to-orange color. There is good prismatic cleavage; the hardness is 3 (Mohs scale) and specific gravity is 4.9. Greenockite and wurtzite, ZnS, are isostructural, and a complete solid-solution series exists between the two minerals. Although greenockite is the most common cadmium mineral, no deposits of it are sufficiently large to warrant mining it solely as a source of cadmium. See also Cadmium.

Environment

In traprock cavities and in ore veins.

Crystals small to minute, complex, and interesting as examples of an infrequent symmetry class: hemimorphic hexagonal. Crystals are very rare, however; greenockite usually manifests as a yellow, pollenlike dusting over other minerals, especially sphalerite and calcite and even in smithsonite, staining it yellow.

Yellow to brown or red. Luster adamantine to resinous; hardness 3-3Ɖ; specific gravity 4.9-5.0; streak yellow; fracture conchoidal; cleavage good prismatic and poor basal. Brittle; transparent to translucent.

Cadmium sulfide (77.8% Cd, 22.2% S).

In closed glass tube the yellow powder turns red when hot and back to yellow or brown when cool. Gives a reddish brown coating on charcoal in the reducing flame. Soluble in hydrochloric acid, releasing hydrogen sulfide gas (rotten-egg smell).

Likely to be mistaken for sphalerite when in crystals, but can be distinguished by its crystal form and by the closed-tube test. The yellow films might be confused with uranium minerals, but the association with zinc minerals should suffice for correct identification.

Crystals are very rare, and were first found at Greenock, Scotland, in cavities in traprock, associated with prehnite. Even the largest are not much over ƈ in. (6 mm) long. A few distorted crystals have been found in the Paterson, New Jersey, traprock area. Orange-red microscopic crystals have been described from Llallagua, Bolivia, associated with pyrite and tin ores. Yellow films are common in the Joplin District (Missouri, Kansas, and Oklahoma) and in the Illinois-Kentucky fluorite region.

It is the only "ore" of cadmium, but that metal is actually recovered only as a by-product of lead and zinc refining. The cadmium is separated in the purification of the other metals. Because of its hemimorphic symmetry, unusual electronic properties make laboratory manufacture of some interest.

Greenockite
General
Category	mineral
Chemical formula	CdS
Identification
Molar 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)
Crystal system	Hexagonal (6mm) Space Group: P 63mc
Cleavage	[1120] Distinct, [0001] Imperfect
Fracture	Uneven - Flat surfaces (not cleavage) fractured in an uneven pattern.
Mohs Scale hardness	3.5-4
Luster	Adamantine - Resinous
Streak	orange yellow
Specific gravity	3.98 - 5, Average = 4.49
Optical properties	Uniaxial, a=2.506, b=2.529, bire=0.0230
Other characteristics	Nonmagnetic, non-radioactive

Greenockite is a rare cadmium mineral that consists of cadmium sulfide, CdS, in crystalline form. Greenockite crystallizes in the hexagonal system. It occurs as massive encrustations and as 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 4.5 to 5.

Greenockite occurs with other sulfide minerals such as sphalerite and galena. It is the only ore mineral of cadmium, most cadmium is recovered as a byproduct of zinc and lead mining.

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. It is also known from the lead-zinc districts of the central United States.

Crystal structure of greenockite

References

• Mindat
• Webmineral
• Mineral galleries
• Glasgow museum site

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