GREENOCKITE
Greenockite – CdS – is a rare sulfide. It is the only ore mineral of Cd, but Cd is obtained as a by-product in the mining of Pb, Zn, and Cu. Greenockite was used as a yellow pigment until it was recognized that Cd is very toxic.
It is classified in the Wurtzite Group, being dimorphic with hawleyite (cubic), which occurs with the same colors, habits, and parageneses as greenockite. Therefore, it is easy to confuse greenockite with hawleyite and vice versa.
Greenockite may contain Zn, and there is a zinciferous variety that exhibits yellow-orange fluorescence under ultraviolet light. Amorphous CdS under X-rays, lemon-yellow in color, is designated xanthochroite. At high temperatures, greenockite is isostructural with wurtzite, and at low temperatures, it is isostructural with sphalerite.
CAUTION! Because greenockite contains Cd, it is very toxic. Always wash your hands after handling, do not inhale dust when breaking it, never lick or ingest it.
1. Characteristics
Crystal system: Hexagonal, dihexagonal pyramidal.
Color: Yellow to red. Yellowish-green, honey-colored, light yellow, green, brown. Many different colors!
Habit: Hemimorphic pyramidal crystals up to 3 cm. Barrel-shaped crystals. Usually as crusts or earthy masses.
Cleavage: {11-22} distinct, {0001} imperfect.
Tenacity: Brittle.
Twinning: By {11-22}, rare, with 3 or 4 crystals.
Fracture: Conchoidal.
Mohs Hardness: 3 – 3.5
Parting: No.
Streak: Yellow-orange to brick red.
Lustre: Adamantine to resinous.
Diaphaneity: Transparent.
Density (g/cm³): 4.8 – 4.9
2. Geology and Deposits
Greenockite is an uncommon mineral found in sulfide deposits. It forms by alteration of sphalerite and wurtzite, forming earthy crusts over these minerals, especially over sphalerite. May form coatings upon sphalerite, pyrite, franckeite and canfieldite.
It occurs in Zn sulfide deposits, preferentially at the boundary between oxidation zones and the primary ore. Some wurtzites show high Cd contents due to fine rhytmic precipitating of alternating, in part almost submicroscopic, layers of almost pure ZnS and CdS.
It rarely occurs as idiomorphic crystals in cavities of mafic igneous rocks.
It is also found in some high-temperature hydrothermal vein deposits; it may be associated with schalenblende.
It can occur as an alteration of sulfides that replace fossil woods.
3. Mineral Associations
It associates with common minerals (quartz, calcite, barite, fluorite) and various Zn minerals such as sphalerite, smithsonite, hemimorphite, hydrozincite, willemite, and franklinite.
With some Mn minerals such as rhodonite, pyrolusite, and pyroxmangite.
With some Sn minerals such as cassiterite, franckeite, stannite, canfieldite, and herzenbergite.
With sulfides such as pyrite, marcasite, pyrrhotite, chalcopyrite, galena, arsenopyrite, miargyrite, and tetrahedrite.
With prehnite, goethite, low-temperature zeolites, gypsum, hydrocerussite, and Pumpellyite Group minerals such as julgoldite-(Fe2+).
4. Transmitted Light Microscopy
Refraction indices: nω: 2.529 nε: 2.506
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism:Weak pleochroism.
Relief: Very high.
Cleavage: {11-22} distinct and {0001} imperfect.
Habits: Generally earthy crusts. Pyramidal hemimorphic crystals.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: maximum birefringence of 0.023, corresponding to colors up to the end of the first order: gray, yellow, orange, red, and blue.
Extinction: Probably paralell.
Elongation sign: No information available.
Twins: Rare, as “trillings” or “fourlings”, by {11-22}.
Zoning: No information available.
CONVERGENT LIGHT
Character: U(-) for blue-green to blue light and U(+) for red to blue-green light. Isotropic at 523 nm.
2V angle: No.
Alterations: No information available.
May be confused with: Several other minerals. In their identification, even if tentative, considering paragenesis is essential.
5. Reflected Light Microscopy
Reflected Light Microscopy is obviously not the recommended analytical method for identifying greenockite. However, it is important to create a polished slide or section to identify the opaque minerals that occur associated with greenockite, such as pyrite and galena.
Sample preparation: Greenockite takes on an excellent polish, even when it occurs as an earthy material composed of very small crystals; careful impregnation is necessary. Its hardness upon polishing is lower than that of sphalerite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Greenish gray.
Compared to the color of sphalerite, the color of greenockite is very similar, slightly lighter and with a subtle bluish tint.
Pleochroism: Weak to absent.
Reflectivity: 18.61%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: It does not exhibit anisotropy. Internal reflections mask the features.
Internal reflections: Common and abundant, ranging in color from very pale lemon-yellow to reddish-brown and orange, reflecting the wide variation in colors it exhibits macroscopically.
May be confused with: Some other minerals.
Hawleyite is very similar, often confused with greenockite.
Monteponite may look like greenockite.
Chromite is very similar, but occurs in completely distinct parageneses.
General Characteristics:
Cleavage is not observed.
Replacement of sphalerite by greenockite may occur.