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Sulfides

Sulfide minerals, such as sphalerite, galena, pyrite, and millerite, contain elements combined with the element sulfur.


Galena

Crystal system: isometric. Cleavage: perfect cubic, although octahedrons are common as truncations to the cube. Color: silver to lead gray. Hardness: 2.5. Streak: grayish black. Luster: bright metallic. Specific gravity: 7.5. Tenacity: sectile. Uses: batteries, gasoline, type metal, solder, low-fusion alloys, telephone and telegraph cable coverings, protective shieldings against radiation.

Galena is the chief ore of lead. It is recovered as a by-product of the fluorspar industry in western Kentucky. It has been recovered from the Columbia Mines in Crittenden County and Silver and Royal Mines in Livingston County. It also occurs to a limited extent in the central Kentucky vein system, where it used to be mined near Gratz in Owen County, and in vein deposits in Scott and Franklin Counties. In both of these areas galena crystals can be found with barite and sphalerite crystals. Galena also occurs with calcite, dolomite, fluorite, and pyrite.


Pyrite, Chalcopyrite, Bornite, Marcasite

Crystal system: isometric. Cleavage: when present, crystals usually cube-shaped; crystal faces commonly marked by parallel lines called striations. Color: pale brass yellow to golden yellow. Hardness: 6.0-6.5. Streak: brown to black. Luster: metallic. Specific gravity: 5.0. Tenacity: very brittle. Uses: sulfuric acid.


When scratched or crushed, pyrite has a sulfury smell


Pyrite has a brown to black streak (white unglazed tile is a streakplate)

Because of its brassy-yellow color, pyrite is often mistaken for gold; hence its common name, "fool's gold." In Kentucky, pyrite and marcasite are very common in several different rock types and in geodes, especially in strata associated with coal beds and in the Chattanooga Shale. Cubic pyrite and pyritohedrons can be collected from coal mines, and pyrite cubes occur with millerite at Halls Gap in Lincoln County. Pyrite can be massive with no crystals visible, or it can have euhedral cubic crystals. In some states pyrite can be mined, but there are no deposits large enough to be mined in Kentucky.

Pyrite occurs frequently with chalcopyrite, bornite, and marcasite. Chalcopyrite, also known as fool's gold, is commonly tarnished and iridescent. Bornite is known as "peacock ore" because it is very iridescent and produces a pleochroic array of colors.

Marcasite crystals have a pale-bronze color, hardness of 6.0 to 6.5, and a tabular cleavage that resembles cockscombs or spearheads. Marcasite, also called white iron pyrite, readily tarnishes yellow to brown and decomposes to a white powder. Immersion in epoxy will retard the decomposition process.


Millerite, Honessite

Crystal system: hexagonal; individual crystals can be twisted, spiraled, or offset along the crystal axis. Cleavage: usually does not occur in cleavable masses. Color: brass yellow. Hardness: 3.5. Luster: metallic. Specific gravity: 5.5.

Millerite is an acicular (needle-like) mineral that occurs in interwoven groups of radiating crystals, usually in geodes and cavities. Millerite hydrates (water is incorporated into its chemical composition) to honessite(NiS·H2O), which has the same mode of occurrence as millerite, but is commonly green to brown and acicular. Capillary pyrite has also been associated with millerite.

Since the early 1960's, millerite from Halls Gap near Stanford in Lincoln County has been a prized collector's item. Most mineral books have pictures of millerite, honessite, and capillary pyrite collected from the Halls Gap locality. Millerite and honessite occur in the Wildie Member of the Borden Formation. Anatase has also been reported (Alan Goldstein, 1992, personal communication) in the upper section of the Halls Gap locality. Most millerite geodes are small, but some are as big as softballs.

Pyrite cubes and calcite crystals have also been observed attached to millerite hairs, and a pyrite ring was observed in one geode.


Sphalerite, Greenockite

Crystal system: isometric. Color: yellowish brown to black. Hardness: 3.5-4.0. Streak: pale yellowish brown. Luster: resinous to adamantine. Specific gravity: 4.0. Tenacity: brittle. Uses: galvanizing iron, brass, die castings; associated cadmium used for batteries and in the electronics industry. Associated minerals: germanium and gallium used to produce transistors, diodes, and rectifiers.

Sphalerite, is also called zinc blende or blackjack and is a common vein-forming mineral in numerous mines in central and western Kentucky, where it has been mined. It is predominantly associated with galena, but also occurs with barite, fluorite, calcite, pyrite, and chalcopyrite.

Sphalerite has been mined from the Hutson Mine in Livingston County, and the Lexington Quarry Company has mined sphalerite from a vein deposit in Jessamine County for several years, producing very fine, dark-brown to black, tetrahedral crystals. The sphalerite there is associated with calcite, barite, and gypsum.

Cadmium, a trace element commonly found in sphalerite, forms the mineral greenockite. Greenockite occurs as orange or yellow crystals or coatings associated with sphalerite. Germanium and gallium, also trace elements in sphalerite, are produced as by-products of sphalerite in the fluorspar area in western Kentucky.