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Carbonates

Carbonates are minerals combined with carbon and oxygen, including calcite, dolomite, smithsonite, malachite, and cerrusite.

Calcite, Aragonite, Travertine

Crystal system: hexagonal. Cleavage: three-directional in rhombohedron-shaped fragments. Color: usually white, but may be tinted various colors by impurities. Hardness: 3.0. Luster: glassy. Specific gravity: 2.71. Acid test: effervesces strongly when treated with dilute hydrochloric acid. Uses: in the form of limestone, as aggregate for road surfacing and concrete, agricultural lime, flux, building stone, sulfur sorbents, and in cement; in the form of Iceland spar in optical instruments to obtain polarized light.

Calcite is one of the most abundant and widespread minerals in Kentucky. It may occur in a multitude of crystal forms, of which scalenohedrons and rhombohedrons are the most common; it may also be granular, massive, earthy, or fibrous. Calcite is the chief mineral in limestone, where it is generally in the form of small, broken fragments of fossil shells. Large, well-developed crystals are generally found in veins, commonly associated with faults. Numerous quarries throughout the State contain various nodules and geodes with calcite crystals. The Lexington Quarry Company in Nicholasville and the Caldwell Stone Company in Danville have discovered numerous scalenohedral calcite crystals along veins in the quarries. Vein calcite is usually associated with fluorite, barite, galena, and sphalerite. Calcite often appears as twinned crystals.

Iceland spar, a clear, pure, colorless variety of calcite, has been found at the Twin Chimneys Mine in Mercer County; other localities near Mundys Landing and the Chinn Mine might contain optical calcite as well.

Aragonite is a polymorph of calcite and is distinguished from calcite by a higher specific gravity (2.95) and hardness (3.5 to 4), and a lack of rhombohedral cleavage. Aragonite usually occurs as radiating crystals and is commonly found in reniform, columnar, and stalactitic forms.

Travertine, often called cave onyx, has properties similar to calcite. It is usually a shade of yellow or brown, and exhibits banding because of intermittent deposition and iron oxide impurities. Travertine is redeposited calcium carbonate. The best known deposits in Kentucky are in caves near the Mammoth Cave region. It also occurs as a surface deposit associated with ground-water seeps and springs, and may be collected around crevices in most limestone quarries. An attractive deposit of travertine may be seen at Elk Lick Falls in Fayette County, where it forms the impressive Petrified Falls. Travertine may be found in stalactitic, stalagmitic, columnar, nodular, encrusting, and many other forms. Travertine is the chief scenic attraction of caves and it should not be destroyed, because it grows very slowly.

Dolomite, Ankerite, Barytocalcite

Crystal system: hexagonal. Cleavage: rhombohedral with curved or saddle-shaped faces. Color: white or pink, rarely gray, green, yellow, or black. Hardness: 3.5-4.0. Streak: colorless. Luster: glassy or pearly. Specific gravity: 2.85. Acid test: effervesces slowly in cold dilute hydrochloric acid.

Dolomite is the chief mineral in dolostone. It is also associated with calcite, galena, sphalerite, barite, and fluorite. Dolomite is found in nodules or geodes in some limestone quarries and in some of the vein deposits in the State.

Iron substitutes for magnesium to form ankerite (cleavage: rhombohedral; color: yellowish brown; hardness: 3.5; specific gravity: 2.9), and barium substitutes for magnesium to form barytocalcite (crystal system: monoclinic; color: white to greenish; hardness: 4; specific gravity: 3.67). Barytocalcite is a polymorph of alstonite and may fluoresce under ultraviolet light. Its high specific gravity makes it feel heavy, and it may resemble "heavy calcite." Barytocalcite has been observed in some mineral veins in Cumberland County.

Cerussite

Crystal system: orthorhombic. Cleavage: orthorhombic crystals are commonly tabular, prismatic, and pyramids; good cleavage in four directions. Color: white when pure, but often gray to very pale blue. Hardness: 3.25. Streak: white. Luster: resinous to adamantine. Specific gravity: 6.5. Tenacity: brittle. Acid test: effervesces in nitric acid. Uses: lead ore.

Cerussite is distinguished from other carbonates by its high specific gravity; it is distinguished from anglesite (PbSO4) by effervescing in nitric acid and by its crystal form. It also occurs in fibrous, granular, and earthy forms. It was formed by the reaction of carbonated water on galena. It occurs in noncommercial quantities near the Big Four Mines and Dike Eaton area in the Western Kentucky Fluorspar District. Cerussite has also been observed in vein deposits in north-central Kentucky. It is associated with galena, sphalerite, anglesite, limonite, and smithsonite.

Malachite, Azurite

Crystal system: monoclinic. Cleavage: perfect, but commonly massive. Color: green (malachite), blue (azurite). Hardness: 3.5-4.

Malachite is rare in Kentucky, but does occur as microscopic grains or green coatings as a weathering/oxidation product of chalcopyrite in some vein deposits in central and western Kentucky. Azurite is extremely rare in Kentucky, although it could have the same mode of occurrence as malachite.

Aurichalcite, a zinc-copper carbonate associated with smithsonite, also occurs in oxidized zinc/copper deposits with malachite and azurite. It is distinct because of its acicular blue crystals. Aurichalcite from a deep well in western Kentucky represents the only known occurrence of the mineral in Kentucky. It was formed from the oxidation of drill steel in cuttings from the well.

Iron and Other Carbonates

Smithsonite, Hydrozincite

Crystal system: hexagonal. Cleavage: perfect. Fracture: uneven. Color: white to greenish brown. Hardness: 5.0. Streak: white. Luster: glassy. Specific gravity: 4.37. Acid test: effervesces rapidly in dilute hydrochloric acid.

Smithsonite occurs usually in reniform, botryoidal, and stalactitic forms and as crystalline encrustations. It is formed by the reaction of carbonated water on sphalerite, and occurs in calcareous rocks associated with galena, sphalerite, cerussite, limonite, fluorite, and calcite. It is distinguished from most other carbonates by its greater hardness and weight (it is distinctly harder than cerussite, but lighter in weight).

Another zinc carbonate, hydrozincite (crystal system: monoclinic; color: white to gray; hardness: 2.5; specific gravity: 3.5), is a massive mineral that forms as earthy, compact crusts in oxidized zinc deposits. It commonly will luminesce blue under ultraviolet light.

One of the better occurrences of smithsonite and hydrozincite is at the Old Jim Mine in Crittenden County, where the deposit is associated with an igneous intrusive dike. Smithsonite can be found at numerous mine sites in western Kentucky and has been observed at the Shrylock Ferry Vein in Woodford County in central Kentucky.

Strontianite

Crystal system: orthorhombic. Cleavage: good. Color: white to gray to green. Hardness: 3.5. Luster: vitreous. Specific gravity: 3.7. Acid test: effervesces in dilute hydrochloric acid. Uses: fireworks, flares, rockets.

Strontianite generally occurs as tufts and radiating acicular crystals. It also occurs as columnar or fibrous crystals. It gives off a crimson flame on ignition, and occurs in hydrothermal veins with barite and celestite. It is found in the north-central part of the Central Kentucky Mineral District in Franklin and Scott Counties. Strontianite balls have been observed in the Caldwell Stone Company Quarry.

Witherite

Crystal system: orthorhombic. Cleavage: indistinct. Fracture: uneven. Color: white to pale yellow. Hardness: 3.5. Streak: white. Luster: glassy. Specific gravity: 4.3. Tenacity: brittle. Acid test: effervesces rapidly in dilute hydrochloric acid.

Witherite is a relatively rare mineral, but is found in central and western Kentucky, where it occurs in veins associated with barite and galena. Elsewhere a minor source of barium, it is noncommercial in Kentucky.