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KGS Home > Rocks and Minerals
Oxides

Oxides are elements combined with oxygen. Oxide minerals are:

Goethite

Crystal system: orthorhombic. Color: yellowish brown to dark brown. Hardness: 5.0-5.5. Streak: brownish-yellow. Luster: dull to adamantine. Specific gravity: 4.37.

Goethite usually occurs in massive, stalactitic, fibrous, or botryoidal (grape-like) forms, and exhibits radial growth. Goethite weathers from pre-existing iron minerals. In western Kentucky, goethite is associated with limonite in porous earthy masses called bog iron ore. Goethite is an ore of iron in some areas; it is noncommercial in Kentucky.

Hematite

Crystal system: hexagonal. Cleavage: indistinct. Color: brownish red, steel gray, or black. Hardness: 5.5-6.0. Streak: cherry red. Luster: dull to metallic, opaque. Specific gravity: 5.2.

Hematite occurs in platy, compact, granular, or earthy masses. When individual grains are red and about the size and roundness of fish eggs, the mineral is called oolitic hematite; when it occurs in plates or scales as mica does and is steel gray it is called micaceous or specular hematite; and when it occurs as a fine powder that can be mixed with oil and used as a paint pigment, it is called red ocher. Hematite is often the red cementing material in sandstones, and is found to a lesser extent in red clays and shales. It is readily distinguished from all other minerals by its cherry red streak.


Specular hematite is the sparkling silver-gray band (arrows) in this limonite concretion.

Hematite is the most important ore of iron, and also the most important metallic mineral in the world. The value of the pig iron and steel manufactured from it every year exceeds the value of any other metal.

During the late nineteenth century, oolitic hematite was mined in Bath County in northeastern Kentucky. In this area, hematite is found in the Brassfield Dolomite of Silurian age and the Boyle Limestone of Devonian age. The average thickness of the iron-bearing beds is 3 feet, and the higher grade ore ranges in iron content from 46 to 57 percent. Iron ore of this type occurs in Silurian rocks from New York to Alabama. Because of its limited thickness in Kentucky, and because of vast supplies of hematite ore in the Great Lakes region, it became unprofitable to mine in Kentucky.

Hematite crystals have been observed lining the cavities of geodes from Estill County. These crystals are dark brown, bladed, and approximately ∓frac12; inch long.

Hematite can also occur in association with fossils in eastern Kentucky. These productid brachiopods were replaced by siderite, which oxidized to hematite, giving the brachiopods a silver-gray color. The yellow mineral on the right hand brachiopod is the mineral limonite, another iron oxide.

Ilmenite

Crystal system: hexagonal. Color: black to brownish black. Hardness: 5.0-6.0. Streak: black to brownish red. Specific gravity: 4.3-5.5. Magnetism: slightly magnetic; more magnetic when heated. Uses: pigment for white paints.

Ilmenite is considered a heavy mineral and commonly occurs with rutile in the McNairy Sands in western Kentucky and in peridotite dikes in eastern and western Kentucky. It commonly occurs in ancient, near-shore sandstone environments. Although the mineral is very noticeable in beach sands as black specks, it is used as a pigment for white paints; the refining process removes traces of iron and other deleterious materials to create a white refined product.

Pyrolusite

Crystal system: tetragonal. Color: black. Hardness: 2-6.

Pyrolusite is commonly found as a secondary mineral and forms in a black, dendritic pattern in sedimentary rocks. It is commonly called wad. The black, soft, sooty mineral is associated with hematite, siderite, and limonite, and is found in the Givens Vein in the Western Kentucky Fluorspar District and in sandstones in eastern Kentucky.

Rutile

Crystal system: tetragonal. Color: red to black. Hardness: 6. Streak: yellow or brown. Specific gravity: 4.3. Uses: source of titanium.

Rutile generally occurs with ilmenite and has been noted in the peridotite dikes in eastern and western Kentucky. Rutile also occurs in the McNairy Sands in the Jackson Purchase Region and has been mined in Tennessee, Florida, and parts of the East Coast.

Uranium Minerals, Uraninite

Crystal system: isometric. Color: black to dark gray. Hardness: 5-6. Specific gravity: 9-10.

Some radioactive minerals have been reported in the black shale that crops out around the Blue Grass Region. Other occurrences of uranium minerals have been reported in eastern Kentucky, including one at Bell Falls in Menifee County. These radioactive minerals are detected by Geiger counter or scintillometer, since usually the minerals themselves are too small to see with the naked eye. Uraninite generally occurs as massive, botryoidal, or banded coatings. The microcrystalline variety of uraninite is called pitchblende. Because of the rapid oxidation, radioactive decay process, and numerous hydrous phases, uraninite can have many chemical variations.