Silicates are arrangements of the elements silicon and oxygen with a wide variety of other elements. The most common silicates are quartz and feldspars. Quartz is abundant in Kentucky, and some of the feldspars occur as very fine grains in sandstones in Kentucky.
Crystal system: orthorhombic. Cleavage: tabular. Color: clear to white, with yellow or red iron staining common. Hardness: 4.5-5. Luster: vitreous. Specific gravity: 3.4.
Calamine is an older name for the rare mineral hemimorphite. It forms in oxidized zinc deposits. Hemimorphite is commonly associated with smithsonite and cerussite, and occurs in clusters of radiating, acicular crystals. A massive variety shows worm-like shapes. Hemimorphite occurs in zinc-bearing vein deposits in the Western Kentucky Fluorspar District in Crittenden and Livingston Counties.
Crystal system: monoclinic and triclinic. Cleavage: good at 90 degree angle. Hardness: 6.
The feldspars are an important rock-forming group of minerals, but their occurrence in Kentucky is limited to very small detrital fragments of sandstones and cement that are only visible with microscopes.
Crystal system: isometric. Color: various colors. Hardness: 6.5-7.5. Luster: vitreous. Specific gravity: 3.5-4.3. Uses: semi-precious stone, industrial abrasive saws, polishing tools.
The garnet group of silicates has a diverse chemical composition consisting of calcium, aluminum, magnesium, and chromium silicates. Some minerals included in the garnet class are pyrope (red to black), almandine (red), grossularite (green yellow), and uvarovite (green). It is translucent to transparent. Pyrope and almandine occur abundantly in ultramafic dikes in Elliott County, and many can be obtained by panning in the alluvial sediments near the dikes. Garnets can also be found in some glacial erratics in northern Kentucky and in metamorphic rocks in the Appalachian Mountains in Virginia and North Carolina.
Crystal system: monoclinic. Cleavage: cleaves into sheets; flexible when bent. Color: dark gray to black (biotite), white to light brown (muscovite). Hardness: 2.5 (biotite)-3.0 (muscovite). Specific gravity: 2.7 (muscovite)-3.0 (biotite).
Muscovite and biotite generally occur in igneous and metamorphic rocks, but in Kentucky they are commonly found as detrital sediment in sandstones, shales, and clays. Mica minerals are commonly mistaken for gold because of their golden color and cleavage, which causes light to be reflected easily. The magnesium-rich phlogopite has similar physical characteristics as biotite and muscovite, and is found in kimberlite dikes in Elliott County.
Crystal system: orthorhombic. Fracture: conchoidal. Color: green. Hardness: 6.5-7. Luster: glassy. Specific gravity: 3.3-4.3.
Olivine is a common igneous rock-forming mineral and occurs in both basaltic rocks and the kimberlite dike in Elliott County. Transparent gem-quality varieties are known as peridots. Olivine alters readily to serpentine.
Fracture: uneven. Color: black to dark brown. Hardness: 5.5. Luster: metallic. Specific gravity: 4.
Perovskite can occur as cubic crystals and massive or reniform (kidney-shaped) masses. It is found in ultramafic rocks and in the peridotite dikes in eastern and western Kentucky.
Crystal system: hexagonal. Fracture: conchoidal. Color: colorless or white, but may be tinted various colors (e.g., purple, amethyst). Hardness: 7.0. Streak: colorless. Luster: glassy. Specific gravity: 2.65. Uses: jewelry; prehistoric arrowheads, knives (flint); gravel (chert).
Quartz is the hardest, most resistant mineral found in abundance in Kentucky. It is the main constituent in sandstones and geodes, and also occurs as vein quartz. Crystals usually consist of six-sided hexagonal prisms capped by pyramids on one or both ends. Quartz crystals are found in geodes that occur in several different rock types, particularly limestone. In south-central Kentucky, valleys and stream beds downslope from the Warsaw-Salem Formation are filled with geodes, some containing amethyst (another variety of quartz).
Several cryptocrystalline (microscopic crystals) varieties of quartz occur in Kentucky. They are commonly recognized on the basis of their fibrous texture and granularity. The fibrous varieties include chalcedony, agate, onyx, and jasper, and granular varieties include chert and flint.
Agate has delicate and varying shades of color arranged in layers. In the typical occurrence the bands are irregular, curved, or in concentric patterns. Agate is used as an ornamental material or in semi-precious jewelry. The color banding is usually related to chemical impurities; for example, iron gives a red or orange color and manganese or calcium give black or blue colors.
For the past decade, beautiful specimens of red, black, yellow, and gray banded agate have been discovered in Estill, Jackson, Powell, Madison, and Rockcastle Counties. These Kentucky agates are derived from the Renfro-Borden Formation of Early Mississippian age and can be collected along some river drainages where the Borden is exposed to weathering. Many of these agates are displayed at local rock shows. To see more pictures of Kentucky agates see the Kentucky Agate section of this web site.
Flint is dark brown to black and breaks with a conchoidal fracture into fragments with sharp cutting edges. It is found in limestones or in soils derived from limestones.
Chert (and Jasper)
Chert and flint are cryptocrystalline varieties of quartz. Chert is usually gray to white; flint is dark brown to black. Chert and flint are very hard and break with a splintery fracture. Chert is usually associated with dolostone and limestone and occurs as lenses, irregular layers, and nodules, but some rock units are composed almost entirely of chert. Chert abounds in the upper St. Louis Limestone in south-central Kentucky between Glasgow and Somerset. The Boyle Dolomite of Silurian age also contains abundant gray, blue, and black chert. Any roadcuts or active or abandoned quarries in this region in the St. Louis Limestone or Boyle Dolomite would contain numerous chert nodules. Chert and flint were the most common silicate minerals used by early Native Americans for making arrowheads. Because chert and flint have a conchoidal fracture, they are easily shaped into arrowheads.
Jasper is an impure variety of quartz that has been colored some shade of red by iron oxide inclusions. The name "Jasper" may also be used for some siliceous agate material that has replaced organic material in petrified wood. It is used as an ornamental stone and in jewelry.
Opal is an amorphous, massive silicate that exhibits a conchoidal fracture and has a characteristic play of colors caused by its water content. Opal is not common in Kentucky, but may occur in some siliceous fossils, particularly in the black shale, and in microscopic amounts in cherts and chalcedony. Opal is not a stable mineral, and in geologic time alters to other silicate minerals.
Many fossils found in Kentucky are silicified. This means the original material of the fossil has been replaced with quartz. These small brachiopod fossils from Lexington are silicified. Click on the image to see the quartz inside. When the brachiopods are broken open you can see the white to clear quartz inside. In some cases the quartz is massive, in others crystalline.
Crystal system: monoclinic. Color: green. Hardness: 3-5. Luster: greasy, wax-like. Specific gravity: 2.5. Uses: asbestos.
Serpentine occurs in both a platy and a fibrous variety. The most common variety is chrysotile, which is the chief source of asbestos. In Kentucky, serpentine occurs in peridotite dikes.
Clay minerals are a subgroup of silicates that comprise the various claystones, such as ball clay, flint clay, and fuller's earth.
Illite is the constituent of many shales and is an intermediate clay between montmorillonite and muscovite. It has more potassium than montmorillonite, but is not expandable or absorptive. It is structurally similar to chlorite, but chemically different.
Crystal system: amorphous. Color: green. Streak: colorless or greenish, but lighter than the grains themselves. Luster: earthy to dull. Specific gravity: 2.3. Tenacity: brittle. Uses: fertilizer, soil conditioner.
Glauconite, a variety of illite, occurs disseminated in shales, sandstones, and limestones, and is commonly associated with phosphate pebbles and iron sulfides. The Floyds Knob Bed of the Borden Formation (Mississippian) is a glauconitic siltstone that crops out in a semicircle around the Outer Blue Grass.
Crystal system: monoclinic. Hardness: 2-2.5. Luster: earthy. Specific gravity: 2.6.
Kaolinite is the chief component of ball clay and flint clay. It has a sheet structure, and is therefore not as absorbent as montmorillonite.
Halloysite is a hydrated variety of kaolinite (crystal system: amorphous; fracture: conchoidal; color: white, yellowish-white, gray, to green; hardness: 1.5; streak: white; luster: earthy to pearly; specific gravity: 2.1; tenacity: brittle) with little or no plasticity. Halloysite has a distinctive tube-like structural appearance.
Crystal system: monoclinic. Hardness: 1-1.5. Specific gravity: 2.5.
Montmorillonite is very fine grained, and visible only with powerful microscopes. It is the main mineral in bentonite and fuller's earth. Montmorillonite is called an expanding clay because the arrangement of its crystal lattice allows frequent and extensive substitution of additional minerals; actual composition may vary depending on iron, magnesium, zinc, aluminum, and silicate ratios.
Vermiculite is a related montmorillonite clay mineral that has the absorbent but not the expandable characteristics of typical montmorillonite.