Part 6

MOLDING SAND (18)

MOLDING SAND is a mixture of sand and clay or other bonding material and is used to make molds in which metal is cast into various useful shapes. There are two kinds of molding sands, synthetic and natural bonded. The first is an artificial mixture of silica sand and clay; the second is a naturally occurring mixture of sand and a bonding material.

For casting, molding sand is first moistened with water and shaped into a mold of the metal part to be reproduced. The mold is then allowed to dry and the molten metal poured into it. The bonding material in the sand must be strong enough to keep the mold in shape during these operations.

Natural bonded molding sand is produced in Fayette, Bond, Bureau, Carroll, Kankakee, and Rock Island Counties, Illinois.

TRIPOLI (19)

TRIPOLI, called amorphous silica in southern Illinois, is a white or light brown, powdery substance that rubs off on the hands like chalk. It consists mostly of very small particles of quartz that result from the weathering of calcareous chert or highly siliceous limestone.

It is finely ground and used as “white rouge” for polishing optical lenses, as a filler in paints, in making ceramic products, as a component of buffing compounds, and as a fine abrasive.

Tripoli occurs in Alexander and Union Counties and is milled at Elco and Tamms in Alexander County.

FULLER’S EARTH (20)

FULLER’S EARTH is clay or silty clay material that contains very fine silica. It is soft, nonplastic, opaque, has a greasy feel when wet, and does not readily break up in water. Its color varies from blue-gray to yellow or buff.

Fuller’s earth is valuable for its unique property of absorbing and decolorizing substances. The material was first used to “full” or remove grease from woolen cloth, hence its name. It also has been used to filter and bleach mineral and vegetable oils by absorbing dark organic matter.

In Pulaski County in extreme southern Illinois the Porter’s Creek Formation contains deposits of clay that were at one time the source of fuller’s earth and still afford clay whose absorbent properties make it useful as litter and as sweeping and cleaning compound.

QUARTZ (21)

QUARTZ is the most common of all minerals, making up about 12 percent of the earth’s crust. There are two main types of quartz—crystalline quartz and dense, crypto-crystalline (submicroscopic) quartz. Many dense varieties occur in Illinois, probably the most common are chert or flint.

Crystals of quartz are typically six-sided, elongated, have sharply pointed pyramid-like ends, and are apt to grow together forming twins. Good crystals are rare in Illinois, and the crystal structure is not apparent in the commonly occurring grains and masses.

Quartz is brittle and hard. It may be colorless or tinted, transparent or translucent, but more commonly it is white and nearly opaque. Transparent quartz looks much like ordinary glass, but it scratches glass easily. It has a glassy to brilliant luster and breaks irregularly or with a good conchoidal fracture.

Some varieties of quartz that are used for semiprecious gems are chalcedony, agate, onyx, and jasper. Chalcedony is waxy, smooth, generally translucent, white to gray, blue, brown, or black. Agate is a form of chalcedony that has a mottled or variegated banded appearance and may be yellow, green, red, brown, blue, gray, or black. Onyx is agate with parallel bands that as a rule are brown and white or black and white. Jasper, an impure opaque quartz, generally is red.

Quartz occurs as rock crystal (colorless, transparent), milky quartz (white, nearly opaque), and smoky quartz (smoky yellow to gray or brown) in geodes from the Warsaw and Keokuk Limestones of the Nauvoo-Hamilton-Warsaw area and as vein and cavity fillings associated locally with fluorite, sphalerite, and galena in extreme southern Illinois. It also occurs as vug (cavity) fillings in limestones and sandstones.

FELDSPAR (22)

FELDSPAR is the name applied to a group of minerals that are the second most common of all the earth’s minerals. All feldspars are composed of aluminum, silicon, and oxygen, combined with varying amounts of one or more metals, particularly potassium, sodium, calcium, and lithium.

The minerals are hard, have a smooth glassy or pearly luster, and cleave along two planes nearly at right angles to each other. Feldspars are fairly light weight. The streak is white, but the color of the mineral is highly variable, although potassium and sodium-bearing feldspar are commonly white or pink and most plagioclase feldspar is gray.

Feldspars are essential parts of the crystalline igneous rocks. Their decomposition products are present in most soils. In Illinois relatively small feldspar crystals can be found associated with quartz and other minerals in granite and gneiss boulders and pebbles in glacial drift.

MICA (23)

MICA is the name of a family of complex aluminum silicate minerals that can be split easily into paper-thin, flexible sheets. If broken across the grain at right angles to the flat, smooth surface they fracture raggedly. In a single mica crystal the sheets range from more or less transparent to translucent and are arranged one on top of another like a deck of cards.

Micas are tough and somewhat elastic, soft enough to be split and scratched by a fingernail, and are light weight. They have a nonmetallic, glassy or pearly luster, although yellow mica may appear to be metallic. Color and streak depend upon the chemical composition of the mineral. Muscovite, or white mica, contains potassium and makes a colorless or white streak. Biotite, or black mica, contains iron and magnesium and is commonly dark green or black, although it may be shades of yellow or brown; its streak is uncolored.

Mica is abundant as tiny, shimmering flakes in Illinois sands, sandstones, and shales (which are then said to be micaceous). It also is common in many varieties of igneous and metamorphic rocks. White or yellow flakes may show a brilliant luster and may be mistaken for silver, platinum, or gold, but those minerals are heavy and malleable whereas mica is not.

CALCITE (24)

CALCITE, a common rock-forming mineral, consists of calcium carbonate. The mineral is white or colorless, but impurities may tint it shades of yellow or gray. Transparent calcite is more rare than the tinted varieties.