Clays


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Clays

 

sedimentary rocks consisting mainly of clay minerals and having plastic properties. Plasticity implies the capability to form a paste with water that assumes any form under pressure and retains it on drying. After firing, clays take on the strength of rock. The main components of clays are SiO2(30-70 percent), A12O3 (10-40 percent), and H2O (5-10 percent). Smaller quantities of TiO2, Fe2O3, FeO, MnO, MgO, CaO, K2O, and Na2O are also present. Clays contain mainly kaolinite, montmorillonite, and hydromicas, and sometimes polygorskit and saponite. Other admixtures are also present in the form of fragments of various minerals and rocks, vegetable or animal remains, and newly formed minerals (carbonates, gypsum, iron oxides, and other minerals). Large amounts of admixtures lead to transitions from true clays to other sedimentary rocks, such as sandy clays and marl. Clays may be of three types, depending on the nature of the predominant mineral component: kaolinitic, montmorillonitic, and hydromica.

Particles smaller than 0.01 mm predominate in clays. A gradual transition from clays to sands takes place with an increase in the number of coarser sandy particles.

All of the most important physicochemical and technological properties of clay rocks (plasticity, swelling, shrinkage, caking, fire resistance, warping, and adsorption) depend mainly on the mineral, granulometric, and chemical composition.

Clays and clay shales comprise more than half of all sedimentary rocks of the earth’s crust. Favorable conditions for the formation of the most valuable industrial clays are created in platforms in which the weathering processes occurring during long exposure to a continental regime lead to the formation of large weathering crusts. The erosion of the weathering crusts on platforms usually leads to the formation of well-elutriated clays in basins of sedimentation. In geosynclinal regions, the rapid mechanical destruction of products of weathering, together with the absence of conditions for their fractionation, leads to predominant formation of relatively little-fractionated clays. Clays formed at the location of the original rock formation (weathering-crust clays) are called primary clays. The clays formed by redeposition of material are called secondary clays. Secondary clays are found among depoists of all types (continental, including lacustrine, littoral lagoon-type, and marine). Lacustrine clays frequently have a monomineral, kaolinitic composition. Pure montmorillonitic clays (so-called bentonites) are usually formed from volcanic ash and pumice. Clays are minerals that are of great practical importance and are mined in large quantities.

The four important clay groups in terms of their composition and the type of industrial application requirements are coarse ceramic, fire-resistant and refractory, kaolins, and adsorption and highly disperse montmorillonites.

Coarse ceramic clays are usually of the iron-montmorillonitic type, are rich in SiO2 (up to 65-70 percent), and contain numerous impurities. Large pebbles and sand cause the formation of cracks in items made from these clays. Limestone and gypsum inclusions are transformed into lime during firing, and this product combines with water, which gives rise to swelling and destruction of the articles. These clays are readily fusible; their caking temperature is 900°-1000° C (lower in the presence of iron oxides); and they are used in the raw state (soil packing and adobe), as well as in the production of coarse ceramic articles, such as dishes, drain pipes, red structural brick, and roofing shingles. The plastic types of clay are also used for pottery, for figurines and other articles, and for sculpting original works of art which are either reproduced in other materials (bronze, marble, or porcelain) or are hardened by firing. The pliability of clay, which records the slightest motion of the sculptor’s hand, makes it possible for the sculptor to achieve a great variety of light and dark effects, as well as flowing and pictorial effects, and to consider in detail the textural possibilities of the material in which the sculpture was conceived. Clays are also widely used in the production of keramzit, a filler for lightweight concrete.

Fire-resistant and refractory clays are predominantly of the kaolin type with various impurities and are characterized by a high alumina content (30-42 percent). The content of undesirable components should not exceed 3-4 percent for Fe2O3, 1-2 percent for TiO2, 0.8 percent for CaO, and 0.2-0.3 percent for SO3. These clays have high plasticity, binding capacity, and fire resistance (not less than 1600°-1670° C) and are the raw materials for various ceramic industries. The most important of these are the production of fire-resistant supplies for metallurgical and glass furnaces, fireboxes for steam boilers, the manufacture of various fine ceramic articles, and electroceramics. Fire-resistant clays are part of many mold compositions that are used in the foundry industry. There are deposits in the USSR (the Ukraine, the Urals, the region of Voronezh, the Moscow basin, and other locations), as well as abroad (England, Czechoslovakia, the German Democratic Republic, the Federal Republic of Germany, and China).

Kaolins are clays of low plasticity. They are prepared from raw kaolin by purification, elutriation in water, or air separation to remove coarse sandy impurities. Elutriated or purified kaolin should not contain sand particles. Its iron oxide content should not exceed 1-1.5 percent; the best grades should contain less than 0.5 percent. Kaolin is used in the paper, rubber, plastic, and perfume industries as a filler, as well as in the ceramic industry as a component of porcelain-faience compositions. Deposits are found in the USSR (the Ukraine and the Urals) and abroad (England, China, Czechoslovakia, the German Democratic Republic, the USA, and other countries).

Montmorillonite clays are characterized by high plasticity and are used in the petroleum industry as a component of drilling muds, in metallurgy as a binder in the production of iron-ore briquets and in preparing molding sand, and in the food, chemical, and pharmaceutical industries. The properties required of clays are uniform particle size, water absorption, and binding, as well as adsorption capability. Montmorillonitic deposits are found in the USSR (the Volga region, the Ukraine, the Crimea, Georgia, Armenia, Turkmenia, Azerbaijan, and Kazakhstan) and abroad (in the USA, Mexico, Italy, and Rumania).

REFERENCES

Merabishvili, M. S. Bentonitovye gliny. Moscow, 1962.
Grim, R. E. Mineralogiia i prakticheskoe ispol’zovanie glin. Moscow, 1967. (Translated from English.)
Kaolinovye mestorozhdeniia i ikh genezis. Moscow, 1968.
Millot, G. Geologiia glin. Leningrad, 1968. (Translated from French.)
Sivokon’, V. I. Izuchenie i otsenka mestorozhdenii pervichnykh kaolinov. Moscow, 1969.
Kurs mestorozhdenii nemetallicheskikh poleznykh iskopaemykh. Edited by P. M. Tatarinov. Moscow, 1969.
Gliny, ikh mineralogiia, svoistva i prakticheskoe znachenie. Moscow, 1970.

V. P. PETROV

References in classic literature ?
Tom went first, cutting rude steps in the clay hill as he descended.
These also he set in clay and when the house had been entirely completed he applied a coating of the clay to the entire outside surface to the thickness of four inches.
I will take the black clay with me, also the pencil cuttings.
The brickyard paid ten cents a cubic yard for the clay.
For a time they sat about, all pulling at clay pipes and chirruping and laughing in queer thin falsettos at the events of the night and the previous afternoon.
He was too occupied with his own vision, and vividly burned before him the sordid barrenness of a poorhouse ward, where an ancient, very like what he himself would become, maundered and gibbered and drooled for a crumb of tobacco for his old clay pipe, and where, of all horrors, no sip of beer ever obtained, much less six quarts of it.
And so, according to the clay of his nature and the pressure of his surroundings, his character was being moulded into a certain particular shape.
Few phenomena gave me more delight than to observe the forms which thawing sand and clay assume in flowing down the sides of a deep cut on the railroad through which I passed on my way to the village, a phenomenon not very common on so large a scale, though the number of freshly exposed banks of the right material must have been greatly multiplied since railroads were invented.
Dear it is, my clay to me; dear it has been to others.
However, considering the heat of the climate, I did not doubt but if I could find out any clay, I might make some pots that might, being dried in the sun, be hard enough and strong enough to bear handling, and to hold anything that was dry, and required to be kept so; and as this was necessary in the preparing corn, meal, &c.
Here he went through the not very difficult process of winking upon the company with his solitary eye, to the enthusiastic delight of an elderly personage with a dirty face and a clay pipe.
But as clay in the abstract, when too much moistened, becomes of a weak and uncertain consistency, breaking down in unexpected places, retaining impressions but faintly, and preserving no strength or steadiness of character, so Mr Swiveller's clay, having imbibed a considerable quantity of moisture, was in a very loose and slippery state, insomuch that the various ideas impressed upon it were fast losing their distinctive character, and running into each other.