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).