Primary clay

More than 20 different t) es of clay can be actually distinguished. Those most appreciated for making ceramics, for example, kaolinite, are built up of combinahons of the basic structural xxnits described above. The parhcles of most consist of platelets (very small, flat sheets) that, when stacked together, form layered arrangements having extensive surface areas, much like the pages of a book. Other common clay parhcle shapes are fibrous or tubular. 

Clays occur naturally either in a relahvely pure condition or mixed with other materials and they are therefore classified into one of two large groups primary and secondary clays. Primary clays are quite pure, xmcontaminated by other materials, and have a rather xxniform composihon. Secondary clays are mixtures of clay with other minerals such as quartz, talc, mica, iron oxides, and even organic matter (the latter derived from the decay of dead plants and animals) the particles of most of the contaminating materials are generally of similar size to those of the clay (Kingery et al. 1976). 

Primary clay is also known as residual clay, indicating that they are either the in situ residue of one type of weathered rock or the transported residue of many types of rocks most primary clay deposits occur, however, in situ, at the location where the clay particles were formed. The clay is usually quite pure and colorless or white, but very small relative amounts of minerals mixed with the clay, such as quartz and/or iron oxides, may impart to it a yellow, brown, or green color. Primary clay is also characterized by the extreme fineness of its particles, which usually measure below 2 micrometers (0.002 mm) in diameter. The more than 20 different types of primary clay minerals can be distinguished by their chemical composition, which varies widely, and by their physical properties. Primary clays that have been used for making ceramic objects are listed in Table 55. 

Kaolin, China clay, terra alba, argille, porcelain clay, and white bole are the generic names used to refer to primary clays that include three distinct white minerals - kaolinite, nacrite, and dickite - all of which share a very similar composition but differ slightly in their structure. Kaolin is rarely found pure, but as a natural mixture with other varieties of clay together, the various clays make up over 95% of the total weight of the mixture, other earthy 

Kaolinite White platelets the type of clay most widely used for making ceramics 

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Primary clay, for example kaolin, is colorless, and when such clay is heated to a high temperature it produces white ceramic materials. Most pottery, however, is colored its color is due to the fact that most of it was, and still is, made not from primary but from secondary clay. Secondary clay contains minerals other than clay, and colored metal ions in them endow the pottery with their color. Iron ions (in iron oxides), for example, tend to make pottery yellow, brown, or red, and manganese ions (in pyrolusite, a mineral composed of manganese oxide) make it either dark or black. 

It is important to realise that clay can be exposed to many processes after it has been depos-ited. The periods are the Miocene Epoch (26 - 7 million years old), the Pliocene Epoch (7-2 millions years old), the Pleistocene Epoch (2 million to 15,000 years old) and the Holocene Epoch (up to approximately 15,000 years old). In The Netherlands no primary clay deposits are found, i.e. all clay is secondary, i.e. formed elsewhere and transported by wind, ice or water. 

Primary clay, known as kaolin, is found in the same place as the parent rock. Kaolin is formed by the weathering of feldspar. Kaolins are coarse in particle size and therefore nonplastic compared to most sedimentary clays. Kaolins are relatively free of mineral impurities such as iron. 

Throughout the areas studied we observe in rocks of different granulometry, from sandstones to rocks of the terrigenous-chemical complex, primary clay mineral associations. The main components of the associations are a number of varieties of Mg-chlorites, primary hydromicas grouped together under the term ferruginous illites, trioctahedral swelling minerals and mixed-layer clays. 

In order to obtain good interfacial adhesion and mechanical properties, the hydrophilie elay needs to be modified prior to its introduction in most polymer matriees, which are organophilie. When nanometric dispersion of primary clay platelets is obtained, the aspect ratio of the filler particle is increased and the reinforcement effect is improved [75-25],... 

Primary Clay or Residual Clay. A clay still remaining in the geographical location where it was formed in the UK such a clay is typified by the china clay (q.v.) of Cornwall (cf. SEDIMENTARY CLAY). 

Residual Clay. See primary clay. Resinous Cement. A term used in chemical engineering for an acid-proof cement, based on synthetic resin, for jointing chemical stoneware or acid-resisting bricks. The cement, when set, is impervious and very hard. 

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