The Constituents of Clay

like most other solid components of the Earth s crust, are largely made up of one or more mineral species. A mineral species is defined as  

A naturally occurring, inorganic, homogeneous solid, having a definite (but not necessarily fixed) chemical composition and a fixed ordered internal structure. 

When a mineral is produced in favourable condition, its ordered internal strueture is reflected in its external morphology and the mineral has a c/yxto/// e form. Otherwise a mineral is cryptocrystalline, i.e. the crystalline form is not perceptible externally but can be detected by scientific analyses like X-Ray Diffraetion. Amorphous natural solids like coal, volcanic glasses etc. do not qualify as minerals. 

A number of mineral species having similar chemical composition or internal structure constitute a mineral group. A number of groups, based on structural or chemical resemblances, constitute one mineral subclass. A number of mineral subclasses form a mineral class, which is the highest hierarchical unit in mineral classification. All the minerals belonging to a particular mineral class have the same dominant anion or aniortic group. The complete hierarchical order in the classification scheme of minerals is given below  

Same anionic group CLASS + Structural/chemical similarity SUBCLASS + Chemical/structural similarity to greater degree GROUP 

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However, it must be emphasized that there has to be some attempt to recognize the limitations of the method before any projection relating to the mineral composition of coal is possible. For example, the high temperature required for the ashing may result in the loss of the volatile constituents of the minerals or the mineral constituents will undergo a chemical change. In the former case, certain of the mineral elements will escape detection while in the latter case the constituents of clays or shale (to cite an example) will lose water of hydration or the carbonate minerals will lose carbon dioxide and the oxides so produced may even undergo further reaction with sulfur oxides or with silica to produce completely different mineral species ... 

All the above aspects of clay are discussed in this book. The present chapter lays a foundation for this discussion - it presents a comprehensive explanation for the universally accepted definition of clay describes the constituents of clay, and elucidates their formation in nature. A brief description of important types of clay is given at the end of the chapter. 

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