A material the way it would be if its crystal structure perfectly matches the unit-cell drawings you find in textbooks. In nature, materials are always contaminated to some degree. Calcium carbonate is never pure, feldspar never has an ideal 1:1:6 relationship between fluxes:alumina:silica, kaolin particles are never crystal-perfect, etc. Some materials are theoretical in physical properties but have no theoretical formula (e.g. ball clays) because they are a mix of many minerals and have a definition that can encompass a broad range of products. When learning ceramic chemistry students usually use theoretical materials (e.g. potash feldspar, kaolin). However when they begin working in the lab of a company they use the actual chemistry of real world materials.
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