Although silica is the commonest constituent of the earth’s crust, the study of it and its compounds baffled investigators for many decades. There are several reasons for this. Firstly, silicates are almost all insoluble in anything except hydrofluoric acid, so that they cannot be separated or investigated by solution methods. Secondly, their thermal reactions of transition, inversion, melting or freezing are sluggish and ill-defined so that no information about compounds and their purity can be inferred from thermal curves. Thirdly, the distinctions between compounds and solid solutions and mixtures are not clear and phase diagrams are more difficult to plot than usual.