X-ray fluorescence (XRF) has been successfully used for more than 20 years as an on-line analysis method for controlling various industrial processes where the concentrations of different elements must be known in the process streams. In many industrial processes, however, the knowledge of the concentrations of different minerals, rather than those of the elements, is of prime importance. Examples of such processes are numerous; plants concentrating apatite for fertilizer and detergent manufacturers, paper filler producers manufacturing kaoline, titanium dioxide (rutile) and talc, potash concentrators and different crystallization processes.