Geochemical Diagram

(also paragenesis diagram), a graphic representation of the sequence of crystallization and subsequent transformations of minerals, as well as of their paragenetic associations. A geochemical diagram usually shows the successive formation of minerals in any actual rock, deposit, or type of ore. (See Figure 1.)

Figure 1. Geochemical diagram

The crystallization temperatures of the minerals are laid out along the horizontal axis, and the individual minerals, from top to bottom in order of their formation, are laid out along the vertical axis. The temperature graduation is done according to geological thermometers, which are minerals with definite melting points (corrected for pressure) or having known polymorphic transformation temperatures. The times of beginning and end of formation of a mineral on the diagram are shown by the figures that extend horizontally. The more copious the formation of a mineral, the wider the figure vertically. Several successive figures for the same mineral denote the existence of several generations of the mineral. An asterisk at the end of a figure denotes the solution of that mineral or its replacement by others. The end of the broken line indicates the beginning of formation of the replacing mineral. A definite paragenetic association of minerals corresponds to each phase (vertical plots). The sequence and degree of fixation of chemical elements in the minerals that have formed may be established by comparing the chemical compositions of the minerals on the geochemical diagram.

Geochemical diagrams are compiled in geological prospecting and surveying of deposits, when studying their genesis, and in classifying ore types. The method of compilation of geochemical diagrams was developed by Academician A. E. Fersman.