A simple cartographic exercise allows to understand the influence of the chemical composition of rocks on their paragenesis. It also helps to appreciate, in a concrete way, the usefulness and power of the graphic representation of these parageneses.

Here is a small portion of a geological map showing 3 lithologies of close chemical composition in the system: Al2O3 - SiO2 - MgO; the chemical composition of these rocks A, B and C and the minerals they are likely to contain are shown in the appropriate triangle:

Lithological map

The map is crossed by 2 isograds: that of the reaction Sapph + Qtz = Opx + Sill illustrated in this Figure and that of the reaction Opx + Sill + Sp = Sapph..

After drawing the 3 triangles on either side of the isograds, draw the metamorphic map by colouring the different sectors according to their paragenesis.

q or qtz : quartz ; saph or spr : sapphirine ; sill : sillimanite ; opx : orthopyroxene ; sp : spinel.

Here is the answer ...

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