1.5 Choosing a Composition Diagram.

As shown on the previous page, the major minerals found in the contact metamorphosed rocks can be plotted on a CaO-SiO₂-MgO composition diagram. However, the minerals plot rather close to one another, crowding into a small area on this diagram as emphasized in the above graph. Petrologists can choose for convenience the chemical components used to plot a composition diagram. Any three non-colinear compositions on the CaO-SiO₂-MgO composition diagram can be used. Click on the radio buttons on the diagram to view some other possible component selections.

Although the spread of the minerals is different on the several diagram choices shown, the relative positions of the minerals remain the same. To show the cmSerpentinite assemblages, we will use the "Di-Tlc-MgO" diagram because it spreads out the relevant mineral compositions and plotting position calculations are relatively straightforward.

Note that Qtz is "pushed off" the composition triangle for two of the selections shown. To calculate where Qtz plots, you will need to express its composition using the three chosen corner compositions, then follow the plotting instructions for ternary diagrams. Are you up for the challenge -- and a surprise?

Plot the Assemblages