Presentation of Rock Analyses

Rock analyses should be recalculated into cation percentages plus the associated anions. In this way the mode is easily calculated, and various kinds of norms are easily constructed, for instance, catanorm, mesonorm, and epinorm for metamorphic rocks of the catazone, mesozone, and epizone, respectively. In most rock-making minerals the volume per oxygen atom has a rather constant value around $$12 cm.^{3}$$, regardless of the nature and number of the associated cations. Thus rock units of the same number of oxygens display approximately the same volume. Again, many rocks contain approximately 160 oxygens per 100 cations, and consequently there are many examples of rock series in which the cation percentages of the several rocks with fair approximation represent rock units of equal volumes. In other cases, if one wants to compare rocks of equal volume, it will be necessary to use the unit of the standard cell ( = the same number of oxygens). A simple and quick method for graphical presentation of rock analyses is described. On a plane projection it shows the quantitative (potential) content of, on the one hand, quartz, feldspars ± feldspathoids, and, on the other hand, the content of pyroxene ± olivine (or, alternatively, the content of amphiboles, garnets, biotite, etc.).