Return to Alkaline Rocks
If the mineralogical composition of a rock cannot be established with certainty, but its chemical analysis is available, petrologists use this so-called "total alkali-versus-silica" (or TAS) diagram. Rocks plotting in a specific field on the diagram can be further categorized on the basis of their texture (just like we did for plutonic rocks), normative composition (since we have no idea what the real minerals are), and chemical characteristics. For example, a sample of lava containing 12 wt.% Na2O+K2O and 45 wt.% SiO2 (normalized to 100% on a volatile-free basis), and > 20% normative nepheline should be termed nephelinite (i.e. volcanic equivalent of nephelinolite). The dashed red line on the diagram separates (very approximately) alkaline volcanic rocks (e.g., alkali basalt, if you are in the basalt box) from their subalkaline counterparts.