Return to Alkaline Rocks
Most alkaline rocks contain variable amounts of feldspathoid minerals, feldspars and ferromagnesian silicates. The volumetric proportions of alkali feldspars (A: sanidine, orthoclase, microcline and low-Ca albite), plagioclase (P) and feldspathoids (F) serve as the basis for this classification scheme endorsed by the International Union of Geological Sciences. On this diagram, plutonic rock names are given in plain font, and volcanic ones in italics. For example, if the rock in question is intrusive and contains 25% nepheline, 30% microcline and 5% pure albite, it will plot in the foid syenite field along the A-F side of the triangle. Given that nepheline is the only foid mineral in this rock, the more appropriate petrographic term for it would be nepheline syenite. This root name can be further refined through the adition of modifiers, i.e. adjectives and nouns that reflect some specific textural and mineralogical characteristics of the rock. For instance, the nepheline syenite shown at right shows a trachytoidal texture and, in addition to nepheline and feldspar crystals (colorless), contains about an equal proportion of ferromagnesian silicates left out of the APF diagram. In our example, these minerals are represented by aegirine (bright green) and amphibole (bluish green to brown). Our rock can now be rebaptized into mesocratic trachytoidal aegirine-amphibole nepheline syenite. Some of these awkwardly long names can be replaced with an old (traditional) name, which would typically allude to the geographic locality where the rock has been initially described from. For instance, our mesocratic blah-blah-blah syenite may be alternatively called lujavrite, for the Lujavr plateau (Lovozero Mts) in the Kola Peninsula. The downside of using such locality names is that you have to know a priori what they all mean.