Principles of Igneous and Metamorphic Petrology (1990), Anthony R. Simon & Schuster's Guide to Rocks and Minerals As many volcanic rocks are too fine-grained or vitreous to be point-counted, those rocks are named according to texture. The above names for volcanic rocks should only be used if a modal analysis of the rock can be completed. feldspathoid monzonite tephritic phonolite diorite/ gabbro/ anorthosite andesite/ basalt monzodiorite/ monzogabbro latite-andesite/ mugearite alkaline feldspar syenite alkaline feldspar trachyte alkaline feldspar granite alkaline feldspar rhyoliteį. In this case, the determining factor may be the presence of a specific mineral, or in the case of diorite/gabbro/anorthosite, the calcium content of the plagioclase. Some zones of the diagram show two or more names for a rock type." biotite granite" or " olivine muscovite granodiorite"). The description of a rock sample as found in the QAP/FAP diagram may be improved with a few qualifiers, usually the name of the dominant mafic mineral (i.e.The first method is an estimation based upon the phase diagram grid. We are going to take the diagram apart to see how it works. For example a is read by measuring the distance from the side bc. Thus, formations are usually described as a range (i.e. A ternary diagram is a triangle with each of the three apexes representing a composition such as sandstone shale and limestone. In most igneous complexes, modal compositions may vary throughout a rock formation, depending on several dozen factors. The names for each of the rock types are arbitrary, and should only be used for qualitative description of a specific rock sample.Quartz and feldspathoid minerals are mutually exclusive, allowing for the combination of the QAP and FAP ternary diagrams into one plot.Ī few notes should be made about the following diagram: Rocks with less than 90 per cent mafic minerals can be plotted on the QAP/FAP diagram below. These classification schemes will be covered in another write-up, and at a later date. Rocks containing more than 90 per cent mafic minerals - ultramafic rocks - are classified using the ratios between the most abundant mafic minerals. M - mafic minerals (including, but not limited to pyroxene, amphibole, mica, sulfides, zircon, apatite etc.) Minerals present in the rock are grouped into five main categories: Usually, this is accomplished either by using a petrographic microscope to perform a point count or via instrumental analysis. The scheme requires that the modal composition be known for a particular rock sample. Most students of geology will recognize the ternary diagram (included below, in glorious ASCII) as a form of the QAP diagram. This scheme is easy to understand, and now enjoys widespread acceptance. As with most classification systems, this proposal sought to standardize the language spoken by petrologists, as many systems for naming igneous rocks were in limited, regional use at the time. In 1976, a subcommission of the International Union of Geological Sciences ( IUGS) proposed a classification scheme for plutonic and volcanic igneous rocks.
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