The advanced methods of igneous rock classification all use a three-point (or ternary) diagram for the identification of the rock samples. A three-point diagram is a triangle with three different mineral types at each of the points, generally quartz, alkali feldspar, and plagioclase. The triangle has a grid set in 10 percent increments for each mineral, with the apex of that mineral being 100 percent. To read the diagram you either estimate, or actually calculate, the percentages of the three different mineral groups (the percentage must add up to 100 percent). Once that is done, the sample is plotted to determine its location within the diagram. For example, if we had a sample that we estimated contained 30% orthoclase, 30% albite, and 40% quartz, we would plot it by counting over 30% for the plagioclase side (plagioclase is along the bottom of the diagram), count down 30% for the alkali feldspar side (alkali feldspar is on the left side of the diagram) and up 40% for quartz (quartz is on the right side of the diagram). Where the three grid lines intersect is the location of our sample on the diagram.
There are two diagrams used for identification of igneous rocks. Both are set up the same with quartz, alkali feldspar, and plagioclase as the apexes. There is one diagram for intrusive (plutonic) igneous rocks, and one diagram for extrusive (volcanic) igneous rocks. Knowing what type of rock you are looking at is the first step in using these diagrams. Basically, if the rock has a phaneritic texture (the crystals can be seen without the aid of a hand-lens) it is plutonic. If the rock has an aphanitic texture (the crystals cant be seen unaided) then it is volcanic. Both diagrams are divided into blocks based on the percentages of minerals present. It was the work of Albert Streckeisen who determined the need for an acceptable igneous rock classification, and who published these diagrams after much correspondence with other professionals. His work was later elaborated by the International Union of Geological Sciences (IUGS), and today we use this international accepted system for the classification of igneous rocks.
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