10.1 Historical Context

The mineralogy and texture of an igneous rock are a complex function of its chemistry and crystallization history. Two igneous rocks with the same magma source and chemical composition might look very different in hand sample, especially if one is plutonic and the other is volcanic. Cooling rate, water content, nucleation and growth kinetics, and many other factors contribute to the mineralogy and texture of the rock. To help see similarities, differences, and patterns among igneous rocks, in spite of their different crystallization histories, a procedure was developed to reduce all igneous rocks to the same basis — a basis that depends on chemistry only. The result of this procedure is a norm, a "standard mineral composition", not to be confused with the mode, the "actual mineral composition" for an igneous rock. Most geologists calculate CIPW norms, named after Whitman Cross, Joseph P. Iddings, Louis V. Pirsson, and Henry S. Washington who devised the procedure early in the 20th century (Cross and others, 1903; Washington, 1917). More than 100 years later, petrologists continue to use CIPW norms. If you want examples, click the following button:   Petrologists use Norms

Chemical analyses of rocks are reported as weight percentages of the oxides of the elements. This reporting protocol was developed when the analytical techniques involved weighing and "wet chemistry" as rocks as well as standards were dissolved in strongly acidic solutions. Less weighing is done today as part of the largely spectroscopic techniques used for rock analyses, such as x-ray fluorescence (XRF), but oxide weight percentages persist. Oxygen is the principle anion in rocks, and because minerals and rocks must be charge-balanced, oxygen is determined from cation measurements rather than measured directly. A CIPW norm is a recasting of a chemical analysis given in weight percents of the oxides into an equivalent represention in weight percents of normative minerals. Cross and others developed a list of normative minerals and a procedure for assigning oxide values to one or more of the normative minerals until all the oxides have been fully assigned. To avoid differences based on water content, which is very sensitive to crystallization history, only anhydrous minerals are used.

The CIPW norm calculation instructions are long and tedius (see Washington's Revised Version (Washington, 1917, Appendix 3)). But the instructions are simply an algebraic word problem that computers can be programmed to perform. As student of petrology, you should work through norm calculations for several rocks so that you understand the principles and logic behind them. Once you have this understanding, let computers do the work and focus on the interpretation of the resulting norms.   More about norms