Normative mineralogy

Normative mineralogy is a calculation of the composition of a rock sample that estimates the idealised mineralogy of a rock based on a quantitative chemical analysis according to the principles of geochemistry. Normative mineral calculations can be achieved via either the CIPW Norm or the Barth-Niggli Norm (also known as the Cation Norm). Normative calculations are used to produce an idealised mineralogy of a crystallized melt. First, a rock is chemically analysed to determine the elemental constituents. Results of the chemical analysis traditionally are expressed as oxides (e.g., weight percent Mg is expressed as weight percent MgO). The normative mineralogy of the rock then is calculated, based upon assumptions about the order of mineral formation and known phase relationships of rocks and minerals, and using simplified mineral formulas. The calculated mineralogy can be used to assess concepts such as silica saturation of melts. Because the normative calculation is essentially a computation, it can be achieved via computer programs. The CIPW Norm was developed in the early 1900s and named after its creators, the petrologists Charles Cross, Joseph Iddings, Louis Pirsson, and the geochemist Henry Washington. The CIPW normative mineralogy calculation is based on the typical minerals that may be precipitated from an anhydrous melt at low pressure, and simplifies the typical igneous geochemistry seen in nature with the following four constraints: The magma crystallizes under anhydrous conditions so that no hydrous minerals (hornblende, biotite) are formed. The ferromagnesian minerals are assumed to be free of Al2O3. The Fe/Mg ratio for all ferromagnesian minerals is assumed to be the same. Several minerals are assumed to be incompatible, thus nepheline and/or olivine never appear with quartz in the norm. This is an artificial set of constraints, and therefore the results of the CIPW norm do not reflect the true course of igneous differentiation in nature.