Normative Mineralogy - Cautions

Cautions

The CIPW Norm or Cation Norm is a useful tool for assessing silica saturation or oversaturation; estimations of minerals in a mathematical model are based on many assumptions and the results must be balanced with the observable mineralogy. The following areas create the most errors in calculations;

• Cumulate rocks. While a normative mineral calculation isn't necessarily improper for use on cumulate rocks, the information gleaned is of doubtful value because a cumulate rock does not represent the melt from which it was extracted. However, if the groundmass of a cumulate can be analysed, it is valid to use a normative calculation to gain information about the parental melt.

• Oxidation state. Because the normative calculation divides Fe between oxide phases and availability for silicate phases, based on estimates of the ratio of Fe2+/Fe3+, miscalculating the ratio for the rock in question may produce erroneous amounts of magnetite or hematite, or alter the silicate mineralogy. If the Fe2+/Fe3+ ratio is known for the sample, the resulting calculation should match the observed mineralogy more closely.

• Pressure and temperature. Because the CIPW Norm is based on anhydrous melts and crystallisation at fairly low pressures, the resultant normative mineralogy does not reflect observed mineralogy for all rock types, especially those formed within the mantle. The normative mineralogy is not entirely accurate at reflecting the mineralogy of rocks formed at high pressures where, for instance, phlogopite may substitute for amphibole, amphibole for olivine and so forth. Altered normaive calculations have been developed that more correctly reflect the particular pressure regimes of the deep crust and mantle.

• Carbon dioxide. The influence of CO₂ on most silicate melts is fairly minor but in some cases, especially carbonatite, but also certain lamprophyre type rocks, kimberlite and lamproite, the presence of carbon dioxide and calcite in the melt or accessory phases derives erroneous normative mineralogy. This is because if carbon is not analyzed, there is excess calcium, causing normative silica undersaturation, and increasing the calcium silicate mineral budget. Similarly, if graphite is present (as is the case with some kimberlites) this can produce excess C, and hence skew the calculation toward excess carbonate. Excess elemental C also, in nature, results in reduced oxygen fugacity and alters Fe2+/Fe3+ ratios.

• Halides. The presence of some halides and non-metallic elements in the melt alter the resulting chemistry. For instance, boron forms tourmaline; excess chlorine may form scapolite instead of feldspar. This is generally rare, except in some A-type granites and related rocks.

• Mineral disequilibrium. Similar to cumulate rocks, a rock may contain extraneous minerals inherited from earlier melts, or may even contain xenoliths or restite. It is improper to calculate normative mineralogy on an igneous breccia, for instance.

For this reason it is not advised to utilise a CIPW norm on kimberlites, lamproites, lamprophyres and some silica-undersaturated igneous rocks. In the case of carbonatite, it is improper to use a CIPW norm upon a melt rich in carbonate.

It is possible to apply the CIPW norm to metamorphosed igneous rocks. The validity of the method holds as true for metamorphosed igneous rocks as any igneous rock, and in this case it is useful in deriving an assumed mineralogy from a rock that may have no remnant protolith mineralogy remaining.