Normative mineralogy

Normative mineralogy is a geochemical calculation of the whole rock geochemistry of a rock sample which estimates the idealised mineralogy of a rock 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.

The process blends the excitement of geochemistry with that of spreadsheet accounting. Because the normative calculation is essentially a computation, it can be achieved relatively painlessly via computer programs.

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CIPW Norm

The CIPW Norm was developed by the petrologists Cross, Iddings, Pirsson and the geochemist 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;

1. The magma crystallizes under anhydrous conditions so that no hydrous minerals (hornblende, biotite) are formed.
2. The feromagnesium minerals are assumed to be free of Al2O3.
3. The Fe/Mg ratio for all feromagnesium minerals is assumed to be the same.
4. Several minerals are assumed to be incompatible, thus nepheline and/or olivine never appear with quartz in the norm.

This is, of course, an artificial set of constraints, and means that the results of the CIPW norm do not reflect the true course of igneous differentiation in nature.

The primary benefit of calculating a CIPW norm is determining what the ideal mineralogy of an aphanitic or porphyritic igneous rock is. Secondly, the degree of silica saturation of the melt which formed the rock can be assessed in the absence of diagnostic feldspathoid species.

The silica saturation of a rock varies not only with silica content but the proportion of the various alkalis and metal species within the melt. The silica saturation eutectic plane is thus different for various families of rocks and cannot be easily estimated, hence the requirement to calculate whether the rock is silica saturated or not.

This is achieved by assigning cations of the major elements within the rock to silica anions in modal proportion, to form solid solution minerals in the idealised mineral assemblage starting with phosphorus for apatite, chlorine and sodium for halite, sulphur and FeO into pyrite, FeO and Cr₂O₃ is allocated for chromite, FeO and equal molar amount of TiO₂ for ilmenite, CaO and CO₂ for calcite, to complete the most common non-silicate minerals. From the remaining chemical constituents, Al₂O₃ and K₂O are allocated with silica for orthoclase; sodium, aluminium and potassium for albite, and so on until either there is no silica left (in which case feldspathoids are calculated) or excess, in which case the rock contains normative quartz.

The full CIPW normative calculation and rules governing its use are best studied in a textbook as the above is only a rough overview. See below.

Normative and modal mineralogy

Normative mineralogy is an estimate of the mineralogy of the rock. It will usually differ from the visually observable mineralogy, at least inasmuch as the types of mineral species, especially amongst the ferromagnesian minerals and feldspars, where it is possible to have many solid solution series of minerals, or minerals with similar Fe and Mg ratios substituting, especially with water (eg; amphibole and biotite replacing pyroxene).

However, in aphanites, or rocks with phenocrysts which are clearly out of equilibrium with the groundmass, a normative mineral calculation is often the best way of gaining an understanding of the evolution of the rock and its relationship to other igneous rocks in the region.

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 will not reflect observed mineralogy for all rock types, especially those which are 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 which 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 within the melt or accessory phases will derive erroneous normative mineralogy. This is because if carbon is not analyzed, there will be 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 will also, in nature, result in reduced oxygen fugacity and alter Fe²⁺/Fe³⁺ ratios.

• Halides. The presence of some halides and non-metallic elements in the melt will alter the resulting chemistry. For instance, boron will form 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 would be 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 which is very 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 which may have no remnant protolith mineralogy remaining.

See also

• Geochemistry
• Mineralogy
• Wüstite
• Oxygen fugacity

References

• Hess, P. C. (1989), Origins of Igneous Rocks, President and Fellows of Harvard College (pp. 276-285), ISBN 0-674-64481-6.
• Blatt, Harvey and Robert Tracy (1996), Petrology, 2nd ed., Freeman (pp. 196-7), ISBN 0-7167-2438-3.