The CIPW Norm

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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The CIPW norm is named for the four petrologists, Cross, Iddings, Pirsson and Washington, who devised it in 1931

What is a Norm and Why is it Needed?

A norm is a means of converting the chemical composition of an igneous rock to an ideal mineral composition. It often reveals similarities in rocks that have quite different modes, or observed mineral assemblages. Some of the factors that can cause such variations are:

The CIPW norm calculates mineral composition as if the magma were anhydrous (water is simply treated as a separate phase) and at low pressure. Other more complex norms have been devised for high-pressure or very hydrous situations. In an ideal world, a norm would match the observed mineral phases perfectly. To create such a norm, we would have to understand magma crystallization perfectly (and we don't), and the norm would be extremely complex to calculate.

Logic of Calculation

The logic of calculating a norm is simple even if the steps are tedious. Deal with the simplest and most predictable minerals first, then apply corrections if the rock is silica-deficient.

Calculating the Norm

Preliminary Comments

Allot Common Substitution Elements to Major Elements

Allot Minor Elements to Accessory Minerals

Allot Titanium

Provisionally Allot Alkalis and Aluminum

Begin Alloting Iron and Magnesium

Create Provisional Pyroxenes

Deal With Silica Excess or Deficit

Convert Provisional Minerals to Remove Silica Deficit

Form Olivine

Form Perovskite

Form Nepheline

Form Leucite

Form Calcium Orthosilicate (Rare)

Form Kaliophilite (Rare)

If there is still a deficit, convert Leucite to Kaliophilite: Kaliophilite = D/2, Leucite = Leucite - D/2, D = 0, Calculation Finished

Convert all remaining provisional minerals, if any, to final quantities

Reference

Myron G. Best; Igneous and Metamorphic Petrology, Freeman, 1982, Appendix E, pp. 616-619

Acknowledgement

Thanks to Dr. Chris McLaren for catching several errors in the original version of this page.

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Created 10 Nov 1997, Last Update 22 November 1999

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