CIPW standard
The CIPW standard is a method in petrology to convert a chemical rock analysis into a normative mineral inventory. It was introduced in 1902 by geoscientists Charles Whitman Cross , Joseph Paxson Iddings , Louis V. Pirsson, and Henry S. Washington and later named after them. Even today, the CIPW standard calculation in its original form is one of the basic standardized methods of representing rock analyzes, even if it has been supplemented by various additions for special applications over the course of time.
Areas of application
The CIPW standard is mainly used for volcanic rocks . When classifying volcanic rocks according to the mineral composition - for example the QAPF representation - there is often the problem that the rocks are not or not completely crystallized due to rapid cooling and are partially or completely in the form of volcanic glass . In order to be able to compare rocks with different content of glass and minerals, the CIPW calculation converts the total chemistry of the rock to proportions of certain standard minerals, which are selected so that they reflect the crystallization at low ambient pressure (volcanic environment). The calculation is based on the assumption that the rock is anhydrous, i.e. no minerals such as B. amphiboles or mica are present. When calculating a holocrystalline (completely crystallized) rock, an extensive approximation between the modal (actual) mineral inventory and the calculated normative mineral inventory is achieved.
Standard minerals
The minerals occurring in the standard are those which would crystallize out of an anhydrous rock melt with slow cooling and low ambient pressure. However, it is only a simplified model . Some of the compounds found in the standard have no direct equivalent in the natural system and are used as a pure calculation variable, such as sodium carbonate . All chemical elements , which usually occur in significant quantities in a rock analysis, are assigned to one or more standard minerals (for example, fluorine is assumed to be entirely in the form of fluorite ). Not all standard minerals appear in the calculation of a specific rock analysis, for example quartz and the representatives of the foids are mutually exclusive, which can also be observed in real rocks.
| Standard mineral | Abbreviation in the standard | Standard mineral | Abbreviation in the standard |
|---|---|---|---|
| quartz | q | Wollastonite | Where |
| corundum | c | Hypersthene | hy |
| Zircon | z | Olivine | oil |
| Orthoclase | or | Calcium silicate | cs |
| Albite | from | Magnetite | mt |
| Anorthite | on | Chromite | cm |
| Leucite | lc | Ilmenite | il |
| Nepheline | no | Hematite | Hm |
| Kaliophilite | kp | Titanite | tn |
| Halite | St. | Perovskite | pf |
| Thénardite | th | Rutile | ru |
| sodium | nc | Apatite | ap |
| Akmit | ac | Fluorite | fl |
| Sodium metasilicate | ns | Pyrite | py |
| Potassium metasilicate | ks | Calcite | cc |
| Diopside | di |
calculation
The calculation of the CIPW standard is based on the molar ratios of the elements. Therefore, a routine rock analysis, which is usually given in percent by weight of the oxides of an element, must first be converted using the molecular masses . The different oxides are now gradually combined according to a precisely defined scheme. For example, K 2 O is combined with Al 2 O 3 and 6 SiO 2 to form two formula units of the standard mineral orthoclase, KAlSi 3 O 8 . This continues until one of the three species is used up. The remaining components are then combined with other oxides to form further standard minerals. The calculation according to the comprehensive CIPW calculation rule is usually carried out by spreadsheet programs.
literature
- W. Cross, JP Iddings, LV Pirsson, HS Washington: A quantitative chemicomineralogical classification and nomenclature of igneous rocks. In: Journal of Geology. 10, 1902, pp. 555-690.
- JD Winter: An introduction to igneous and metamorphic petrology. Prentice Hall, Upper Saddle River, New Jersey 2002, ISBN 0-13-240342-0 .