Difference between normative and modal composition of a rock
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Two types of analysis are useful when examining Igneous Rocks: Modal analysis and Normative analysis.
Modal Analysis - This type of analysis requires only a thin section. It produces an accurate representation of the distribution and volume percent of the mineral within a thin section. Allows counting of each mineral occurrence along a series of traverse line across a given thin section. It gives the maximum and minimum grain sizes. It gives real proportions of minerals in vol. %. The methods used are point counting of standard thin sections or polished stained slabs, Image analysis, and Powder X-ray diffraction.
Normative analysis - It involves the CIPW norm - Cross, Iddings, Pirsson and Washington 1902. The proportions of ideal mineral end-members are calculated as wt. %. The rocks need to be divided by density and recast to 100% to get percentages in vol. %. The calculation involves a hierarchical list of rules that tend to be often ambiguous. This analysis does not include hydrous minerals, yields phases often not matching modal mineralogy in the studied igneous rocks, especially acidic ones.
Modal Analysis - This type of analysis requires only a thin section. It produces an accurate representation of the distribution and volume percent of the mineral within a thin section. Allows counting of each mineral occurrence along a series of traverse line across a given thin section. It gives the maximum and minimum grain sizes. It gives real proportions of minerals in vol. %. The methods used are point counting of standard thin sections or polished stained slabs, Image analysis, and Powder X-ray diffraction.
Normative analysis - It involves the CIPW norm - Cross, Iddings, Pirsson and Washington 1902. The proportions of ideal mineral end-members are calculated as wt. %. The rocks need to be divided by density and recast to 100% to get percentages in vol. %. The calculation involves a hierarchical list of rules that tend to be often ambiguous. This analysis does not include hydrous minerals, yields phases often not matching modal mineralogy in the studied igneous rocks, especially acidic ones.
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