The total alkalis—silica diagram TAS

Volcanic rocks of the subalkaline series have been further subdivided on the basis of their concentrations of K2O and SiO (Peccerillo and Taylor, 1976). Le Maitre et al. (1989) propose a division of subalkaline rocks into low-K, medium-K and high-K types and suggest that these terms may be used to qualify the names basalt, 

The norm of a rock may be substantially different from the observed mineralogy — the mode. The calculations assume that the magma is anhydrous thus minerals such as biotite or hornblende are not permitted. Farther simplifying assumptions are that no account is taken of the minor solid solution of elements such as Ti and Al in ferromagnesian minerals and that the Fe/Mg ratio of all ferromagnesian minerals is assumed to be the same. The normative mineralogy is based entirely 

Rules for the CIPW norm calculation after Kelsey (1965) and Cox et at. (1979) 

It is helpful to use a standard form when calculating a norm by hand. This may be done by laying out the oxides (with their molecular weights and therefore the molecular proportions) as columns along the top of the page and the more common normative minerals as rows along the left-hand margin. The boxes of the fable are filled as the calculation is made and can be used to keep a check on when an element is totally used up. Constants used in the CIPW norm calculation are listed below. 

Oxide Molecular weight Normative mineral Formula Molecular weight 

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