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Calculation of the CIPW norm: New formulas
that are available for calculation of the norm. The
SINCLAS implementation of Verma et al (2002, 2003) which is based and modied after Kelsey (1965) is the most reliable and consistent of all.
Additionally, there is no unique way of xing the FeO/Fe2O3 ratios (oxidation ratios) of igneous rocks which in turn, signicantly change the nature and amount of the normative minerals of rocks (Middlemost 1989). Therefore no standardization is possible without evolving a universally agreed procedure to determine the oxidation ratio. Oxidation ratios obtained from chemical analysis either are not acceptable as most of the igneous rocks show higher Fe2O3 in their analysis because of the oxidation postdating their consolidation. Until a method is evolved to uniquely x the oxidation ratio, standardization can only be achieved for the details of the calculation procedure. The SINCLAS system of Verma et al (2002, 2003) even oers three options on how to x the FeO/Fe2O3 ratios and involves an amount of subjectivity on the part of the user.
At the end of the calculation of norm for a rock it is customary to report the sum of weights of
Keywords. CIPW norm; igneous rock classication; oxidation ratio; standard igneous norm; normative minerals.
J. Earth Syst. Sci. 118, No. 1, February 2009, pp. 101113
Printed in India. 101
Kamal L Pruseth
Institute Instrumentation Centre, Indian Institute of Technology Roorkee, Roorkee 247 667, India. A completely new set of formulas, based on matrix algebra, has been suggested for the calculation of the CIPW norm for igneous rocks to achieve highly consistent and accurate norms. The suggested sequence of derivation of the normative minerals greatly deviates from the sequence followed in the classical scheme. The formulas are presented in a form convenient for error-free implementation in computer programs. Accurate formulas along with the use of variable molecular weights for CaO and FeO; corrected formula weights for apatite, pyrite and uorite; and suggested measures to avoid signicant rounding o errors to achieve absolute match between the sum of the input weights of the oxides and the sum of the weights of the estimated normative minerals. Using an analogous procedure for determining the oxidation ratios of igneous rocks as used in the SINCLAS system of...