The estimation of modal composition from bulk rock and mineral chemistries as examplified by granitic rocks in the Hiei, Hira and Kurama areas, central Japan

Abstract

The granitic rocks in the Hiei, Hira and Kurama areas of central Japan are characterized by their chemical compositions, especially the quantities of the alkali and the alkaline earth elements. The mineral analysis and the modal analysis were carried out to address how and which mineral species controled the difference in the chemical compositions of the whole granitic rocks.
     The modal analysis adopted in this study is same as the Ferry's method which was based on mass balances between chemical compositions of metamorphic minerals and those of whole rocks. However, the Ferry's method was modified to apply to the granitic rocks, that is, the numbers of mineral species and the equations were simplified, and a method of least squares was applied to solve the equations.
      The modal results were obtained secondarily using the results of chemical analysis of rocks and minerals, however this method is useful in checking up whether the analytical results were agreeable or not and in estimating the quantities of the main constituent minerals by weight percent.