Chemical analyses of rocks with the petrographic microscope

The chemical composition of the granite (G-1) was calculated from quantitative miner-alogical (modal) analyses of petrographie thin-sections. The percentages of the major constituents, SiO₂, A1₂O₃, Fe₂O₃, FeO, MgO, CaO, Na₂O and K₂O were computed. These calculations were compared with the results of wet chemical analyses made by 30 chemists in 25 analytical laboratories. Each constituent calculated from the thin-section analysis of the rock standard lies within the range reported by the chemists. The arithmetic mean of the percentage of each constituent determined by the chemical analyses agrees closely with the analysis calculated from the mode

A norite sample was also analyzed by petrographic and chemical procedures. Agreement between the two techniques for this sample was close.

This study indicates that ior rocks in which the composition of the individual minerals has been obtained by optical measurements, the major constituents can be determined with the petrographic microscope by modal analyses, and the results may be considered reliable.