A common basis of petrographic descriptions of acid igneous rocks rests upon the quantitative determination of quartz content and feldspar types and contents. With these fundamental determinations almost all of these igneous rocks may be placed in their correct categories. In general, the quantitative determination of the ferromagnesian and other accessory minerals is not essential to the classification of these rocks. Much of this basic information necessary for rock classification is now obtained at the Kennecott Research Center by use of the infrared spectrophotometer. From standard calibration curves obtained by using individual pure minerals, and with the aid of synthetic mixtures of the diagnostic minerals, the quantities and types of minerals present in the unknown samples can be rapidly determined. To test the validity of the analytical results obtainable by this method, the standard granite (G-1) was used, which had been originally prepared and distributed as a powder by the U. S. Geological Survey for spectrographic and chemical analyses. This, therefore, constituted a good rock sample whose mineralogical composition was known, and was particularly useful as most igneous rocks associated with ore bodies are of this acidic type - granites, granodiorites, monzonites, and quartz monzonites.