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the EDITOR'S

column L.T.Hallett, Editor

SE OF THE infrared absorption spectrophotometer for inor­

ganic and mineralogical examina­tions represents a considerable de­parture from its usual application in structural studies and analysis in organic chemistry.

During the past three years, for example, the Kennecott Research Center, Salt Lake City, Utah, has prepared 4000 infrared curves of standard minerals, rocks, and other inorganic materials. Results of these studies were reported by R. J. P. Lyon, W. M. Tuddenham, and C. S. Thompson at the Rocky Mountain Minerals Conference. This meeting was sponsored by the Utah Section of AIME.

In their paper, "Quantitative Mineralogy in 30 Minutes," the au­thors said that the infrared tech­nique can yield both qualitative and quantitative results in mineralogical examinations. The technique has the added advantage of produc­ing reproducible results rapidly. Technicians can do this work.

The authors use a portion of the assay pulp sample (at minus 200 mesh) which has been prepared for wet chemical analysis. In addition to eliminating the need for prepar­ing a second sample, the pétro­graphie results from the infrared studies are obtained on the same sample used for the wet analysis.

Unlike x-ray diffraction tech­niques, which are concerned with preferred orientation problems, in­frared absorption curves are not modified by grain size as the sam­ples are ground to —1 micron. This is a particular advantage in pétrographie examinations of fine­grained and even cryptocrystalline rocks. The authors have even ob­tained excellent spectra from sam­ples that would be considered amorphous by x-ray diffraction.

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VOL. 3 1 , NO. 3, MARCH 1959 · 5 5 A

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The infrared method also allows use of samples consisting of a single grain, a rock chip, or sample from hundreds of feet of drill core.

The method does not allow de­termination of texture which is re­quired to deduce the full pétro­graphie history of the rock. It will, however, give the present compo­sition of the rock, regardless of the degree of alteration or degree of weathering the sample has under­gone. The infrared method is sen­sitive for minerals formed by altera­tion or weathering.

Pétrographie descriptions of rocks are based on the quantitative determination of quartz content and feldspar types and contents. Based on these fundamental deter­minations, almost all rocks may be placed in their correct categories. The quantitative determination of black ferromagnesian and other ac­cessory minerals is not essential to classify rocks.

At the Kennecott Research Cen­ter standard calibration curves are prepared with the infrared spectro­photometer using individual pure minerals. By using synthetic mix­tures of minerals, the quantities and types of minerals present in un­knowns is determined rapidly.

Validity of analytical results ob­tained in this manner has been checked against the standard gran­ite (G-l) samples distributed by the U. S. Geological Survey. This has been found to be a good sample type to use, as most igneous rocks associated with ore bodies are of this acidic type.

The infrared method is insensi­tive to less common components such as biotite, muscovite, opaque and nonopaque accessories found in the standard granite sample. The technique used involves pre­paring KBr disks consisting of 2.50 mg. of a preground sample blended into 1.00 gram of KBr and weighing out enough of the blend to form a 12-mm.-diameter disk of the de­sired thickness. The disks are formed in a vacuum die under 12.5-ton pressures. The spectrophotom­eter used has rock salt optics. The so-called low-cost infrared spectro­photometer can be used in this work.

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