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GEOCHEMICAL PROBLEMS A S STUDENT PROJECTS1 MORRIS F., STUBBS New Mexico Institute of Mining and Technology, Socorro
OPPORTUNITY to undertake supervised research should be made available t o all qualified chemistry majors. An independent problem serves not only to stimulate interest in the student's chosen field, but also affords a means of introducing him to literature searching, independent experimental work, and report writing. If at all possible, such problems should be selected from an area in which the student has developed some interest. Since all chemistry majors at our institution complete a one-year course in the principles of geology as sophomores, several have developed an interest in geochemistry and have chosen t o undertake problem in this relatively new field which is concerned with the application of chemical methods to the solution of geological problems. A brief account of some aspects of recent work in this area done by students in our laboratory may, therefore, be of general interest. Two of our seniors have recently completed a biogeochemical investigation to determine if plants common to semiarid regions could be utilized to detect anomalous amounts of copper. I t is well known that certain plants growing in ore-bearing regions often contain larger amounts of valuable elements present in the ore than do the same plants growing in nonmineralized areas. Such anomalies are being utilized increasingly for locating are bodies and as an aid t o geological mapping. Usually the metal concentration found must 'Based on a paper presented before the Division of Chemioal Education a t the 133rd Meetine of the American Chemical Society, San Francisco, April, 1958. u
VOLUME 35, NO. 10, NOVEMBER, 1958
be two to three times that of the normal content for the area t o be of significance. A study of the ash content of five common desert plants growing in the Copper Flat area, near Hillsboro, New Mexico, has shown that in each case plants growing in a location known to contain little or no copper mineralization were found to have a much lower copper content than the same plants growing in mineralized areas. The differences were largest in the cases of the Spanish Bayonet, Prickly Pear, and Emory Oak, suggesting that these desert plants might possibly be used as copper concentration indicators. Spectrograms of the ashed samples were first made and compared with standards. Copper was also determined by titrating acid solutions prepared from the ashes with d i t h i z ~ n e . ~ . ~ Sampling and analysis of surface soils is usually the most successful method for geochemical exploration. This relatively new method is used chiefly as an aid to finding mineral deposits which may underlie primary residual soils. A knowledge of the basic geology of an area is essential before work is started. Soil samples are commonly obtained a t shovel depth along a series of traverses a t 50 to 100 foot intervals. The samples are hand pulverized and collected in heavy paper bags. Our students have made both spectrographic and 'WARREN,H. V., R. E. DELAVAULT AND R. I . IR~SH, Bull. Geol. Soc. Am., 63, 435-84 (1952). E. B., "Colorimetric Determinations of Trsees of SANDELL, Metds," Intersoienoe, New Yark, 1950, pp. 100-106.
chemical analyses of soils in two mineralized areas and found interesting anomalous amounts of copper and manganese. We have used a modification of the improved carbamate method of Cheng and Bray4 in analyzing for copper. This method is based on the fact, that copper and bismuth are the only metals sequestered from versenates by sodium diethylthiocarbamate. The transmittancy of the resulting golden-brown CCla solution was measured at 5000 A. with a Beckrnan Model B spectrophotometer. The parts per million of copper in the samples were determined from a previously prepared calibration curve. The manganese determinations were made by oxidizing the sample solutions to permanganate and comparing the colors obtained with standard solutions. One of our students has studied the relation of the uptake of trace elements by the Creosote Bush to pH and the concentration of other elements in the soil in which the plant grows. Others have carried out studies to determine if deficiencies of trace elements necessary for plant and animal metabolism exist in the soil in this region of the Rio Grande Valley. These two studies did not require any previous knowledge of geology. Our projects have all developed lively interest, and the students who have participated have obtained much valuable experience. We are now extending our methods of geochemical exploration to include the
application of chromatographic methods of analy~is.~ Other projects which might be developed include: Geochemical prospecting for other heavy metals, such as lead, zinc and uranium; investigation of the distribution and association of trace elements, including the reasons for any relationships found. Although we have included spectrographic and spectrophotometric methods in various phases of our projects, it is not necessary to have expensive equipment to engage in geochemical prospecting or traceelement detection. The content of many heavy metals in plants, stream waters, and soils may be estimated with surprising accuracy through the use of simple, inexpensive equipment and the utilization of visual colorimetric comparison with suitable standards. The principles of chelation, solvent extraction, and chromatography may be illustrated with these simplified methods. Robertson6 gives methods of geochemical prospecting recommended hy the United States Geological Survey and a bibliography of geochemical prospecting. WHENO, K. L., AND R. H. BRAY,Anal. Chern., 25, 6 5 5 6 9 (1953). $HUNT,E. C., A. A. NORTH, A Y D R. A TELLS, Analyst, 80, 172-94 (1955). @ROBERT~ON, F., "Georhemicsl Prospecting by Soil Analysis in Montana." Bulletin 7. Montana Bureau of Mines and Geoloev.
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