174 Subsurface Geologic Methods 



Rapid Method of Heavy-Mineral Separation ^^ 



One of the major drawbacks to heavy-mineral studies is the time 

 required in making clean separations. A competent worker with the proper 

 setup can make a complete heavy-mineral separation, including the per- 

 manent micro-mount, in a lapsed time of 10 minutes by following the steps 

 outlined and illustrated below. 



Necessary Equipment 



Figure 72 shows the equipment required in the procedure for making 

 a single separation. The articles shown are bromoform, a standard 3-inch 

 evaporating dish, stainless-steel teaspoon, filter paper, product to be con- 

 centrated, wash bottle with alcohol, adjustable wooden rack, and glass 

 funnels and beakers for filtering. The setup may be varied to fit the size 

 of the problem. If a number of separations are to be carried on, the 

 process may be speeded up by having the workbench arranged for multi- 

 ple equipment. 



Procedure 



Figure 73, steps a, h, c, d, e, and /, illustrate the stages of making a 

 separation. Step a: The prepared and weighed sample is placed in the 

 evaporating dish with sufficient bromoform (diluted to desired density) 

 to float the light fraction freely. The material is then stirred thoroughly 

 with the teaspoon in order that the "heavies" may sink. Step h: After 

 the sample is stirred, it is allowed to settle for one-half to one minute, 

 and the larger part of the floating light fraction is spooned off into the 

 filter. Step c: The remaining "lights" are then carefully poured off in 

 successive order with a few seconds of gentle circular motion (panning) 

 between each pouring. Step d: The filter containing the bromoform and 

 "lights" is then held above the evaporating dish to allow the bromoform 

 that is filtering out to wash down the sides of the dish. This step is repeated 

 until the separation is complete and the spout or pouring side of the dish is 

 free from the light fraction. Step e: The final concentrate is washed from 

 the dish into the second filter with alcohol from the wash bottle. The 

 "'heavies" are then washed clean of bromoform, filtered, and dried on a 

 hot plate. Step f: The final, dried, heavy fraction is then divided and 

 mounted as desired. 



If proper caution is exercised throughout the procedure, a clean sepa- 

 ration is obtained. Bromoform is expensive and care should be taken to 

 avoid wastage. 



Preparation of Thin Sections 



A thin section is a slice of rock or mineral 0.03 millimeter thick 

 mounted on a glass slide for examination under the petrographic micro- 

 scope. The nonopaque, rock-forming minerals have a high degree of trans- 



*' Adapted from procedure teughl the senior author by Dr. J. L. Anderson, Department of Geology, 

 Johns Hopkins University. 



