Subsurface Laboratory Methods 133 



stirred gently in such a way that the heavy minerals are worked down the 

 slope of the funnel and into its stem. Then a vigorous stirring will free 

 additional heavy minerals that are trapped with the light minerals at the 

 top of the funnel. 



Selection of a proper filter paper is also important. The most porous 

 paper that permits the heavy minerals to be caught and recovered should 

 be used. The writer uses Whatman No. 4. A less porous paper, which 

 filters more slowly, may double the time required for a heavy-mineral 

 separation. 



The writer weighs the heavy-mineral fractions of most samples to 

 the nearest half -milligram on an analytic balance. This weighing permits 

 computation of the hydraulic ratio ^^ if that is needed. For most routine 

 work, weighing the heavy-mineral separates will not be necessary. 



The heavy-mineral fractions are split to 1,000 to 1,500 grains with 

 an Otto miscrosplit ^^ and mounted in Canada balsam on 1- by 2-inch glass 

 microscope slides. Often more than one mount is needed, especially when 

 much pyrite or barite is present in the heavy-mineral fraction. The sample 

 number, size grade, and slide number are scratched on each slide with a 

 diamond pencil. This provides a permanent mount, to which reference 

 can be made at a later date. For easier identification, splits of the heavy- 

 mineral fraction may be mounted in oils of various refractive indices. 



Mineral Identification 



Heavy minerals are usually studied with a petrographic microscope, 

 using medium power (5 X ocular and an 8-mm. objective) for most work. 

 The properties most useful in rapid identification of the mineral grains 

 are opaqueness or translucence, appearance in reflected light (particularly 

 for opaque minerals, rutile, and tourmaline), color, relief as compared 

 to the mounting medium, pleochroism, inclusions and alteration products, 

 crystal form, grain shape, cleavage, birefringence, extinction angle, and 

 isotropic or anisotropic character. Milner's ^^ book probably is the best 

 for mineral identification. Krumbein and Pettijohn"*^ and Russell ^° also 

 have tables that may be used. These identification tables list many prop- 

 erties of the minerals, but those given above are most useful. 



If a large number of samples is to be studied, time and effort can be 

 saved by mounting the heavy-mineral suites in oils of various indices of 

 refraction and identifying all species that are present. Ordinarily minerals 

 that are very rare in the heavy-mineral separate may be neglected, as their 

 presence or absence in any amount is a matter of chance. Then simple 

 criteria can be set up by which each mineral can be recognized at sight or 



''■' Rittenhouse, Gordon. Transportation, and Deposition of Heavy Minerals: Geol. Soc. America Bull., 

 vol. 54, no. 12, pp. 1725-1780, Dec. 1, 1943. 



'^ Otto, G. H., Comparative Tests of Several Methods of Sampling Heavy Mineral Concentrates : 

 Jour. Sedimentary Petrology, vol. 3, no. 1, pp. 30-39, April 1933. 



*^ Milner, H. B., Sedimentary Petrography, 3d ed., 666 pp., London, Thomas Murby & Co., 1940. 



** Krumbein, W. C, anJ Pettijohn, F. J., Manual of Sedimentary Petrography, 549 pp.. New York, 

 D. Appleton-Century Co., 1938. 



■** Russell, R. D., Tables for the Determination of Detrital Minerals: Nat. Research Council Co 

 on Sedimentation Kept., 1940-1941, pp. 6-8, 1942. (Separate copies of tables, 50 cents.) 



-.omm. 



