132 Subsurface Geologic Methods 



If aggregates of particles still persist, the sample may be placed in 

 the sieves and shaken by hand for about one minute. The sediment in each 

 sieve is removed to a square of heavy brown paper, and the aggregates 

 are broken down by roiling the end of an iron pestle gently over the ma- 

 terial. At intervals the sand is resieved by hand to remove disaggregated 

 particles. The completeness of disaggregation is checked by examination 

 with a hand lens or binocular miscroscope. This method of disaggregation 

 causes little breakage of heavy minerals, even if the sandstone is very 

 quartzitic. The heavy-mineral grains, being of different composition, come 

 loose like peas from a pod. The sample is recomposed and sieved, the 

 procedures outlined in the discussion of that subject in this section being 

 followed. 



Acetylene tetrabromide of specific gravity, about 2.93, or bromoform 

 of specify gravity, about 2.86, is placed in a glass funnel, to the stem of 

 which is attached a piece of rubber tube closed by a pinch clamp. The 

 sand sample or sieve separate is introduced, and the mixture is stirred at 

 intervals until the heavy minerals have settled into the stem of the funnel. 

 The pinch clamp is opened, and the heavy fraction is washed onto a filter 

 paper in a second glass funnel. After the excess heavy liquid has been 

 filtered into a receptacle and returned to the stock bottle, the filter paper 

 is washed several times with alcohol. The light minerals and the remain- 

 ing heavy liquid are then drained onto another filter paper, the heavy 

 liquid is filtered off, and the light minerals are washed with alcohol. Both 

 heavy and light minerals are dried in an oven at 205° F. The alcohol- 

 heavy liquid washings are saved for recovery of the heavy liquid. 



When a large number of heavy-mineral separations are to be made, 

 mass-production methods can be used. The writer has employed two bat- 

 teries of six separation units each, and one assistant could make as many 

 as 24 separations a day. Most of these were of sand of one-eighth- to one- 

 sixteenth-millimeter size, requiring about two hours of alternate stirring 

 and settling to effect a satisfactory separation. For coarser sands that 

 settle more rapidly, more separations a day would be possible. 



Acetylene tetrabromide and bromoform are the most commonly used 

 heavy liquids. The writer prefers tetrabromide because its greater spe- 

 cific gravity reduces the number of altered grains, rock fragments, car- 

 bonates, micas, and chlorites in the heavy-mineral fraction. The acetylene 

 tetrabromide or bromoform is reclaimed from the alcohol washings by 

 shaking it with an excess of water and decanting the alcohol-water mix- 

 ture. This process is repeated several times, and the acetylene tetrabro- 

 mide clarified by filtering. 



Attention to two apparently minor details will save much time in 

 making heavy-mineral separations. When the heavy minerals settle, most 

 of them come to rest on the sloping sides of the funnels. If the suspension 

 is stirred vigorously, most of these heavy minerals go into suspension and 

 settle again on the funnel slopes. Consequently the suspension should be 



