Subsurface Laboratory Methods 131 



The color of the sunglasses was a disadvantage. It should be possi- 

 ble to obtain two nearly colorless polaroid discs. Place one on the micro- 

 scopic stage, mount the grains in oil on a glass microscope slide above it, 

 and mount the second polaroid disc on a cardboard support that could 

 be slipped in above the slide as needed. With this arrangement, however, 

 rotation of the grains might be difiicult. 



Heavy Minerals 

 General 



Heavy minerals are minerals of high specific gravity (2.86 to 2.96), 

 which occur in minor amounts in all sands and sandy limestones. Even 

 though present in small amounts, such heavy minerals as tourmaline, 

 zircon, hornblende, and staurolite may be exceedingly useful in correlat- 

 ing sands, outlining petrographic provinces, indicating sources and past 

 history of the source material and helping to decipher geologic history. 

 To facilitate their examination, heavy minerals are separated from the 

 quartz and other light minerals with which they are associated. 



The study of heavy minerals has the following disadvantages: (1) 

 The preparation and study of the samples are time-consuming, and conse- 

 quently the costs are relatively high. (2) The ability to use a petrographic 

 microscope is necessary. (3) An understanding of principles that govern 

 the size distribution and post-depositional modification of heavies is neces- 

 sary for the correct interpretation of the results. A lack of such under- 

 standing has been partly responsible for the present low opinion of heavy- 

 mineral studies in the oil industry. 



Preparation of Sample 



The heavy minerals of the entire sample or of one or more sieve 

 separates may be studied. H the sieve separates are used, particular care 

 should be given in cleaning the sieves, so that contamination will be kept 

 to a minimum. When marked differences in amounts or kinds of heavy 

 minerals occur in different samples, it may be well to sieve and discard 

 a preliminary sample. In carbonate rocks or in sandstones with carbonate 

 particles or carbonate cement, the carbonate should be removed prior to 

 sieving by boiling in dilute hydrochloric acid. 



Indurated rocks must be broken down to free the heavy-mineral grains. 

 A test sample of 5 to 100 grams may be soaked in water overnight and 

 then, after the excess water has been removed by siphoning or decantation, 

 the sample is boiled for ten minutes in 3 to 4. normal hydrochloric acid to 

 remove carbonates and iron oxides. The sample is transferred to a 1,000-cc. 

 beaker, and water is added to fill the beaker. After being stirred, the mix- 

 ture is allowed to settle one minute, and the upper 800 cc. of water and 

 sediment suspension is siphoned off. This washing process is repeated until 

 the water is clear after the one-minute settling period. This procedure re- 

 moves the fine silt and clay (less than about 0.01 mm.). 



