176 Subsurface Geologic Methods 



parency in thin slices, and their different reactions to transmitted, polarized 

 light constitutes the basis of optical mineralogy. Because the optical 

 properties of the minerals differ with the thickness of the section, 0.03 

 millimeter has been selected as an arbitrary standard thickness. 



Thin sections may be made of well-consolidated rocks, of friable or 

 less well-consolidated rocks, of individual minerals, of fragments of min- 

 erals or rock, or of a heavy-mineral concentration. The friable or poorly 

 consolidated material is impregnated with a bonding cement before sec- 

 tioning, whereas the fragmental material or heavy-mineral concentrate 

 requires a special technique that will be described later. 



The equipment necessary for preparing thin sections in the labora- 

 tory include a diamond power saw, power-lap wheels of cast iron, glass 

 plates, a hot plate, and a microscope. Materials needed are mounting 

 cement, abrasives, glass mounting slides, and cover glasses. 



The number of laps and grades of grinding compound used depends 

 upon the technique to be followed. At least two grinding laps are re- 

 quired, one each for course and fine abrasives. The technique briefly out- 

 lined below is used at the Colorado School of Mines; it may be varied to 

 suit the needs of individual specimens. 



Sawing and Grinding 



From the rock specimen to be studied, saw a piece 3 to 5 millimeters 

 thick with the two parallel, flat faces. Trim the edges until the final slice 

 measures approximately 30 x 22 x 3 mm. Now choose the smoothest side 

 and, with 80- to lOO-mesh abrasive on the course lap wheel, grind this 

 face until all traces of the saw marks disappear. Wash the slice thor- 

 oughly, transfer to the 320-mesh abrasive on the fine lap wheel, and polish 

 the ground surface. The polished surface is to be mounted next to the 

 glass slide and, therefore, must be perfectly flat. A final polish with 600- 

 mesh abrasive is often required. The worker must learn from experience 

 when this final polish is necessary. 



When the grinding is completed, wash the slice thoroughly to remove 

 all abrasive and foreign material. A tooth brush is helpful for this. The 

 shaped and polished piece now has one flat, smooth surface ready for 

 mounting.''^ 



Mounting the Slice 



Place the rock slice on a hot plate w^ith the polished side up and ex- 

 clude all moisture. When drying is complete, place a standard petro- 

 graphic glass slide (45 x 25 mm.) beside the specimen and allow the tem- 

 perature of both to become the same. (If a controlled-temperature hot 

 plate is available, keep it at about 300° F.) Cover the upper surface of 



^ The abrasives are kept in kitchen-size salt shakers. Abrasive is applied to the wet lap wheel as 

 needed. Some workers mix it with water in a bottle provided with a glass tube through the cork in order 

 that the contents of the bottle may be shaken on to the lap as required. Experience is necessary before 

 the operator can obtain the right mixture of water and abrasive that will produce the most efficient 

 cutting. 



