eluded here are brief, but references to more thorough treatment are given at the 

 end of this chapter. 



THIN-SECTION The methods of thin-section preparation have 



PREPARATION not changed in the last 100 years, although 



the techniques have been improved. The main 

 steps in preparing a thin section still include sawing a chip, cementing it to a 

 glass slide, reducing it to proper thickness, and protecting it with a cover glass. 

 With suitable equipment, thin sections may be prepared in nearly any size, 4 x 

 5 inches being not uncommon; but for most work, standard 26 x 45-millimeter 

 petrographic slides are satisfactory. The equipment needed to prepare thin sec- 

 tions depends on whether mass or occasional production is desired. Except for 

 research departments or for special projects, facilities for occasional production 

 are adequate. Most thin sections are made now by professional men who can 

 produce large numbers at a moderate price ($1.25-$1.75) . 



Reed and Mergner (1953) give a thorough account of thin-section prepara- 

 tion as followed in the United States Geological Survey laboratory. They also 

 describe techniques used for special rocks and for repairing damaged slides. 

 Meyer (1946) describes a method that involves sawing off as much of the excess 

 material as possible after the specimen has been cemented to the slide. Rowland 

 (1953) describes a method involving use of a surface grinding machine that 

 has a vertically arranged diamond-impregnated grinding wheel. By this change 

 from conventional horizontal-turning laps, sections can be ground faster and 

 more accurately. 



The equipment necessary for thin-section preparation includes a rock saw, 

 grinding laps, hot plate, plate glass, and a polarizing microscope. A large variety 

 of rock saws is available, but all are similar to the one shown in Figure 4-1. Al- 

 though it is possible to prepare a thin-section with a single grinding lap, it is 

 more efficient to have three or even four. Conventional laps have a horizontal 

 turn and are mounted in basins to catch cuttings and excess abrasive material. 

 The setup shown in Figure 4-2 is typical. Nearly any type of hot plate having 

 a heat-control rheostat is satisfactory. A small plate has the advantage of heating 

 and cooling quickly; a larger one may be better controlled. An ordinary plate 

 glass will serve as a base for final grinding, but it must be discarded at the first 

 sign of concavity. A polarizing microscope is required to determine proper 

 thickness of the thin section (0.03mm) , but one no longer suitable for regular 

 petrographic work is usually adequate. Some of this equipment is shown in 

 Figure 4-3. For those who desire compact, inexpensive equipment, combination 

 machines that can be used for both sawing and grinding are available. 



Friable sedimentary rocks must be impregnated with a binding material be- 

 fore being sectioned. Most rocks that are coherent enough to yield cores are 



60 



