A few drops of hydrofluoric acid are placed on a thin section, or on 

 grains mounted in Canada balsam with their upper surfaces exposed, 

 and allowed to remain for 1 or 2 minutes before being gently washed 

 off. The acid produces a thin, gelatinous film of aluminum fluoro- 

 silicate on the feldspar and other aluminous minerals, but leaves the 

 quartz clear. After washing, the specimen is immersed in a water 

 soluble organic dye for about five minutes and then again washed. 

 Fuchsine, methylene blue, safranine, or malachite green may be used 

 as a stain . . . The degree of gelatinization, and therefore the depth of 

 color retained on staining, is greatest with anorthite; becomes succes- 

 sively lighter with less calcic feldspars; and is lightest with orthoclase 

 or microcline. 



Clay Minerals 



Staining methods can be used effectively in clay mineralogy. Mielenze, 

 King, and Schieltz (1951) discuss the procedures and results of many tests. 



Care should be taken in applying stain solutions to clay minerals because 

 impurities, improper or inconsistent procedure, and the complex nature of 

 the clay minerals may cause extremely variable results. Staining methods are 

 used principally as aids in petrographic analysis. They are also very useful in 

 routine examination of suites of samples when representative samples have 

 been X-rayed. 



Certain impurities affect many of the stain tests. For example, ferrous iron 

 and other reducing agents may prevent a color change, whereas manganese 

 oxide will give a blue color with benzidine even if clay minerals are absent. 

 On the other hand, quartz, feldspar, carbonates, gypsum, muscovite, biotite, 

 sericite, chlorite, iron oxides, glauconite, volcanic glass, and other nonclay 

 minerals do not affect the tests. 



The sample (about 20 grams) should first be pulverized, then divided into 

 two equal parts by quartering. To one portion hydrochloric acid is added and 

 heated to about 50C for 2 hours. It is then washed thoroughly with distilled 

 water. The washing is continued until silver chloride fails to form when silver 

 nitrate is added to the wash water. The sample is then dried in an oven at 105C 

 for about 24 hours. A small portion (about 1 milligram) of the dried sample 

 is then placed on a glass slide; and after staining, one evaluates the resulting 

 color by comparing it with a standard color chart. Comparisons are made with 

 petrographic microscope equipped with a comparator eyepiece. 



Tests are run on both the untreated and the acid treated portions of the 

 sample. The portion of the sample treated with hydrochloric acid is tested with 

 a nitrobenzene, solution of safranine "y" and malachite-green. The portion not 

 acid treated is tested with an aqueous solution of benzidine. Three or four 

 drops of the solution are added to the material on the glass slide and stirred 

 to wet the material. In the benzidine test, one should wait about 5 minutes before 



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