Subsurface Laboratory Methods 197 



by the followftig procedure: (1) Pulverize the sample or disaggregate the 

 sandstone to minus-SO-mesh; (2) boil the material for one minute in 

 hydrofluoric acid; (3) wash the acid-treated sample gently in distilled 

 water; (4) boil the washed sample for ten minutes in a water-saturated 

 solution of eosine y; and (5) carefully wash the sample and remove the 

 excess dye solution. 



The plagioclase grains are coated with a medium- to dark- to 

 orange-red film. Orthoclase and quartz are not stained. The combination 

 of this test and the cobalt-nitrite test for orthoclase serves as a basis for 

 rapidly estimating the feldspathic content of sands and sandstones. The 

 sodic feldspars (albite, oligoclase, and andesine) are not noticeably af- 

 fected by the eosine test. 



Eosine dye may be used to identify nephelite and cancrinite (ortho- 

 silicates) . Nephelite assumes a light-pink discoloration, whereas can- 

 crinite attains a much darker pink. The discoloration is produced within 

 the grain and not as an exterior film as with the calcic plagioclases. 

 Sodalite is not affected by the dye. 



Clay-Mineral Stain Tests 



In recent years considerable work has been devoted to clay miner- 

 alogy. Mineralogic analyses have indicated three important groups of 

 the clay minerals (kaolinite, montmorillonite, and illite). Identification 

 of these clay-mineral species within each group is extremely difficult owing 

 to their minute size. Chemical, optical, X-ray, electron-diffraction, and 

 differential thermal-dehydration methods are required for precise deter- 

 mination. 



Several dye tests are employed for assisting in differentiating vari- 

 ous clay groups. Extreme care should be exercised in applying these tests, 

 for results may be extremely variable because of impurities, complex 

 mineralogic associations, and inconsistent preparation procedure. The 

 stain results may be observed in reflected light under either a petrographic 

 or binocular microscope at magnifications from 30 to 120 diameters. 



Benzidine Test — A saturated water solution of the organic compound 

 benzidine (or benzidine hydrochloride) produces a blue coloration in 

 contact with clay minerals of the montmorillonite and illite groups, al- 

 though the benzidine solution itself is slightly pink. The sample is not 

 treated with hydrochloric acid prior to application of the stain solution. 

 It has been reported that manganese dioxide and organic matter may 

 cause formation of a blue coloration in the absence of bentonite, and that 

 ferrous iron or other reducing agents may prevent the development of 

 coloration. ^^ Gypsum has a pronounced effect on the benzidine test. This 

 effect may be minimized by first boiling the material in water, pouring 

 off the fine fraction, thoroughly drying it at 105° C, and then applying 

 the stain solution. 



^' McConnell, Duncan, Notes on Properties and Testing of Bentonites: U. S. Bur. Reclamation, 

 Denver, Laboratory Kept. Pet-44B, 1946. 



