198 Subsurface Geologic Methods 



Crystal-Violet Test — The crystal-violet dye solution (25 cc. of nitro- 

 benzene, 0.1 gram of crystal violet) causes acid-treated montmorillonite 

 first to stain green and then greenish yellow or orange yellow. lUite as- 

 sumes a rather dark-green color. Kaolinite merely absorbs the violet stain. 



Safranine y Test — Another stain applicable for identifying clays of 

 the montmorillonite and illite groups is the safranine y (nitrobenzene 

 saturated with safranine y) . McConnell ^^ summarizes this test as follows: 



(1) A small representative sample (about 20 grams) is selected, crushed, 

 and placed in a beaker; (2) strong hydrochloric acid is added in amounts 

 four or five times the volume of earth material. If significant amounts of 

 carbonates are present the quantity of acid is proportionally increased. The 

 sample in acid is retained at elevated temperatures for an hour or two. Suitable 

 temperatures can be obtained by placing the sample on top of a small labora- 

 tory oven; (3) the acid-treated sample is washed five times, using 200 milli- 

 liters of distilled water for each washing. The earth material is then trans- 

 ferred to a filter paper, which is placed in a dish and oven-dried at about 

 105° C. ; (4) the dried material is examined and one or more samples are 

 removed from the filter paper for staining. Considerable care must be exer- 

 cised in the selection of this sample (or samples) because stratification invar- 

 iably takes place in the funnel during washing; (5) three or four drops of 

 nitrobenzene saturated with safranine y are added to the mineral powder and 

 the quantities of colorless, red, purple, and blue grains are estimated. The 

 quantity of blue and purple grains compared with the total number is an 

 indication of the amount of bentonitic material present. 



This test is apparently capable of giving anomalous results in rare 

 instances but is probably subject to interferences no more frequently than 

 the benzidine test. 



Kaolinite is unaffected by safranine y. Minerals of the montmoril- 

 lonite group become blue when the dye is applied, whereas illite grains 

 tend to exhibit a more bluish-purple to purplish hue. 



Malachite-Green Test — After being acidized with hydrochloric acid 

 the clay minerals of the kaolinite group, when in contact with malachite- 

 green solution (25 cc. of nitrobenzene, 0.1 gram of malachite green), 

 become a bright apple-green. The montmorillonite and illite minerals 

 commonly become pale yellow or greenish yellow. 



Summary of Clay-Stain Results 



In table 7 results of clay-stain tests are given in summary. 

 For favorable results in clay-stain tests the following precautions 

 should be observed. 



1. The acidization (HCl) procedure should be complete and uni- 

 form. Best results are obtained if the material is pulverized and passed 

 through a 200-mesh screen. 



2. After step (1), the sample should be thoroughly washed free of 

 the acid with distilled water; otherwise, consistent stain results cannot be 

 obtained. 



' McConnell, Duncan, op. clt. 



