If the mineral takes flakes of steel off the needle and shows no abrasion, the 

 hardness is approximately 6, or greater. Quartz, feldspar, and many other sil- 

 icates fall in this category of hardness. If the hardness is less than 6, the mineral 

 will show abrasion, and no steel will be cut from the needle. In this case, next 

 try the brass wire, whose hardness is from 3 to 4. Proceed as above, using 

 very light pressure, until either the wire or the mineral shows abrasion. If the 

 grain is scratched by the brass wire, try the fingernail, whose hardness is about 

 2i/2- Selenite (gypsum) is easily scratched by the fingernail; anhydrite is not. 

 By working carefully between these gauges, one can make accurate estimates 

 of the hardnesses of minerals from 1 to 6. After considerable practice and experi- 

 ence, one can estimate the hardness with reasonable accuracy by scratching 

 with a steel needle. 



The hardness of small grains imbedded in very fine matrix is judged by 

 rubbing the flat surface of a knife blade over the area. If the grains are harder 

 than the metal, they will gouge tiny flakes of metal from the tool. The side of the 

 needle or brass wire can be used in the same manner. 



Solubility: The acid reactions of limestones and dolomites have been de- 

 scribed in detail on page 31. It is sometimes desirable to observe the general 

 characters of insoluble constituents of carbonates, although time and lack of 

 facilities will not permit a standard insoluble-residue analysis. A satisfactory 

 substitute method consists in placing a small chip in the acid dish and, when the 

 reaction has ceased, re-examining the insoluble portion under the microscope. 

 In order to save time, regular examination of samples down the hole is continued 

 while the acid reaction is in progress. When such tests are frequent, several acid 

 containers can be used to advantage. The insoluble residue may be examined 

 under the microscope while still immersed in the acid, provided the effervescence 

 has ceased and only low-power lenses are used. When the insoluble portion does 

 not break down, the aggregate may be carefully lifted out of the acid with the 

 forceps, lightly rinsed in water, and laid on blotting paper for examination. 



The water solubility of certain salts has been discussed. Barite in finely 

 divided particles may appear much like some varieties of anhydrite. The 

 material should be boiled in hydrochloric acid. Anhydrite is slowly soluble, 

 barite is not. Then the solution may be tested with a few drops of barium 

 chloride solution. If a sulphate is in solution, a heavy white precipitate is 

 formed. 



Ignition: Solid hydrocarbons and carbonaceous materials such as gilsonite 

 and coal rarely occur as redeposited grains in sandstones and shales. To test, 

 one should hold a chip in the forceps in the flame of a match. After the chip 

 is heated for a moment, it is removed from the flame and the odor of the fumes 

 noted. Liquid or solid hydrocarbons and coal have quiet different odors on 

 ignition. Coals leave a powdery ash; hydrocarbons leave a black film or a 

 spongy mass. 



39 



