It is not always possible to distinguish between anhydrite and gypsum by 

 observation. Mixtures of the two are very common, and alteration of anhydrite 

 to gypsum may occur under favorable conditions in a matter of a few weeks. 

 Ordinarily, the distinction can be made on the basis of the following character- 

 istics : 



Anhydrite is often amorphous-appearing though it does possess perfect 

 cleavage. Its hardness is considerably greater than that of gypsum; it cannot 

 be scratched with the fingernail. It tends to be translucent with a pearly luster. 

 Gypsum occurs as a fibrous-to-lacy mass of selenite crystals, as a glassy solid 

 mass with a subvitreous luster, or as a snowy earthy -to-massive rock (alabaster). 

 Any form of gypsum can be scratched easily with the fingernail. Anhydrite is 

 brittle; gypsum is usually spongy. One should note the manner in which the 

 chip breaks when crushed on the microscope stage. 



Rock salt (halite) can usually be identified by its taste, solubility in water, 

 and perfect cubic cleavage. When it occurs in finely disseminated crystals, 

 it might be confused with barite, which has a prismatic cleavage. Barite is in- 

 soluble in water and almost insoluble in cold or hot hydrochloric acid. 



There are certain other evaporitic salts of sodium and potassium, but their 

 importance as rock makers is comparatively minor. When such minerals are 

 suspected, and if it is important to identify them, quantitative chemical analyses 

 should be employed. 



As mentioned in an earlier paragraph, anhydrite sometimes alters readily 

 to gypsum. This process sometimes occurs when anhydrite is exposed to 

 certain types of fluid in the course of drilling a well. The alteration to gypsum, 

 which is accompanied by an increase in volume, results in swelling into the hole. 

 When this process is going on, abundant anhydrite cavings are liable to appear 

 in the samples some time after the formation has been drilled. When anhydrite 

 or gypsum recurs in the samples associated with lithologies which do not 

 suggest evaporitic conditions, the drilling records should be checked to deter- 

 mine if trips were made at the depths where the evaporites appear. These cavings 

 generally are caused by the shutdown of the mud pumps. 



SOME PRECAUTIONS IN As has been suggested in the foregoing pages, 



SAMPLE EXAMINATION there are numerous situations wherein the 



well-sample examiner might be led into er- 

 roneous interpretations of the samples. For the benefit of those to whom the 

 process of examining well samples is a new undertaking, the more common 

 pitfalls are listed below. The purpose of this check list is to put the novice on 

 guard against errors that are made all too often, occasionally even by those 

 having considerable experience. 



36 



