Subsurface Laboratory Methods 195 



termining these minerals and for evaluating their interrelationships in a 

 rock are here briefily outlined. 



Calcite and Aragonite 



Meigen ^^ developed a method of distinguishing aragonite from cal- 

 cite by immersing a polished rock surface or thin section of each for 20 

 minutes in a solution of boiling cobalt nitrate and observing the resulting 

 color. Aragonite stains a light purple in the initial stages but upon con- 

 tinued boiling assumes a violet hue. Calcite attains a similar color 

 only after several hours of immersion. For fine-grained rocks the two 

 minerals are difficult to differentiate owing to the spreading of the stain. ^- 

 Aragonite grains treated with cobalt nitrate, when immersed in a solution 

 of ammonium sulphide, become coated with a film of black cobalt sul- 

 phide. 



Calcite and Dolomite 



Several staining methods are used for distinguishing calcite from dolo- 

 mite. The results of these stains differ, some being more dependable and 

 exacting than others. These tests are given in the order of preference. 



Fairbanks Method ^^ — In the Fairbanks method the solution to be 

 used is prepared by mixing 0.24 grams of haematoxylin, 1.6 grams of 

 aluminum chloride, and 22 cc. of water and bringing the mixture to a boil; 

 the solution is cooled and, after the additions of a small quantity of hydro- 

 gen peroxide, filtered. Calcite, upon being immersed in the solution, 

 rapidly stains dark purple, whereas dolomite remains unaffected. The ad- 

 vantage of this test is its rapidity and dependability. The polished surface 

 or thin section is allowed to remain in the stain o'nly thirty seconds, and is 

 then removied and carefully washed in water. Boiling the rock in the stain 

 solution is not required. This method is exceptionally favorable for evalu- 

 ating limestone and dolostone fragments in well samples. 



Copper Nitrate Method — Calcite boiled in a concentrated solution of 

 copper nitrate assumes a medium-green color; dolomite is not affected. 

 The color may be fixed by immersing the sample in ammonia. This test 

 is effective and yields consistent results. 



Silver Chromate Method — The polished surface or thin section is 

 immersed three or four minutes in a boiling ten-percent solution of silver 

 nitrate. The nitrate is then washed free from the sample, which is sub- 

 sequently treated with a saturated solution of potassium chromate. Cal- 

 cite and aragonite grains are stained reddish-brown; dolomite retains its 

 original color. This method is accredited to Lemberg ^^ and may be con- 

 sidered as giving dependable results. 



*^ Meigen, W., Eine einfach Reaktion zur Unterscheidung von Aragonit Kalkspath: Centralb f. 

 Min., etc., pp. 577-578, 1901. 



^ Twenhofel, W. H., and Tyler, S. A., Methods of Study of Sediments, p. 129, New York, McGraw- 

 Hill Book Company, Inc., 1941. 



^^ Fairbanks, E. E., A Modification of Lemberg's Staining Methods: Am. Mineralogist, vol. 10, pp. 

 126-127, 1925. 



** Lemberg, J., Zur microchemischen Untersuchung einiger Minerale: Zeitachr, geol. Gesell., Band 

 44, pp. 224-242, 1892. 



