■4:76 Johnston, Merwin) and Williamson — 



lower than this, one must pay particular attention to the homo- 

 geneity of the sample if a valid conclusion as to its constitution 

 is to be drawn from the test. Sorby* used the density as a 

 means of distinguishing calcite from aragonite in shells, etc.; 

 his conclusions are indisputable in all cases in which he 

 observed a high density, in materials which are essentially pure 

 calcium carbonate. 



Several so-called chemical tests have been proposed, and two 

 of them have been quite generally adopted. These tests we 

 shall now discuss briefly, for we have found that they cannot 

 be relied on in all circumstances, and least of all where good 

 tests would be most desirable, i. e., for differentiating the 

 several forms when in a very fine state of division or as aggre- 

 gates of fine particles. The two most generally used were 

 suggested by Meigen ;f they are as follows : 



I. Aragonite immersed in a dilute solution of cobalt nitrate 

 slowly becomes lilac and the color develops rapidly on boiling ; 

 calcite in a similar solution remains uncolored in the cold but 

 slowly becomes blue on heating.;}: 



II. In a solution of ferrous sulphate calcite causes a yellow 

 precipitate,§ aragonite a dark green precipitate. 



Two other tests have been suggested by Thugutt.| 



III. Aragonite is turned pink by congo red or alkaline 

 alizarin, but calcite remains uncolored. 



TV. Aragonite turns red if treated for a second with 

 0"1 JV AgN0 3 followed, after washing, by a 20 per cent solution 

 of K 2 Cr 2 7 . 



V. .Niederstadt^f proposed to use as a more quantitative test 

 the rate of precipitation of carbonates of various metals when 

 calcium carbonate in one or other form is added to their salts ; 

 aragonite is stated to be more active in precipitating Mn, Zn 

 and Fe, calcite in precipitating Cu, Pb and Ag. He asserts, 

 for instance, that if one gram of calcite or aragonite powder (its 

 degree of fineness and homogeneity of size are not specified) 

 is added to a boiling solution of manganese sulphate, then in 

 five minutes 74 per cent of the theoretical amount will be pre- 

 cipitated if the powder is aragonite but only about 1 per cent 

 if it is calcite. 



Of these, the test which has been most frequently employed 

 is the cobalt nitrate test. Some of its limitations should be 



*H. C. Sorby, Quart. J. Geol. Soc, xxxv, 56, 1879. 



fW. Meigen, Centralblatt Min. Geol., 1901, 577. 



% According to Lange (Inaug. Diss., Freiburg, 1904) the differences in color 

 are due to the formation of basic cobalt carbonates differing in composition. 



§ In some books the erroneous assertion is made that this is ferric 

 hydroxide. It is somewhat difficult to state the pi'ecise nature of these two 

 precipitates ; both are probably hydrous or basic ferrous carbonates. 



|| Thugutt, Centralblatt Min. Geol.,. 1910, 786. 



UNiederstadt, Z. angew. Chern., xxv, 1219, 1912. 



