CHAPTER XXIII 



CALCIUM CARBONATE AND SILICA 



Occurrence of Carbonates. We have already seen that chalk 

 consists of calcium carbonate. The production of lime from 

 this source, together with its reconversion into calcium carbonate, 

 have been already studied in Chapter XXI. It occurs naturally 

 in several distinct forms, and many rock masses are largely or 

 entirely built up of this compound. When pure it occurs crys- 

 talline as the minerals calcite and aragonite, which differ only in 

 their crystalline forms and physical properties. Calcite is also 

 known as Iceland spar, calc spar, and by other local names. It 

 is generally quite transparent and somewhat resembles quartz, 

 from which it can be distinguished by its inferior hardness. It 

 is easily scratched by a knife, while quartz is unaffected. It is 

 what is called a doubly-refracting substance. If a clear crystal 

 of Iceland spar be placed upon the page of a book and the print 

 viewed through it, two images of each word will be seen. 

 Calcium carbonate also occurs more or less pure in the earth's 

 crust in a great variety of forms, such as chalk, limestone, 

 stalactite, stalagmite, travertine, &c., some of which have been 

 formed by purely chemical means, others by the aid of living 

 organisms. 



Those formed by chemical means owe their formation to the 

 power, possessed by water containing dissolved carbon dioxide, 

 of dissolving calcium carbonate, which is again precipitated when 

 carbon dioxide escapes. 



Travertine or Calcareous Tufa is precipitated by springs which 

 lose their dissolved carbon dioxide, which is necessary for the 

 solution of the calcium carbonate, as they flow onwards. The 



