250 wells: ARAGONITE FROM SEA-WATER 



were well washed with distilled water, dissolved in acid, and 

 tested for the presence of sulfate. A small amount of sulfate was 

 shown. This agrees with the results of others on natural aragonite 

 formed from the sea.^ No lead could be detected, and hence the 

 possibility that lead carbonate nuclei determined the formation 

 of the aragonitic form seems to be excluded. 



One might account for the deposition of this aragonite on the 

 theory that the water was originally supersaturated with it. 

 It seems more reasonable, however, to assume that some change 

 must have occurred in the water. The most plausible explanation 

 appears to be that very slow attack of the glass by the water 

 caused a series of chemical reactions leading to the deposition of 

 calcium carbonate in the less stable form of aragonite. The carbon 

 dioxide in sea-water exists principally in the form of bicarbonate 

 ions, HCOa", which constitute a very small fraction of all the 

 anions present. Na+ and Cl~ are present in relatively great 

 excess. The alkali dissolved from the glass yields hydroxide 

 ions, OH~. The following reactions may be assumed to have 

 occurred : 



OH- + HCO3- = CO3— + H2O (i) 



CO3— + Ca + + = CaCOg (2) 



Another possible explanation of the deposit is that some of the 

 carbon dioxide was boiled out of the water before the tubes were 

 sealed up. This explanation, however, seems less probable than 

 the other. 



Several writers have pointed out that recently formed deposits 

 of calcium carbonate in nature are likely to be aragonite, whereas 

 the oldest deposits are principally calcite. The facts here noted 

 obviously have a bearing on the deposition of aragonite in nature, 

 the principal difference in conditions being that a loss or gain 

 of carbon dioxide from the water in the tubes was impossible. 

 As there is a large literature on the subject of the deposition of 

 calcium carbonate from sea- water it appeared to the writer worth 

 while to make further analvtical determinations on this water 

 with special reference to the question of the solubility of calcium 



2 Johnston, Merwin, and Williamson. Anier. Journ. Sci. 41: 508. 19 16. 



