•_>.v_' Van Tuyl — New Points on the Origin of DolomiU . 



this theory but regards the magnesia content of the more 

 highly dolomitic portions as having been enriched by reaction 

 with the magnesia of sea water. 



Experimental Evidence. 



On the experimental production of dolomite there is a vol- 

 uminous literature. This has been well summarized by F. W. 

 PfafE (38), and later by Steidtmann (39). Dolomite has been 

 frequently prepared artificially under conditions of high tem- 

 perature or high pressure, or both, but it has been produced 

 in the laboratory at ordinary temperatures and pressures only 

 in rare instances, and then in minute amounts and under con- 

 ditions which doubtfully operate in nature, at least on a large 

 scale. It must be conceded then that these experiments fur- 

 nish little evidence as to the actual conditions obtaining when 

 extensive beds of dolomite are formed naturally. For the pur- 

 pose of obtaining more accurate data on this point, a series of 

 experiments was begun at ordinary temperatures and pressures 

 early in 1912. In this series it was attempted to simulate 

 natural conditions as near as they could be estimated, and to 

 obtain some quantitative measurement of the effect of time 

 and of concentration in the production of dolomite. In one 

 set of experiments it was attempted to reproduce the condi- 

 tions which exist in nature when limestone is altered to dolo- 

 mite beneath the sea by solutions bearing magnesia. In these 

 the effect of solutions of known concentration of MgCl 2 and 

 MgS0 4 , and of mixtures of the salts, both with and without 

 the presence of NaCl, on powdered aragonite was tried. The 

 concentration of the magnesium solutions used ranged from 

 two to ten times the concentration of the magnesia in sea 

 water. After a period of six months, residues from the experi- 

 ments were thoroughly tested for dolomite. The results were 

 entirely negative. No trace of dolomite could be found. 

 Careful re-examination of the residues after a period of nearly 

 three years still gave the same result. The analyses showed 

 that the CaC0 3 had reacted slightly with the solutions, but no 

 MgC0 3 had been deposited. Apparently the soluble trihy- 

 drate of MgC0 3 had been formed. It then appears that dolo- 

 mite cannot be prepared artificially under these conditions. 



In a second set of experiments it was attempted to obtain 

 dolomite as a direct chemical precipitate at ordinary tempera- 

 tures and pressures. First solutions of the bicarbonates of cal- 

 cium and magnesium, after being standardized, were mixed in 

 molecular equivalent proportions so as to give the same ratio 

 of CaC0 3 to MgC0 3 as exists in normal dolomite. The solu- 

 tion was then allowed to evaporate spontaneously during a 



