THE BARITE DEPOSITS OF MISSOURI 71 



extent by mechanical weathering. Chemical weathering is de- 

 pendent upon the relative solubilities of the materials attacked, 

 and their differential removal produces a segregation of those 

 substances which are the least soluble. The greater the differ- 

 ence in solubility the more readily is the segregation brought 

 about, and great insolubility means more complete concentration. 

 The disturbing factor in the chemical concentration is mechanical 

 erosion. If it is especially active, the concentration becomes a 

 question of size and specific gravity, and upon a steep slope even 

 these factors mean merely a retardation of complete removal. 



In discussing the origin of the barite deposits all these fac- 

 tors must be considered. Barite is a very insoluble mineral of 

 high specific gravity. Most statements of its solubility place it 

 at 1 part in 400,000. Smith^ gives the molar solubility as .00013 

 grms. Landolt-Bornstein-Meyerhoffer^ give the solubility in 

 water at 18.3° as 2.4x10-*. These figures show that barite has 

 a very low solubility in water, and, therefore, this is a favorable 

 factor in its residual concentration. 



The weathering 



The weathering of a rock is due to the combined action of 

 mechancial and chemical processes. The mechancial agents are 

 mainly temperature changes. These become less and less effect- 

 ive as the thickness of the mantle rock increases, until they 

 finally cease. Where the slope of the surface is sufficiently 

 great for the water to remove the residual particles mechanically 

 almost as fast as they are produced, the underlying rock is ex- 

 posed more or less continuously and temperature effects are im- 

 portant. Over most of this area there is a thick mantle rock, the 

 result of the rather gentle slopes in the areas underlain by the 

 Potosi and the Proctor formations. In a few places, usually 

 those adjacent to the larger streams where erosion is more active, 

 there is essentially no residual clay. 



On the other hand chemical changes usually proceed slowly 

 and are favored by a slow movement of the surface waters and a 



'Smith, Alexander, General Inorganic Chemistry. 

 'Landolt-Bomstein-Meyerhoffer, Physikalisch-Chemische Tabellen, p, 

 223. 



