188 



University of California Publications. 



[Geology 



The mineral is soft and crushes easily to crystalline flakes 

 and flour with no gritty particles. It is easily fusible, and com- 

 pletely soluble in dilute acid, yielding gelatinous silica on 

 evaporation. Unlike colemanite or neocolemanite, it does not 

 calcine to a white powder and cannot therefore be so readily 

 separated from the gangue and carbonaceous impurities. 



Origin of the mineral. — It is evident from the position of 

 these masses of howlite in the deposit that the silico-borate was 

 formed from the same evaporating solution and simultaneously 

 with the crystallization of the neocolemanite. Granted that the 

 original mineral was a siliceous travertine or marl acted upon 

 by boracic acid then some of the silica became dissolved and 

 formed metasilieic acid, which replaced the water of crystal- 

 lization and boric acid that would otherwise have formed the 

 neocolemanite. These nodular masses of howlite probably repre- 

 sent silico-borate segregations in the solution similar to magmatic 

 segregations in the fused rock mass. 



The similarity of the two minerals is more apparent if we 

 express their composition as follows : 



It would seem from this that the howlite was formed in the 

 presence of silicic acid which was taken up in the place of water 

 and boric acid, and the resultant silico-borate was precipitated 

 more rapidly as finely crystalline masses. 



Chemical composition. — The mineral was dissolved in dilute 

 hydrochloric acid, the boron expelled by evaporation with 

 methyl alcohol, and the silica and calcium determined gravi- 

 metrically. The boric oxide was obtained in the same way as in 

 neocolemanite. The average analysis showed 



B 2 3 



45.56% 



CaO 



28.26 



Si0 2 



14.81 



H 2 



11.37 





100.38 



Specific gravity at 13° C = 2.531. 



Neocolemanite Ca 2 B 4 8 ■ 2B(OH) 3 

 Howlite Ca 2 B 4 8 ■ B(OH) 3 



2H,0 

 H 2 Si0 3 



