8 



PEOCEEDINGS OF THE NATIONAL. MUSEUM 



VOL. C9 



follow SiOa, FcaOs, K2O, and MgO. The number at the head of the 

 column of ratios shows the proportion of the end members in the 

 make-up of that analysis. 



The analyses of glauconite given in the table have been compiled 

 from various sources and the water has not been determined under 

 similarly controlled conditions, and in most of them it has not even 

 been determined separately below and above 100°. For these reasons 

 they give no reliable evidence of the part water plays in the composi- 

 tion of glauconite. However, the water content of the unusually 

 pure glauconite from the Bonneterre dolomite of Bonneterre, Mo., 

 has been carefully determined for different temperatures in the chem- 

 ical laboratory of the United States Geological Survey, and the de- 

 hydration curve is given in Figure 2. 



O 1 Z 3 i S b 7 ro water 



Fig. 2. — ^.Dehydration curvej of Missouri glauconite. 



There is a sharp break in the curve at 280° and below that temper- 

 ature about 3.30, and above about 3.26 per cent of the water is given 

 off, the total water in the sample being 6.56 per cent of the whole. 

 There is good reason for believing that the water above this sharp 

 break is constitutional and below is water of crystallization or ad- 

 sorbed water. (2H2O) seems to represent the constitutional water 

 in the formula for glauconite, since that amount would demand 

 3.06 per cent, and since the ratio of theoretical water to water 

 given off about 280° is 3.06 to 3.26 or 1.06. Glauconite is a finely 

 micaceous mineral and minerals of this type adsorb water freely. 

 For this reason it is probable that the water given off below 280° is 

 adsorbed water and not water of crystallization. The total water 

 in glauconite is somewhat variable, but the average is about SHgO. 



