GRAIN DENSITIES, WEIGHT-HUMIDITY AND WEIGHT-TEMPERATURE RELATIONS 



71 



clays are in equilibrium with water vapor of various 

 concentrations are given in a table on page 25 of the 

 paper cited. 



Weight - Temper atur e Relations 



The samples were put through a 150-mesh sieve 

 ;0.1 mm) and weighed in a light platinum crucible sus- 

 pended from an arm of a balance in a vertical electric 

 furnace heated to various temperatures for at least 24 

 hours at each temperature. The weights tabulated in 

 table 22 are in percentages of that at 50° C as a base. 

 Room humidities do not appreciably affect weights taken 

 at temperatures above 70° C. 



Samples 17 and 19, high in carbonate, lose weight 

 only slightly out to where CO2 is given off; 600° C for 17 

 and 650° for 19. CaCOs as pure crystalline marble, 

 breaks sharply at 600° C. 



The remaining samples 31 to 77 dehydrate very 

 much as do bentonites and glauconites and some other 

 older clays of similar composition. They drop rapidly 

 in weight from room temperature up to 150°, then grad- 

 ually and evenly out to 440 (77) - 580 (31), followed by a 

 steep drop and then a gradual decrease to 800° C. The 

 data on bentonite given for comparison relate to a very 

 pure one from the Vicksburg formation in central Mis- 

 sissippi. The glauconite is also a pure one from Bonne 

 Terre, Missouri. 



These curves are entirely different in character 

 from the dehydration curves for analcite (cf. P. G. Nut- 

 ting, "Standard thermal dehydration curves of minerals," 

 Professional Paper [U.S.G.S.] 197-E, p. 202) or for the 

 zeolites, chabazite, or phiUipsite. In the lower range of 

 temperatures, these zeolites change in weight only slow- 

 ly at first as the higher hydrate starts to break up. At 

 room temperature the curves for both are horizontal in- 

 stead of steep, as are those for bentonites and the bottom 

 samples. 



Chabazite shows no breaks at higher temperatures 

 as additional water is expelled, but phillipsite exhibits a 

 sharp drop at 140° and another at 630° C, the latter cor- 

 responding to one molecule of H2O rather closely and 

 being not unlike the characteristic breaks in the curves 

 for the bottom samples studied and for the bentonites. 

 If either chabazite or phillipsite has formed and is pres- 

 ent, it must form such a minor percentage of the whole 

 as not to be evident as breaks in the dehydration curves. 



Discussion of Nutting's Report 



In attempting to distinguish between various types of 

 clay minerals from their dehydration curves, three im- 

 portant points are to be noted: (1) the temperature at 

 which a break occurs in a curve; (2) the amount of water 

 lost during this sharp drop in the water content, which 

 indicates the number of molecules of essential water in 



Table 21. Vapor pressure versus moisture content 

 (Ratio of moisture content at the humidity indicated to that at zero humidity) 



Relative 



humidity 



in per cent 



19 colloid 



31 colloid 



69 all 



72 all 



73 all 



77 colloid 



77 all 



Table 22. Percentage Of water lost at the temperature indicated 



Sample 

 no. 



100° 150 



200° 300° 400° 450° 500° 550° 600° 650 



700 



800° 



