ART. 15. MINERALOGY OF GOUGE CLAYS—SHANNON, 3 
Optical properties—Under the microscope the material is trans- 
parent, colorless, and entirely crystalline with a finely foliated or 
felted-fibrous structure. The birefringence is moderately high. The 
indices of refraction measured are, parallel to the cleavage y = 1.513; 
perpendicular to the cleavage, a = 1.488, both + .003. The bire- 
fringence indicated is 0.025 + .003. These indices separate this 
clay clearly from both sericite and kaolinite which have much higher 
indices. The Idaho material compares very well with the lowest set 
of values given by Larsen? for leverrierite, the optical data thus being 
in agreement with the chemical evidence as discussed below. 
Pyrognostics—Heated in a closed tube the clay yields abundant 
neutral water at a very moderate temperature. In the forceps it 
decrepitates mildly and fuses with moderate ease on thin edges to a 
white blebby enamel, the fused portion becoming markedly incan- 
descent. 
Composition and chemical properties—Material which had been 
proven free from contaminating impurities by careful optical exami- 
nation was available in quantity for investigation and the analysis 
was carefully made upon ample material. The dehydration phenom- 
ena were not studied in detail as it is planned to do this as part of 
another research later. The loss of water at 110° C. was found to 
be 14.48 per cent, the mineral reaching constant weight for this tem- 
perature with two hours heating. The first hour the loss was 14.28 
percent. During the second hour an additional loss of 0.20 was ob- 
tained while several hours additional exposure to the same tempera- 
ture occasioned no further loss in weight. When exposed again to 
the air of the room, however, the material proved to be exceedingly 
hygroscopic and a sample which had been dried to constant weight 
at 110° C. regained all of its original water content upon standing 
overnight in the air of the room. A similarly dehydrated sample 
regained 2.18 per cent of its original weight upon standing overnight 
in a desiccator over fresh calcium chloride. Samples dehydrated at 
110° C. and allowed to stand overnight over concentrated sulphuric 
acid show a gain of several per cent following which they can not 
again be brought to the original dry weight at this temperature. 
This suggests that sulphuric acid has been adsorbed, an assumption 
which has not as yet been confirmed. This phenomenon has repeat- 
edly been observed by the writer to occur with hygroscopic mineral 
powders, and the use of sulphuric acid desiccators has accordingly 
been discontinued in mineral analysis. 
2Esper S. Larsen. U.S. Geol. Surv. Bull. 679, p. 245, 1921. 
