BENTONITE. 
59 
known as the Mo wry shale, as a rule above it and overlain by 
20 feet of very dark shales containing concretions. At some locali- 
ties it is 5 feet or more thick, but generally is much thinner, in places 
measuring only a few inches. 
* When freshly exposed, bentonite varies in color from light yellow 
to light olive-green, with waxy luster, but most of it becomes of a dull 
cream color on exposure. When freshly uncovered, it appears as a 
bedded joint clay and breaks out with conchoidal fracture in blocks 
varying from roughly rectangular shapes to long, slender prisms. 
The joints are more or less open and here and there contain crystals and 
plates of gypsum and sulphate of soda. The texture is very fine 
grained, no grit being perceptible to the touch and very little when 
the clay is ground between the teeth. Under the microscope it is 
seen to consist of extremely minute more or less rounded particles 
of fairly uniform size and apparently of the same mineral nature, with 
scattered particles of undecomposed labradorite. The clay has a soft 
unctuous or soapy feel, but is brittle and easily quarried. Owing to 
its highly absorbent character it clings strongly to the tongue. In 
weathering it absorbs a large amount of water and increases greatly 
in volume, forming a frothy mantle on the surface which often 
resembles hoarfrost. When this dries it becomes a soft white pow- 
der. Mixed with the proper amount of water it is exceedingly 
plastic and with the addition of more water becomes a paste 
resembling glue. Tests show that it completely absorbs over three 
times its weight or seven times its volume of water and twice as 
much glycerine as diatomaceous earth will absorb. 
COMPOSITION. 
As will be seen from the subjoined table of analyses, bentonite falls 
under the kaolin group of hydrous silicates of alumina. Its resem- 
blance to ehrenbergite has been pointed out by Knight, and Read 
considers it a variety of montmorillonite. 
Analyses of bentonite from the Laramie Basin, Wyoming. 
Taylor/' 
Taylor.^ 
Linscott.c 
Cosgnll.' 
Cassa Mining Co.<* 
Si0 2 
59.78 
15.10 
2.40 
4.14 
.73 
J 
58. 25 
24.70 
2.61 
1.30 
1.61 
66.5 
23.9 
3.1 
1.0 
5 
64.0 
22.9 
3.1 
2.0 
1.0 
64.0 
24.0 
3.2 
1.5 
.(. 
60.18 
j- 26.11 
} 2.54 
.80 
10.26 
60 is 
Al 2 Ov- -- 
Fe 2 3 
26.58 
RlgO 
CaO 
\ 1.01 
\ . 23 
1 1.23 
Na 2 
H 2 
16.26 
11.00 
5.0 
7.0 
6.7 
I 
10 26 
98.51 
94.47 
100.0 
100.0 
100.0 
99.63 
99.49 
" Analyst, unknown. 
'' Analysis by 11. L. Hodges. 
■ \ nalyses by John < >gden 
d Analyses by Thomas T Read. 
