CHARACTER, ORIGIN, OCCURRENCE, AND USES. 
25 
form of free silica or fragmentary quartz, and this being anhydrous 
an analysis of the material shows a smaller percentage of water than 
is found in the purer clay or kaolin. Thus, while kaolin usually 
contains about 12 per cent of water, the clays have less than 8 per 
cent. a 
Kaolin deposits may originate in three ways — (1) by the alteration 
of feldspar in place; (2) by chemical action other than the ordinary 
alteration of feldspar, as illustrated by rectorite; (3) by the washing 
down and redepositing of kaolin formed in place. The great bulk of 
our kaolins, however, are derived directly from feldspar. The proc- 
ess by which feldspar or feldspathic rocks are changed to kaolin is 
kaolinization. Syenite, which is generally known in Arkansas as 
" granite," is made up largely of feldspar, and when the feldspar decom- 
poses, as it always does sooner or later, it forms kaolin. The other 
ingredients of the syenite, however, such as hornblende, mica, iron, 
pyrites, and quartz, remain in the residuary material, so that it does 
not form pure, clean kaolin, but is generally an impure clay. Below 
are analyses of feldspar, of a variety of Arkansas syenite, and of a 
syenite partly changed into clay. 
Analyses of feldspar, syenite, arid kaolin. 
Feldspar.a 
Blue sy- 
enite." 
Decom- 
posed 
syenite, b 
Kaolin, c 
Silica (Si0 2 ) 
66.39 
18.13 
1.44 
60.03 
20.76 
4.01 
.75 
2.62 
.80 
4.48 
5.96 
.07 
.59 
58.50 
25.71 
3.74 
46.27 
Alumina (AI2O3) 
38.57 
Ferric oxide ( Fe20a) 
1.36 
Ferrous oxide (FeO ) 
Lime (CaO) 
.16 
.06 
8.51 
5.36 
.44 
Trace. 
1.96 
1.37 
34 
Magnesia (MgO) 
25 
Potassium (K2O) 
23 
Soda (Na 2 0) 
37 
Phosphoric acid (PaOa) 
Water 
.42 
5.85 
13 61 
100. 47 
101. 07 
97.57 
101. 00 
a From Fourche Mountain, Little Rock, Ark. 
b From Pulaski County, railway cut 2 miles north of Sweet Home. 
<■ From sec. 9, T. 1 S., R. 12 W. 
These analyses show that in passing through the process of decom- 
position from a hard blue syenite to a kaolin the rock loses part of 
all its ingredients except alumina and water, which are proportion- 
ately increased; or perhaps it would be better to say that the removal 
in solution of some of the constituents leaves others predominant. 
The change from feldspar to kaolin goes still further, but the feldspar 
contains less iron and is therefore more liable to end in a clean white 
kaolin than is the ordinary rock. 
* — ■ _ __ — . 
a Of course no reference is here intended to prophyllite or rectorite, which are hydrous silicates of 
alumina and contain less than 8 per cent of water. Those substances, however, arc not likely to be 
mistaken for common clays. 
