276  ONTKIBUTIONS   TO    ECONOMIC    GEOLOGY,  L906,  PART   I. 
NATURE  OF   FULLER'S   EARTH. 
THEORIES    OF    OTHER     WRITERS. 
Almost  as  many  suggestions  have  been  advanced  concerning  the 
nature  of  fuller's  earth  as  there  are  writers  on  the  subject,  but 
apparently  none  of  them  are  based  on  anything  more  than  an  inspec- 
tion of  the  ultimate  analyses.  I  will  now  take  up  these  theories  and 
study  them  in  the  light  of  my  experiments. 
Danaa  states  that  fuller's  earth  has  for  its  base  the  mineral  smec- 
tite, and  possibly  also  maithacite.  Although  it  is  not  so  stated  in 
the  passage  quoted,  the  inference  is  that  he  considers  these  minerals 
the  cause  of  the  bleaching  power.  In  order  to  test  the  truth  of  this 
theory  it  is  necessary  to  have  at  hand  formulae  of  smectite  and  mai- 
thacite, and  these  1  have  calculated  as  here  described.  Dana  (as 
quoted  by  Riesa)  gives  the  following  analyses  of  impure  minerals: 
Analyses  of  smectiU  and  maltTiaciti . 
SiO». 
\l.<> 
II.". 
FeaO 
CaO. 
MgO. 
Smectite         
M ;i It  Illicit  <■ 
51.2] 
12  25 
10.66 
27.  89 
2.07 
3.  I.", 
■_'  13 
25 
4.89 
I  have  considered  the  iron,  lime,  and  magnesia  as  impurities,  and 
have  recalculated  the  analysis  on  the  basis  of  silica,  alumina,  and 
water     loo.     The  resull  is  as  follows: 
Recalculated  analyses  of  smectiU  and  inalthacite. 
Si<>„ 
\l,o. 
BjO. 
Smectite 
56  (i 
51.9 
13.5 
11.0 
:«».  5 
Maithacite 
:<?.  l 
The  formula'  corresponding  most  nearly  to  these  figures  are  for 
smectite  Al,(  >,.7Si< >_..  L2l  I ,( ).  and  for  maithacite  AI,(),.7Si(),.  101 1 ,(), 
and  the  percentage  composition  for  these  formula'  should  be  the 
following: 
Composition  corresponding  to  formula  for  smectiU  and  maithacite. 
SiOj. 
AI,o,. 
II,o. 
Smectite 
57.06 
52. 18 
13.78 
12.40 
29.16 
Maithacite 
35  42 
These  figures  are  used  in 
(p.  270). 
allc 
denial  ions  and  in  the  table  of  minerals 
a  Quoted  in  Seventeenth  Ann.  Rept.  U.  S.  Geol.  Survey,  pt.  3,  continued,  1896,  p.  876. 
