BENTONITE  OF  THE  LARAMIE  BASIN,  WYOMING. 
By    C.    E.    SlEBENTH; 
The  variety  of  clay  known  as  "  bentonite"  occurs  in  many  localities  in  Wyoming,  and 
various  articles"  describing  its  occurrence  and  character  have  appeared  from  time  to 
time.     These  have  been  freely  drawn  on  in  the  preparation  of  the  following  paper. 
PHYSICAL   CHARACTERS. 
In  color  the  clay,  when  freshly  exposed,  varies  from  a  light  yellow  to  a  light  olive-green 
with  a  waxy  luster,  but  assumes  a  dull  cream  color  on  exposure.  In  freshly  uncovered 
outcrops  it  appears  as  a  bedded  joint  clay,  the  blocks  varying  from  roughly  rectangular 
orconchoidal  shape  to  long  slender  pieces.  The  joints  are  more  or  less  open  and  occasionally 
contain  crystals  and  plates  of  gypsum  and  sulphate  of  soda.  The  clay  is  very  fine  grained, 
no  grit  being  perceptible  to  the  touch  and  very  little  when  ground  between  the  teeth. 
Microscopically  it  is  made  up  of  extremely  minute,  more  or  less  rounded  particles  of  fairly 
uniform  size  and  apparently  of  the  same  mineral  nature,  with  here  and  there  particles 
of  undecomposed  labradorite.  The  clay  has  a  soft  unctuous  or  soapy  feel,  but  it  is  quite 
brittle  and  is  easily  quarried,  ft  clings  strongly  to  the  tongue.  In  weathering  it  absorbs 
a  great  amount  of  water  and  increases  much  in  volume,  forming  an  intumescent  reticulated 
mantle  an  inch  in  thickness,  which  later  melts  down  to  a  white  powder  under  the  effects 
of  rainfall  and  frost.  Mixed  with  the  proper  amount  of  water  it  is  exceedingly  plastic 
and  with  the  addition  of  more  water  it  becomes  a  perfect  paste.  Tests  show  that  it  com- 
pletely absorbs  over  three  times  its  weight  or  seven  times  its  volume  of  water,  and  twice 
as  much  glycerin  as  diatomaceous  earth  will  absorb. 
COMPOSITION. 
Bentonite  falls  under  the  kaolin  group  of  hydrous  silicates  of  aluminum,  as  will  be  seen 
from  the  subjoined  table  of  analyses.  Its  resemblance  to  ehrenberglte  has  been  pointed 
out  by  Knight,  while  Read  considers  it  a  variety  of  montmorillonite. 
a  Knight,  W.  C.,  Eng.  and  Min.  Jour.,  vol.  63,  1897,  pp.  600-601;  vol.  66,  1898,  p.  491. 
Merrill,  George  P.,  Ann.  Rept.  U.  S.  Nat.  Museum,  1899,  pp.  340,  .548. 
Slosson,  E.  E.,  Tenth  Ann.  Rept.  Wyoming  College  Agric.  and  Mech.,  1900,  extract,  p.  14. 
Read,  Thomas  T.,  Eng.  and  Min.  Jour.,  vol.  76,  1903,  pp.  48,  49. 
Darton,  N.  H.,  Geologic  Atlas  U.  S.,  folio  107  (Newcastle,  Wyo.),  1904,  pp.  5,  9. 
Merrill,  George  P.,  Nonmetallic  minerals.  1904,  pp.  233,  243. 
Fisher,  C.  A.,  Bull.  U.  S.  Geol.  Survey,  No.  260,  1905,  pp.  559-563. 
Darton,  N.  II.,  Prof.  Paper  U.  S.  Geol.  Survey  No.  32,  1905,  p.  400. 
Bull.  285—06 29  445 
