GILA    RIVER    ALUM    DEPOSITS. 
221 
tat  ion  has  generally  lost  its  fibrous  structure,  becoming  compact  and 
assuming  a  yellowish  color.  Although  the  incrustation  may  be  sev- 
eral inches  thick,  the  individual  fibers  are  rarely  more  than  a  third  or 
a  half  of  an  inch  in  length,  the  crust  being  made  up  of  successive  layers 
of  short  crystals.  Each  layer  probably  represents  the  growth  of  a 
single  season  during  which  the  supply  of  percolating  water  was  rela- 
tively abundant.  The  layers  of  crystals  are  in  places  separated  by  a 
very  thin  film  of  the  rock  which  has  been  split  off  by  the  growth  of  a 
subsequent  layer  of  crystals. 
The  chemical  composition  of  this  fibrous  halotrichite  is  shown  below. 
Analyses  of  halotrichite  from  Gila  River,  New  Mexico. 
A. 
B. 
c. 
FeO.                 .   -  .            : 
7.94 
11.77 
35.25 
45.09 
13.59 
7.27 
37.19 
40.62 
.50 
7.8 
A1203 
11.0 
S03 
34.5 
H20 
46.7 
100.05 
99.17 
100 
A.  Analyst,  W.  T.  Sehaller.    Carefully  selected  fibrous  crystals  from  tunnel  No.  2. 
B.  Analyst,  F.  W.  Clarke.     "Fibrous  mineral  of  silky  luster."     Bull.  U.  S.  Geol.  Survey  No.  9, 1884. 
Contains  a  trace  of  Fe203. 
C  Theoretical  composition  of  halotrichite— FeS04-f  Al2(S04)3+24H20. 
As  shown  in  the  foregoing  analyses,  the  halotrichite  is  the  double 
sulphate  of  aluminum  and  iron,  and  the  alunogen  is  the  sulphate  of 
aluminum  and  is  free  from  iron  except  as  a  minor  impurity.  The 
relations  of  the  two  minerals  may  be  explained  as  follows: 
As  a  result  of  chemical  reactions  within  the  alum  rock  the  double 
sulphate,  halotrichite,  is  formed.  This  is  carried  to  the  surface  in  solu- 
tion by  the  slow  capillary  circulation  and  on  the  evaporation  of  the 
solvent  water  is  deposited  as  a  crystalline  incrustation.  Wherever 
water  subsequently  gains  access  to  this  deposit  it  is  redissolved  and 
carried  downward.  This  solution,  however,  particularly  when  it 
forms  a  thin  film  trickling  dowrn  the  face  of  a  cliff,  affords  abundant 
opportunity  for  oxidation  and  the  iron  is  converted  from  the  ferrous 
to  the  ferric  state  and  thereby  becomes  insoluble.  It  is  deposited  as 
ferric  oxide,  which  accounts  for  the  prevalence  of  red  color  in  the 
rocks  and  soil,  wmereas  the  aluminum  sulphate,  unaffected  by  the 
oxidizing  conditions,  remains  in  solution  and  either  passes  off  with  the 
surface  waters  or  by  the  evaporation  of  the  water  is  deposited  in 
stalactitic  forms  and  incrustations  upon  the  cliffs. 
In  the  report  already  cited,  Blake  suggests  two  methods  by  which 
the  chemical  reactions  necessary  for  the  formation  of  sulphates  within 
the  alum  rock  may  be  produced,  namely,  (1)  the  oxidation  of  the  con- 
tained pyrite  with  the  production  of  free  sulphuric  acid,  and  (2)  the 
Bull.  315—07 15 
