Mg(Al.OH)2(PO4)2.  Magnesium  may  be  replaced  by  iron  and  cal-
cium. Yields  water  in  closed  tube  and  turns  white.  The  blue  color  may  be
restored  by  treatment  with  cobalt  nitrate  solution.  Moistened  with  sulphuric
acid  it  imparts  a  bluish  green  color  to  the  flame.  Not  attacked  by  acid?.
Occurs  at  Werfen,  Salzburg,  Austria ;  various  places  in  Styria ;  Horrs-
joberg,  Sweden;  Crowders  Mt.,  Gaston  County,  N.  C. ;  and  Graves  Mt.,
Lincoln  County,  Ga.
3.     HYDRATED    COMPOUNDS

The  hydrated  compounds  include  normal,  acid,  and  basic  salts.  Very
few  of  these  minerals  occur  in  large  quantities.
Struvite,  NH4MgPO4.6H2O.
Orthorhombic,  pyramidal  class,  a  :  b  :  c  =  0.5667  :  I  :  0.9121.  Ex-
cellent example  of  hemimorphism.  Perfect  basal  cleavage.  Translucent
to  opaque.  Vitreous  luster.  Colorless,  but  often  yellow  or  pale  brown.
Hardness  1.5  to  2.  Specific  gravity  1.66  to  1.75.  Found  in  sewers  and
guano  deposits.
VIVIANITE  GROUP

This  group  embraces  the  ortho  salts  of  magnesium,  iron,  cobalt,  nickel,
and  zinc  containing  eight  molecules  of  water.  Chemically  they  are  iso-
morphous,  but  only  on  three  of  them  have  -crystallographic  investigations
been  made.  These  have  been  referred  to  the  monoclinic  prismatic  class.
a  :  b    :        c            ft

BOBIERRITE,,  Mg.5(PO4)2.8H2O.  103°  Approx.
Vivianite,  Fe,,(PO4)2.8H2O.            0.7498  :  i    :  0.7017,  io4°26'

HOERNESITE,  Mg3( AsO4)2.8H2O.  ?

SYMPI.ESITE,  Fe:!(AsO4)2.8H2O.      0.7806  :  i    :  0.6812,  107°  13'

Erythrite,    Co,(AsO4)2.8H26.        0.7937  :  i    :  0.7356,  105°  9'

Annabergite,  NL(AsO4)2.8H2O.  ?

CABRERITE,  (Ni,Mg)3(AsO4)2.8H2O.  ?

KOTTIGITE,  (Zn,Co)3(AsO4)2.8H2O.  ?

These  minerals  are  all  secondary  formations  and  do  not  occur  very
abundantly.
BOBIERRITE.     Mg3(PO4)2.8H2O,  occurs  in  aggregates  of  very  small,  white  monc
clinic  crystals  and  in  reniform  masses  in  guano  at  Mejillones,  Chile.
17^  DESCRIPTIVE    MINERALOGY

Vivianite,  Blue  Iron-earth,  Fe3(PO4)2.8H2O.
