The  ordinary  light-colored  micas  are  mostly  muscovite,
and  the  black,  mostly  biotite.  The  optic-axial  plane  in
most  micas  passes  through  the  longer  diagonal  of  the  base,
being  perpendicular  to  tne  plane  of  symmetry.  In  a  black-
ish Vesuvian  mica  (meroxene)  and  in  phlogopite  and  zinn-
waldite, it  passes  through  the  shorter  diagonal  of  the  base.
Lepidolite  is  a  light-colored  mica  containing  lithia,  belong-
ing with  muscovite.  Muscovite  and  biotite  are  so  related
that  crystals  of  the  latter  often  occur  that  are  finished  out
uninterruptedly  by  muscovite,  the  axial  lines  of  the  one
continuous  with  those  of  the  other;  and  such  crystals  are
sometimes  several  inches  across;  there  is  here  a  compound
structure  chemically,  but  no  twinning  in  the  crystallization.
When  a  thin  plate  of  mica  is  struck  with  a  pointed  awl  or
other  like  tool  a  symmetrical  star  of  six  rays  is  produced,
the  rays  being  cleavage  lines  parallel  to  the  sides  of  the
rhombic  prism  /  and  the  shorter  diagonal.
Muscovite. — Common  Mica.
Monoclinic.     Usually  in  plates  or  scales.     Sometimes  in

radiated  groups  of  aggregated  scales
(plumose  mica);  rarely  spheroidal.
Colors  from  white  through  green,
yellowish  and  brownish  shades;  rarely
rose-red  or  reddish  violet.  Lustre
more  or  less  pearly.  Transparent  or
translucent.  Tough  and  elastic.
H.  =2-2-5.  G.  =2-7-3.  Optic-axial
angle  44°  to  78°.
Composition.    A  common  variety  has  the  general  formula


MICA   GROUP.  289

(iE3}R),0„Si3,  E  including  K,  and  H^,  and  R,  aluminium
and  some  iron  in  the  sesquioxide  stato.  An  analysis  of  mica
of  this  variety  obtained  Silica  4G*3,  alumina  36*8,  iron  ses-
quioxide 4*5,  potash  9*2,  fluorine  0*7,  water  1*8  =  99*3;  3
to  5  p.  c.  of  water  often  present,  and  passes  thus  to  a  hy-
drous mica.  The  variety  Phengite  contains  more  silica.
