Besides  the  crystals,  it  is  common  to  find  timphibole  in
columnar  and  bladed  forms,  which,  however,  still  show  the
prismatic  cleavage  to  the  skilled  observer,  and,  as  forms
like  these  are  rare  with  pyroxene,  this  serves  to  aid  in  dis-
tinguishing the  two  species.  Further,  it  may  be  coarse  to
fine  fibrous,  and  in  the  latter  case  look  like  flax.
The  hardness  is  5  to  6,  and  the  specific  gravity  varies
from  2.9  to  3.4.  The  luster  is  vitreous,  or  pearly  in  the
fibrous  kinds.  The  color  varies  from  white  to  gray,  green


•DESCRIPTION   OF   MINERAL   SPECIES.  29?

of  many  shades,  and  black,  also  occasionally  pink.  Some
kinds  phosphoresce,  or  give  out  light,  as  when  two  pieces
are  rubbed  together  in  the  dark.
The  common  varieties  correspond  pretty  closely  to  those
of  pyroxene.  Tremolite  is  a  silicate  of  lime  and  magnesia
free  from  iron,  and  is  white  in  color,  or  very  pale  green,
like  the  related  diopside.  Actinolite  is  green,  and  besides
lime  and  magnesia  contains  also  iron;  it  is  often  in  fibrous
or  columnar  masses,  sometimes  with  a  radiated  structure.
Nephrite,  including  much  of  the  green  Oriental  jade,  is  a
closely  compact  variety  of  actinolite;  the  whitish  varieties
of  nephrite,  however,  approach  nearer  to  tremolite.  An-
other kind  of  jade  is  a  different  mineral,  though  having
the  same  appearance;  it  is  a  silicate  of  alumina  and  soda,
related  to  pyroxene,  and  is  called  jadeite.
Aslestus*  is  a  variety  usually  like  actinolite  in  compo-
sition, but  separable  into  very  fine  flexible  fibers,  which
may  be  made  or  woven  into  an  incombustible  paper  or
cloth.  Mountain  leather  is  a  curious  kind,  in  thin  flexible
but  very  tough  sheets,  consisting  of  interlaced  fibers;  it  is
usually  nearly  white  in  color.
