Sillimanite

xpl 20*

ppl 20*

group                                   aluminosilicate
system                                  orthorhombic
formula                                 Al2SiO5
optic                                   biaxial pos
Too-Vee                                 20 - 30¡
relief.1                                high pos
min. index                              1.653 / 1.683
color                                   colorless, brown
pleochroism                             yes
birefringence.1                         0.018 - 0.022
cleavage                                yes
elongation                              yes
extinction                              parallel
angle                                   0¡
alteration                              sericite
habit.1                                 fibrous, prismatic
occurence                               igneous (accessory), metamorphic (medium to high grade), hydrothermal
TwoV                                    1
relief                                  3
pleochroism.2                           yellow, brown, blue
birefringence                           5 8
cleavage.2                              {010} good parallel