Cordierite

xpl 10*

group                                   tectosilicate
system                                  orthorhombic
formula                                 Mg2Al3(AlSi5)O18
optic                                   biaxial pos / neg
Too-Vee                                 40 - 90¡
relief.1                                low neg / pos
min. index                              1.521 / 1.578
color                                   colorless, blue
pleochroism                             yes
birefringence.1                         0.005 - 0.016
cleavage                                yes
elongation                              no
extinction                              parallel
angle                                   0¡
alteration                              pinite (chlorite, muscovite)
habit.1                                 anhedral, granular
occurence                               igneous (granitic), metamorphic (medium to high grade, contact, regional, pelite)
TwoV                                    
relief                                  
pleochroism.2                           blue, violet, yellow
birefringence                           
cleavage.2                              {010} fair, {100} and  {001} poor