Cummingtonite

group                                   inosilicate
system                                  monoclinic
formula                                 (Mg,Fe)7Si8O22(0H)2
optic                                   biaxial pos / neg
Too-Vee                                 65 - 98¡
relief.1                                mod high to high pos
min. index                              1.630 / 1.729
color                                   colorless, green
pleochroism                             yes
birefringence.1                         0.020 - 0.045
cleavage                                yes
elongation                              yes
extinction                              parallel, symmetrical
angle                                   10 - 21¡
alteration                              hornblende lamellae, chlorite, talc
habit.1                                 prismatic, fibrous
occurence                               igneous (volcanic, intermediate), metamorphic (mafic, gneiss)
TwoV                                    2 3
relief                                  2 3
pleochroism.2                           yellow, brown, green
birefringence                           5 8 9 10 11 12 13 
cleavage.2                              {110} good amphibole cleavage at 56 and 124¡