Arfvedsonite

group                                   amphibole
system                                  monoclinic
formula                                 NaNa2Fe2+4Fe3+Si8O22(OH)2
optic                                   biaxial pos / neg
Too-Vee                                 0 - 100¡
relief.1                                high pos
min. index                              1.638 / 1.710
color                                   green, blue, yellow, purple
pleochroism                             yes
birefringence.1                         0.013
cleavage                                yesical amphibole cleav. {110} intersects at 56 and 124 deg.
elongation                              yes.
extinction                              paralled, inclined
angle                                   0-30¡, parallel to (010)
alteration                              may be uralitized (turned into fine-grained, light amphibole)
habit.1                                 columnar, tabular, granular
occurence                               igneous (alkalic)
TwoV                                    1 2 3
relief                                  3
pleochroism.2                           green, yellow
birefringence                           5 6 7
cleavage.2                              {110} intersects at 56 & 124¡, {010} parting