group riebeckite
system monoclinic
formula Na2Mg3Al2Si8O22(OH)2
optic biaxial neg
Too-Vee 10 - 45¡
relief.1 mod to mod high pos
min. index 1.606 / 1.717
color blue
pleochroism yes
birefringence.1 0.010 - 0.020
cleavage yes
elongation yes lent
extinction symmetrical, inclined
angle 0 - 21¡
alteration amphibole
habit.1 prismatic, columnar
occurence metamorphic (medium grade, regional)
TwoV 1 2
relief 2 3
pleochroism.2 blue, green, violet
birefringence 1 4 5 6 7 8
cleavage.2 {110} amphibole cleavge at 55 and 125¡