group amphibole system monoclinic formula see properties optic biaxial neg Too-Vee 66 - 84¡ relief.1 high pos min. index 1.670 color brown, yellow, reddish, greenish pleochroism yeslowish, reddish, brownish birefringence.1 0.044 cleavage yes elongation ** extinction *** angle *** alteration amphiboles, titanaugite, Fe-Ti oxides, chlorite habit.1 columnar, tabular occurence igneous (alkalic, volcanic) TwoV relief pleochroism.2 *** birefringence cleavage.2 ***