Biotite

Rich in Mg - Rich in Fe
nα=1.530 - 1.625
nβ=1.557 - 1.696
nγ=1.558 - 1.696
δ= 0.028-0.070
2V= -(0-25)°

Thin section photos:

Biotite (phlogopite). Zoom 40x, PPL.