Potassium feldspars

Microcline
nα=1.514 - 1.523  
nβ=1.518 - 1.526
nγ=1.521 - 1.530
 δ=0.007  
2V = -(71-84) °

Orthoclase
   Na-free - Na-bearing
nα=1.518- 1.528  
nβ=1.522 - 1.533  
nγ=1.524 - 1.535
δ=0.006 - 0.007  
2V= -70°

Sanidine
   Na-free - Na-rich
nα=1.519- 1.525  
nβ=1.525 - 1.530  
nγ=1.526 - 1.531
δ=0.006 - 0.007  
2V= -(20 - 50)°

This section pictures:

Tabular grains of microcline from alkaline syenite, 
Kola Peninsula. XPL

Microcline from nepheline syenite, Kola peninsula. 40x, XPL.