In ternary feldspars of essentially one phase, calcium content has a dominant influence on the optic axial angle. In such feldspars and also in binary feldspars from sölvsbergite rocks, variations of cooling histories do not significantly affect the optic axial angle. In ternary feldspars which are unmixed into two or three prominent phases, Al/Si ordering has an important effect on the 2V value. A recent suggestion of several writers that in feldspars the alkali structural site may be partially occupied by (H3O)+ ions is applied as a possible way to explain a correlation observed between petrographical features of the rocks and the optic axial angles of their feldspar phenocrysts. In ternary feldspars, calcium content starts to have significant influence on the infra-red absorption when CaO reaches 2% and more. There is a significant parallelism between results obtained by optical and infra-red methods for the same minerals. Infra-red data can be applied to petrogenetic problems.