TABLE FOR BECKE METHOD.[116] Orthoclase │ α │< ω Quartz Microcline │ β │ „ Albite │ γ │ „ ───────────────────────────────┼───────┼─────────────────────────────── Oligoclase, Ab_{4}An_{1} │ α │< ω; γ > ω Quartz. ───────────────────────────────┼───────┼─────────────────────────────── Labradorite, Ab_{1}An_{1} │ α │>ε Quartz Anorthite │ β │ „ „ │ γ │ „ (5) _Becke’s_ method may be employed to identify the feldspar, by determining the relative values of the indices of refraction of the feldspar grain when it lies in contact with a quartz grain (best results) or with the balsam (not such good results.) The grains should have vibration directions in parallel position. Other methods that may be employed are here simply referred to: Determination (in convergent light) of the emergence of an optic axis with reference to a known plane, the basic plagioclases show an optic axis about parallel to _ć_ of the crystal; determination of total reflection by Wallerant’s total reflectometer; determination of the value of the mean index of refraction of crushed isolated grains by Schrœder van der Kolk’s method;[117] determinations by specific gravity separations with use of heavy solutions, and by chemical and micro-chemical tests (for the relative amounts of K, Na and Ca).[118] REMARKS: The plagioclases may have the same two general habits as orthoclase, being glassy and colorless in the younger eruptive rocks, and dull and cloudy in the granular and porphyritic, older, massive and schistose rocks. They occur in rocks of intermediate and basic composition. Albite is found in granite (commonly intergrown with orthoclase), gneiss, etc., and frequently as a secondary constituent (secondary feldspar[119]) in the feldspar-quartz mosaic of mechanically metamorphosed rocks. It may also be present in acid eruptive rocks. Oligoclase is very frequent in granite, syenite, gneiss, diorite, trachyte, andesite, diabase, etc.; and particularly accompanies orthoclase. Labradorite is confined more to the gabbros,[120] basic eruptive rocks and crystalline schists, rich in amphibole and pyroxene. Anorthite occurs in gabbros, the most basic porphyrites, basalts, etc. Chemical corrosion and mechanical deformation[121] may take place as in orthoclase. Anorthite and labradorite are more or less decomposed by hydrochloric acid, while albite and oligoclase are not acted on by the acid. Especially interesting is the alteration of the plagioclase that takes place in gabbros, accompanied by “uralitization” of the pyroxene, forming “_saussurite_.” This consists of a white to greenish confused aggregate, chiefly of zoisite, grossularite, vesuvianite, chlorite, secondary feldspar (albite), etc.