Igneous rock. Such an isotherm at 590 o shows that at this temperature a solid solution having the composition of point B (Or 32 Ab 68) coexists with an alkali feldspar solid solution with the composition of point A (Or 70 Ab 30). Quartz-alkali-feldspar-syenite. The alkali feldspar series behaves quite differently. composition, phase relationships, and structural states of alkali feldspars have been shown by Wright (1967) and Crosby (197 l) to give feasible estimates of these temperatures. Coarse-grained ("plutonic") crystalline igneous rock . The major third component for alkali feldspars is An. Normal crystalline igneous rock. Sanidine is a monoclinic alkali feldspar with small optic axial angle (2V) and commonly occurs in volcanic rocks. In addition, there are distortions to the framework that effect symmetry. It solidifies at a much lower temperature (below 1000C) than plagioclase. The major third component for plagioclase feldspars is Or. The composition of coexisting exsolved phases can be found by drawing an isotherm until it intersects the solvus. Classification of Quartz-alkali-feldspar-syeniteHide. In this system, different ordering of Si and Al on tetrahedral sites changes symmetry of structure. The K-feldspar polymorphs, sanidine, orthoclase, microcline, and adularia have identical chemical compositions. Alkali feldspars have a range of chemical composition and the end members may be written as KAlSi 3 O 8 and NaAlSi 3 O 8. Hide all sections | Show all sections. Alkali feldspar has the formula KAlSi 3 O 8, potassium aluminosilicate. K-FELDSPARS. An alkali-feldspar-syenite with 5-20% quartz on the QAPF diagram, and no foids . Include compositions between KAlSi 3 O 8 and NaAlSi 3 O 8. Rock. In solution, the albite and orthoclase molecules are compatible with each other but they have to separate out as distinct minerals on crystallisation. Alkali fedspar syenite Syenite, from Latin "Lapis Syenitis" (lapis = stone), from Syene (an ancient city of southern Egypt), is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or present in relatively small amounts (5%).The feldspar component of syenite is predominantly alkaline in character (usually orthoclase). This mineral is often called potassium feldspar or K-feldspar because potassium always exceeds sodium in its formula. Usually the process is too fast for completely separate crystals to develop. Use the maximum extinction angle to determine the composition of the plagioclase from the Michel-Levy composition chart.. ALKALI FELDSPARS. Most natural feldspars have minor amounts of other components in their composition. The formula actually is a blend ranging from all sodium (albite) to all potassium (microcline), but albite is also one endpoint in the plagioclase series so we classify it there. Syenitoid. 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