Normal crystalline igneous rock. 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). Use the maximum extinction angle to determine the composition of the plagioclase from the Michel-Levy composition chart.. ALKALI FELDSPARS. Hide all sections | Show all sections. Rock. Classification of Quartz-alkali-feldspar-syeniteHide. The major third component for plagioclase feldspars is Or. Quartz-alkali-feldspar-syenite. In this system, different ordering of Si and Al on tetrahedral sites changes symmetry of structure. This mineral is often called potassium feldspar or K-feldspar because potassium always exceeds sodium in its formula. Sanidine is a monoclinic alkali feldspar with small optic axial angle (2V) and commonly occurs in volcanic rocks. Include compositions between KAlSi 3 O 8 and NaAlSi 3 O 8. 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). It solidifies at a much lower temperature (below 1000C) than plagioclase. Igneous rock. 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. 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. An alkali-feldspar-syenite with 5-20% quartz on the QAPF diagram, and no foids . In addition, there are distortions to the framework that effect symmetry. The K-feldspar polymorphs, sanidine, orthoclase, microcline, and adularia have identical chemical compositions. The composition of coexisting exsolved phases can be found by drawing an isotherm until it intersects the solvus. The alkali feldspar series behaves quite differently. 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. Usually the process is too fast for completely separate crystals to develop. Alkali feldspar has the formula KAlSi 3 O 8, potassium aluminosilicate. K-FELDSPARS. In solution, the albite and orthoclase molecules are compatible with each other but they have to separate out as distinct minerals on crystallisation. The major third component for alkali feldspars is An. Most natural feldspars have minor amounts of other components in their composition. Syenitoid. Coarse-grained ("plutonic") crystalline igneous rock .

alkali feldspar composition

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