The play of colors visible in some feldspar of labradorite composition is due to very fine-grained exsolution lamellae known as Bøggild intergrowth. The immiscibility gaps in the plagioclase solid solutions are complex compared to the gap in the alkali feldspars. Intermediate compositions of plagioclase feldspar also may exsolve to two feldspars of contrasting composition during cooling, but diffusion is much slower than in alkali feldspar, and the resulting two-feldspar intergrowths typically are too fine-grained to be visible with optical microscopes. bytownite (70 to 90) (NaSi,CaAl)AlSi 2O 8.labradorite (50 to 70) (Ca,Na)Al(Al,Si)Si 2O 8.oligoclase (10 to 30) (Na,Ca)(Al,Si)AlSi 2O 8.The plagioclase series follows (with percent anorthite in parentheses): The barium feldspars are monoclinic and include the following: Barium feldspars are sometimes classified as a separate group of feldspars, and sometimes they are classified as a sub-group of alkali feldspars. It is a mineral associated with hydrothermal alteration of the primary feldspar minerals.īarium feldspars form as the result of the substitution of barium for potassium in the mineral structure. Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope.īuddingtonite is an ammonium feldspar with the chemical formula: NH 4AlSi 3O 8. The perthitic textures in the alkali feldspars of many granites can be seen with the naked eye. ![]() Perthite is a typical texture in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition. ![]() Sanidine is stable at the highest temperatures, and microcline at the lowest. Potassium and sodium feldspars are not perfectly miscible in the melt at low temperatures, therefore intermediate compositions of the alkali feldspars occur only in higher temperature environments.
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