The structure of a feldspar crystal is based on aluminosilicate tetrahedra.
The specific gravity in the plagioclase series increases from albite (2.62) to anorthite (2.72–2.75). 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. anorthoclase (triclinic) (Na,K)AlSi 3O 8.Alkali feldspars Īlkali feldspars are grouped into two types: those containing potassium in combination with sodium, aluminium, or silicon and those where potassium is replaced by barium. K-feldspar is the final feldspar to crystallize from the magma. This defines the continuous Bowen's reaction series. Calcium-rich plagioclase is the first feldspar to crystallize from a cooling magma, but the plagioclase becomes increasingly sodium-rich as crystallization continues. The ratio of alkali feldspar to plagioclase feldspar, together with the proportion of quartz, is the basis for the QAPF classification of igneous rock. Albite is considered both a plagioclase and alkali feldspar. Only limited solid solution occurs between K-feldspar and anorthite, and in the two other solid solutions, immiscibility occurs at temperatures common in the crust of the Earth. Solid solutions between albite and anorthite are called plagioclase, or, more properly, plagioclase feldspar. Solid solutions between K-feldspar and albite are called alkali feldspar. potassium feldspar (K-spar) endmember K Al Si 3 O 8.Compositions of major elements in common feldspars can be expressed in terms of three endmembers: The feldspar group of minerals consists of tectosilicates, silicate minerals in which silicon ions are linked by shared oxygen ions to form a three-dimensional network.