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. Compositions of major elements in common feldspars can be expressed in terms of three
endmembers: •
orthoclase endmember
KAlSiO •
albite endmember
NaAlSiO
orthoclase (monoclinic), and
microcline (
triclinic) refer to
polymorphs of K-feldspar. --> The ratio of alkali feldspar to plagioclase feldspar, together with the proportion of
quartz, is the basis for the
QAPF classification of igneous rock. Calcium-rich plagioclase is the first feldspar to crystallize from cooling magma, then the plagioclase becomes increasingly sodium-rich as crystallization continues. This defines the continuous
Bowen's reaction series. K-feldspar is the final feldspar to crystallize from the
magma.
Alkali feldspars Alkali feldspars are grouped into two types: those containing potassium in combination with sodium, aluminium, or silicon; and those where potassium is replaced by barium. The first of these includes: •
orthoclase (
monoclinic) •
sanidine (
monoclinic) •
microcline (
triclinic) •
anorthoclase (
triclinic) 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. Sanidine is stable at the highest temperatures, and microcline at the lowest. Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope.
Ammonium feldspar Buddingtonite is an ammonium feldspar with the chemical formula: NH4AlSi3O8. It is a mineral associated with hydrothermal alteration of the primary feldspar minerals.
Barium feldspars Barium feldspars form as the result of the substitution of barium for potassium in the mineral structure. Barium feldspars are sometimes classified as a separate group of feldspars, The barium feldspars are monoclinic and include the following: •
celsian •
hyalophane Plagioclase feldspars The plagioclase feldspars are
triclinic. The plagioclase series follows (with percent
anorthite in parentheses): •
albite (0 to 10) •
oligoclase (10 to 30) •
andesine (30 to 50) •
labradorite (50 to 70) •
bytownite (70 to 90) •
anorthite (90 to 100) Intermediate compositions of 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. The immiscibility gaps in plagioclase solid solutions are more complex than those in alkali feldspars. The play of colours visible in some feldspar of
labradorite composition is due to very fine-grained exsolution
lamellae known as Bøggild intergrowth. The
specific gravity in the plagioclase series increases from
albite (2.62) to anorthite (2.72–2.75). ==Structure==