Feldspars (KAlSi3O8 – NaAlSi3O8
– CaAl2Si2O8) are a group of rock-forming tectosilicate minerals which
make up as much as 60% of the Earth's crust.
Feldspars crystallize from magma
in both intrusive and extrusive igneous rocks, as veins, and are also present
in many types of metamorphic rock. Rock formed almost entirely of calcic
plagioclase feldspar (see below) is known as anorthosite. Feldspars are
also found in many types of sedimentary rock.
This group of minerals consists
of framework tectosilicates. Compositions of major elements in common feldspars
can be expressed in terms of three endmembers:
Potassium-Feldspar (K-spar) endmember
Albite endmember NaAlSi3O8
Anorthite endmember CaAl2Si2O8
Solid solutions between K-feldspar
and albite are called alkali feldspar. Solid solutions between albite and
anorthite are called plagioclase, or more properly plagioclase 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. Albite is considered both a plagioclase
and alkali feldspar. In addition to albite, barium feldspars are also considered
both alkali and plagioclase feldspars. Barium feldspars form as the result
of the replacement of potassium feldspar.
The alkali feldspars are as follows:
(monoclinic), — KAlSi3O8
(triclinic) — KAlSi3O8
(triclinic) — (Na,K)AlSi3O8
Sanidine is stable at the highest
temperatures, and microcline at the lowest. Perthite is a typical texture
in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions
during cooling of an intermediate composition. The perthitic textures in
the alkali feldspars of many granites can be seen with the naked eye.
Microperthitic textures in crystals are visible using a light microscope,
whereas cryptoperthitic textures can be seen only with an electron microscope.
The plagioclase feldspars are
triclinic. The plagioclase series follows (with percent anorthite in parentheses):
to 10) — NaAlSi3O8
(10 to 30) — (Na,Ca)(Al,Si)AlSi2O8
(30 to 50) — NaAlSi3O8 — CaAl2Si2O8
(50 to 70) — (Ca,Na)Al(Al,Si)Si2O8
(70 to 90) — (NaSi,CaAl)AlSi2O8
(90 to 100) — CaAl2Si2O8
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. The immiscibility gaps in the plagioclase
solid solution are complex compared to the gap in the alkali feldspars.
The play of colours visible in some feldspar of labradorite composition
is due to very fine-grained exsolution lamellae.
The barium feldspars are monoclinic
and comprise the following:
Feldspars can form clay minerals
through chemical weathering.