Feldspar cement includes overgrowths on feldspar framework
grains (Figs. 62, 63) in all types of sandstones, which is by far t^a more common occurence, and also as a complete pore filling mosaic
of grains in arkose. Albite and K-spar are the only known cement
types, and both are extremely pure end members of the alkali and
plagioclase feldspar series (Kastner, 1971). Chemical conditions necessary for feldspar growth include a supply of dissolved silica
and high Na+/H+ or K+/Na+ activities such as might be found in
formation water of high salt concentration (Pettijohn and others,
1972, p. 429). The feldspars form at moderate burial depths and
are not of hydrothermal or metamorphic origin.
Albite cement shows a preference for albite framework grains
as seed crystals, just as K-feldspar prefers orthoclase or microcline.
However, albite will also form on K-feldspar grains and
vice versa. Commonly, twinning of framework grains will be propagated
into the overgrowth also.
The source of feldspar cement is most probably detrital alkali
or plagioclase feldspars of intermediate composition, because
they are probably less stable than pure end member feldspars