Pericline twinning is a type of crystal twinning which show fine parallel twin laminae typically found in the alkali feldspars microcline.
22.
The leucite tinguaites are usually pale grey or greenish in color and consist principally of nepheline, alkali feldspar and aegirine.
23.
The essential components of aplites are quartz and alkali feldspar ( the latter usually orthoclase or microperthite ), microcline and albite.
24.
With the addition of alkali feldspar replacing or becoming more dominant than calcic plagioclase theralites grade into foidal syenite, including shonkinite.
25.
By definition, granite is an igneous rock with at least 20 % quartz and up to 65 % alkali feldspar by volume.
26.
Minerals such as alkali feldspar, leucite, nepheline, sodalite, phlogopite mica, and apatite may be present in the groundmass.
27.
The intergrowth forms by exsolution due to cooling of a grain of alkali feldspar with a composition intermediate between K-feldspar and albite.
28.
If cooling is sufficiently slow, the alkali feldspar may exsolve to form separate grains with near-endmember albite and K-feldspar compositions.
29.
Take the alkali feldspar minerals for example, whose end members are albite, NaAlSi 3 O 8 and microcline, KAlSi 3 O 8.
30.
Within the lava dome rocks lie phenocrysts of amphibole, biotite, plagioclase, quartz, and titanite with some alkali feldspar in some domes.
