FELDSPARS:
As you have learned in lecture, the feldspars (ignoring the rare barium feldspars)
occur in two solid solution series. One series, the alkali feldspars, have compositions
lying between NaAlSi308 (Ab) and KAl Si3O8 (Or). The other series, the plagioclase
feldspars, have compositions between NaAlSi308 (Ab) and CaAl2Si2O8 (An). Where
Ab, Or, and An are the abbreviations for albite, orthoclase, and anorthite. In
addition to variations in composition of the feldspars there are variations in
the structural state of the feldspars. On one extreme are the "high"
temperature feldspars which crystallized from magma and are preserved in rapidly
quenched volcanic rocks. High-albite and high-sanidine are the end members of
the "high" alkali feldspar series, while high-albite and high-anorthite
are the end members of the "high" plagioclase series. At the low temperature
end we have the "low" feldspars that form under diagenetic or low temperature
metamorphic conditions. These include the low-albite-microcline end members of
the alkali feldspars and the low-albite-low-anorthite end members of the plagioclase
series. Intermediate temperature variants also exist. It is these variations in
chemical composition and structural state that create complexities in the optical
properties of the feldspars.
But, while complexities exist, there are some unifying properties of the feldspars,
which you will see as you look at the thin sections in this lab. In general
the feldspars have low birefringence, two good cleavages (001 and 010), low
to moderate relief with respect to quartz (1,2,3,4,5,6) and balsam (some are positive and
some negative with respect to balsam), high 2V (with a few important exceptions),
and often simple to complex twinning. Thus with a little practice you will be
able to identify the major "species" which are introduced in this
lab.
