The feldspar group is a fairly large group with nearly 20 members recognized,
but only nine are well known and common. Those few, however, make up the
greatest percentage of minerals found in the Earth's crust. The following
are some of the more common feldspar minerals:
The plagioclase feldspars:
The K-feldspars or alkali felspars:
The feldspars are a group of minerals that have similar characteristics
due to a similar structure. All feldspars have low symmetry, being only
monoclinic, 2/m, to triclinic, bar 1. They tend to twin easily and one
crystal can even be multiply twinned on the same plane, producing parallel
layers of twinned crystals. They are slightly hard at around 6, and have
an average density at 2.55 to 2.76. They have a rather dull to rarely vitreous
luster. Crystals tend to be blocky. Some feldspars may be triboluminescent.
They have two directions of cleavage at nearly right angles. Feldspars
also tend to crystallize in igneous enviroments, but are also present in
many metamorphic rocks.
The general formula, for the common feldspars, is XAl(1-2)
Si(3-2) O8 . The X in
the formula can be sodium, Na and/or potassium, K and/or calcium, Ca. When
the cation in the X position has a positive one (+1) charge such as with
sodium or potassium, then the formula contains one aluminum and three silicons
ions. If the formula contains the positive two (+2) cation calcium, then
the formula will contain two aluminums and only two silicon
ions. This substitution keeps the formula balanced, because aluminum has
a charge of positive three (+3) and silicon has a charge of positive four
(+4). Basically, the more calcium in the crystal, the more aluminum that
will be needed to balance the charge.
The silicons and aluminums occupy the centers of interlinked tetrahedrons
of SiO4 and AlO4. These tetrahedrons connect at each corner to other tetrahedrons
forming an intricate, three dimensional, negatively charged framework.
The cations that represent the X in the formula sit within the voids in
The different feldspars are distinguished by structure and chemistry.
The potassium or K-feldspars are polymorphs, meaning they have the same
chemistry, KAlSi3 O8 , but different
structures and therefore are different minerals. The plagioclase feldspars
are a set of minerals that are in a series from a sodium rich end member,
albite, to a potassium rich end member, anorthite. The intermediate members
of the series are given arbitrary boundries based on their percentage of
sodium or calcium.
Often, feldspars are simply referred to as plagioclase and orthoclase
(a K-feldspar) because identification to greater precision is difficult
with ordinary methods. Once identified, however, some feldspar mineral
varieties are found to have distinctive characteristics or originate from
a classic locality and on these bases are recognized by mineral collectors
as belonging to a specific feldspar mineral.