Description:
THE FELDSPAR GROUP 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:
Albite, (Sodium aluminum silicate)
Oligoclase, (Sodium calcium aluminum silicate)
Andesine, (Sodium calcium aluminum silicate)
Labradorite, (Calcium sodium aluminum silicate)
Bytownite, (Calcium sodium aluminum silicate)
Anorthite, (Calcium aluminum silicate)
The K-feldspars or alkali felspars:
Microcline, (Potassium aluminum silicate)
Sanidine, (Potassium sodium aluminum silicate)
Orthoclase, (Potassium aluminum silicate)
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 this structure. 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. potassium feldspar; see microcline |