Description:
Albite is a common felspar and is the
"pivot" mineral of two different feldspar
series. It is most often associated with the plagioclase
series where it is an end member of this series. The
plagioclase series comprises felspars that range in
chemical composition from pure NaAlSi3 O8 to pure
CaAl2 Si2 O8 . The various plagioclase feldspars are
identified from each other by gradations in index
of refraction and density in the absence of chemical
analysis and/or optical measurements. Albite is also
an end member of the alkali or K-feldspars whose series
ranges from pure NaAlSi3 O8 to pure KAlSi3 O8. This
series only exists at high temperatures with the mineral
sanidine being the potassium, K, rich end member.
At lower temperatures, the K-feldspars will seperate
from the albite in a process called exsolution. The
albite will form layers inside the k-feldspars crystals.
Some times these layers are discernable to the naked
eye and the stone is referred to as perthite. Albite
by definition must contain no less than 90% sodium
and no more than 10% of either potassium and/or calcium
in the cation position in the crystal structure..
Albite is the last of the feldspars to crystallize
from molten rock. The process of crystallization from
a molten rock body serves to isolate rarer elements
in the last stages of crystallization and therefore
produces rare mineral species. Thus albite is often
found with some lovely rare and beautiful minerals.
Although usually not an exceptional collection mineral
in itself, albite can be a nice accessory mineral
to other mineral species. A variety associated with
tourmaline is called cleavelandite and forms extremely
thin, platy, white and sometimes very transparent
crystals. All plagioclase feldspars show a type of
twinning that is named after albite. Albite Law twinning
produces stacks of twin layers that are typically
only fractions of millimeters to several millimeters
thick. These twinned layers can be seen as striation
like grooves on the surface of the crystal and unlike
true striations these also appear on the cleavage
surfaces. The Carlsbad Law twin produces what appears
to be two intergrown crystals growing in opposite
directions. Two different twin laws, the Manebach
and Baveno laws, produce crystals with one prominant
mirror plane and penetrant angles or notches into
the crystal. Although twinned crystals are common,
single crystals showing a perfect twin are rare and
are often collected by twin fanciers. |
