joseph.herbert@ATT.NET on thu 10 may 01
I recall people looking for feldspar chunks and have a
couple of things to pass along. I am currently in
eastern Kansas and found some "blue Granite Stone" at
the grocery store - sold as landscaping material. this
stuff is dark grey and in 1/2 to 3/4 inch chunks. It is
no doubt granite and is largely feldspar. this is not
the end of the story, however.
When most of us are looking for feldspar we want
Orthoclase - pink potassium feldspar. This material is
recognizable by the 90 degree angle of the prism faces
of large crystals and by cleavage planes at 90 degrees.
Another similar material is called albite and is
(surprise) white. this material is a sodium feldspar.
It does not quite have that 90 degree angle. To make
things a little more interesting, the potassium and soda
feld spars can occur together and the potassium feld
spar can contain some sodium and the sodium feld spar
can contain some potassium.
In the case of the "blue Granite Rock" there is another
thing going on. an additional kind of feldspar, called
plagioclase has a range of compositions running from all
sodium to all calcium in a rather continuous manner.
for the chemistry types out there, this substitution is
accompanied with changes in the silica/alumina ratio to
keep things balanced in the ion sense. These
plagioclases are rather dark, grey or brown.
So you can find feldspar chunks sold as gardening
material. You will probably be happier if you look for
the lightest color material you can find. pink to light
tan would be mostly potassium and sodium feldspar.
these materials are all opaque with cleavage planes
viaible. if it is white or frosty clear with no planes
cleavage visible, it is probably quartz. Look for
blocky crystals that are light color and you may get
feldspar chunks at garden rock prices.
Joseph herbert
--
Joseph Herbert
Instructional Writer
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