Michael Banks on sat 6 nov 99
Don't be confused Craig, Your instincts are right on. Petalite has 2 moles
more silica than K/Na feldspars and, while it is classified as a
feldspathoid due to structural similarities to true feldspars, it is not a
silica-undersaturated feldspathoid like nepheline.
The true silica-undersaturated feldspathoids crystallise from silica-poor,
alkali-rich magma. They have only about 2/3 as much silica as alkali
feldspar and can be regarded as silica starved feldspars. Petalite is not
one of these, crystallising in a silica-rich environment.
The unsaturated feldspathoids include Leucite; KAlSi2O6 (equiv. silica-poor
K- feldspar, nepheline; (Na,K)AlSiO4 (equiv. silica-poor Na-feldspar) and
eucryptite; LiAlSiO4 (silica poor spodumene). These minerals are less
chemically stable than feldspars due to their less saturated oxygen bonds,
for example nepheline is noticeably soluble and breaks down rapidly in
nature, so that it never forms a detrital mineral (unlike feldspar). The
great reactivity of nepheline for silica results in the more powerful
fluxing action of nepheline syenite (a mixture of k-feldspar and nepheline)
compared to the feldspars.
Petalite is actually an over-endowed feldspar and has no soda or potash
equivalents. The highish molar silica (one mole higher than stoichiometric)
in Custer feldspar is probably due to quartz inclusions in the mined product
which are impractical to remove. For the same reason, commercial petalite
can have more than 8 moles of silica.
As a footnote, very pure potash feldspars with over 16% K2O occur around the
world but, premium commercial products rarely exceed 12% K2O. Silica seems
inevitable in processing of this mineral. Nepheline syenite rock never has
any quartz (by definition, as nepheline can't crystallise in the presence of
free silica), so silica dilution of commercial nepheline syenite is not
likely.
Michael Banks,
Nelson,
New Zealand
> ----------------------------Original message----------------------------
> So now I'm confused. I'm under the impression that petalite is a very
> silica rich mineral. I compared petalite to albite, anorthite,
> plagiolcase, blah blah and my references say that it has more silica than
> any of them. I looked at the seger flux unity formulas for all the spars
> and petalite has 8 moles of silica to one mole of flux. The closest
> feldspar to that is Custer, which has 7 moles of silica. What am I
> missing? I looked at all the stuff I have about petalite and as far as I
> can tell it's a naturally occuring substance that isn't altered or
> processed with additions of silica. What gives? :>)
>
> later, Craig Martell in Oregon
>
> ----------------------------Original message----------------------------
>
> >Mary explained:
> >Petalite is a feldspathoid (as is nepheline) Feldspathoids are similar
to
> >feldspars--these minerals have too little silica to be feldspars.
>
Craig Martell on sun 7 nov 99
Hello:
Thanks Michael. I feel much better now!
Thanks as well for all the additional info. I printed your post and put it
in my mineralogy text in the section on silicate minerals. Cool!
May all your lithium glazes come out rosy! Well, unless you want a
different hue.
regards, Craig Martell in Oregon
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