Paul Herman on thu 22 apr 04
Joseph,
Thank you very much for the information on feldspar, I loved it.
Feldspar has been on my mind lately, as there a couple of places near
here where we can get big pink chunks of it. I've been making glaze
tests out of it and they look good (to me).
My question is: What is an appropriate test to determine the
potash/sodium content, for a potter's perspective? The nearby city of
Reno has a thriving mining industry, with many assay labs, etc. so the
technology is readily available. Of course, I want something thorough
enough, but cheap.
I understand from the paragraph below that pink feldspar could contain
up to 30% sodium ion, and would like to find out exactly what is in my
feldspar. Any of your thoughts or suggestion would be much appreciated.
best wishes,
Paul Herman
Great Basin Pottery
Doyle, California US
http://www.greatbasinpottery.com/
----------
>From: Joseph Herbert
> While the sodium/calcium plagioclase family varies in composition smoothly
> between the end compositions (all sodium or all calcium) the same is not
> true for sodium and potassium feldspars. Even though the chemical formulae
> are similar, the size difference between the respective alkali ions is so
> great that they cannot be easily accommodated in the same crystal structure.
> Potassium feldspars with up to 30 percent sodium ion are stable and sodium
> feldspars with up to 32 percent potassium are stable. Other compositions,
> which are stable at high temperature, form closely layered crystals, with
> alternating layers having stable potassium rich compositions and sodium rich
> compositions. The mixtures of potassium and calcium feldspars are unstable
> and do not occur.
Joseph Herbert on thu 22 apr 04
One of our correspondents wrote: "I have a recipe that calls for soda
feldspar. Exactly which feldspar is that? Would it be Custer feldspar?"
There are three kinds of information about feldspars that can be considered:
The chemical composition of feldspar minerals, the occurrence of feldspar
minerals in rocks, and the feldspar presented to us by the mining industry
for our use.
Chemistry and mineralogy
Feldspars, a general class of silica/alumina rock forming minerals, vary in
chemical composition and in gross physical characteristics (like color and
crystal structure) over a well-understood range. For purposes of
formulating ceramic glazes, the variation in alkali metal (Potassium,
Sodium, and Calcium) content and the accompanying variation in Silica and
Alumina content are of interest. There are related minerals, called
feldspathoids, which share some chemical characteristics with feldspars.
The most prominent of these in ceramic use is Nephylene of Nephylene Syenite
fame. The distinction between Nephylene as a mineral name and Nephylene
Syenite as a rock name is something that geologists care about but that
finds surprisingly (for geologists) little interest among other groups.
Orthoclase is the mineral name for potassium feldspar. The mineral is pink
to salmon colored and the crystal form shows a right angle between two of
the faces (that is the ortho in the name). The chemical composition for
this mineral is KAlSi3O8. This mineral is the main source of potassium in
high fire ceramic glazes.
Albite is the mineral name for a sodium rich feldspar. The mineral is
usually white and the crystal form shows a 93 degree angle for cleavage
planes. The chemical composition for this mineral is NaAlSi3O8.
Anorthite is the mineral name for a calcium rich feldspar. The mineral is
usually dark grey or brownish and shows a 94 degree angle for cleavage
planes. The chemical composition for this mineral is CaAl2Si2O8.
There are several interesting relationships between these minerals. Albite
and Anorthite are the end members of a double substitution solid solution
series. Sodium ions hold a single positive charge and Calcium ions hold a
double positive charge. One of the requirements for molecules in the
chemical world is that they have no net charge. If a calcium ion is
substituted for a sodium ion, there is an additional positive charge that
must be offset somehow. In the case of sodium/calcium feldspars, the amount
of aluminum increases at the expense of silicon to balance the charge of the
calcium. As the composition changes from all sodium to all calcium, the
number of aluminum ions doubles while the number of silicon ions decreases
by one third.
The Feldspar minerals that have compositions between all sodium and all
calcium are called Plagioclase feldspar minerals. They are given names on
the basis of alkali ion composition: Albite (100-90% Na), Oligoclase (90-70%
Na), Andisine (70 - 50% Na), Labradorite (50 - 30% Na), Bytownite (30 - 10%
Na), and Anorthite (10 - 0% Na). As the sodium content decreases, so does
the silica content. The weight percent of silicon in Albite is about 32%
and decreases to about 20% in Anorthite. Similarly the weight percent of
aluminum increases from about 10 percent in Albite to about 19 percent in
Anorthite. The melting temperature of Albite is 400 degrees Celsius lower
than that of Anorthite.
By way of contrast, Orthoclase, the potassium feldspar mineral, is about 30
percent silicon and 9 percent aluminum.
While the sodium/calcium plagioclase family varies in composition smoothly
between the end compositions (all sodium or all calcium) the same is not
true for sodium and potassium feldspars. Even though the chemical formulae
are similar, the size difference between the respective alkali ions is so
great that they cannot be easily accommodated in the same crystal structure.
Potassium feldspars with up to 30 percent sodium ion are stable and sodium
feldspars with up to 32 percent potassium are stable. Other compositions,
which are stable at high temperature, form closely layered crystals, with
alternating layers having stable potassium rich compositions and sodium rich
compositions. The mixtures of potassium and calcium feldspars are unstable
and do not occur.
Leucite is a feldspar-like mineral that contains potassium as the alkali ion
and contains silicon and aluminum in the ratio of 2:1. The chemical formula
is K(AlSi2O6). This mineral is low in silica and high in aluminum. If the
original molten material from which the mineral formed had more silica in
it, Orthoclase feldspar would form instead of Leucite. Similarly, Nephylene
is a sodium/potassium feldspar like mineral. The chemical formula is (K,
Na)(AlSiO4). The ratio of silicon to aluminum is 1:1.
Occurrence in rocks
Feldspar minerals are found in nearly all igneous rocks and in many
metamorphic rocks. Igneous rocks are formed by the cooling, either slow or
fast, of melted rock material. Lava is melted rock material flowing on the
surface of the earth and magma is melted rock material inside the earth.
The lava from Hawaiian volcanoes solidifies to form the rock called Basalt
containing calcium-rich plagioclase feldspar minerals. A mass of liquid
rock of similar composition that cooled slowly inside the earth would form a
rock called Gabbro or Diorite.
Potassium feldspar, Orthoclase, is found in the rock called granite. Molten
rock bodies that cooled slowly underground long ago form the central core of
the Rocky and Sierra Nevada mountains. Lately, these rocks have been seen
with increasing frequency at Home Depot stores as possible countertop
materials. Looking at the displays of polished rock, the pink or white
patches are feldspar of some kind. In the same rock, transparent areas are
Quartz and black specks are Biotite mica or Amphibole. Some granitic rocks
contain both potassium feldspar and sodium/calcium feldspar. This can be
seen by the presence of rectangular shaped crystals of different colors,
often pink and white or grey.
An average granite may be 32 percent Quartz, 30 percent Orthoclase, and 29
percent Albite. This rock is almost 74 percent SiO2, 13.5 percent Al2O3, 5
percent K2O, and 3.5 percent Na2O. An average basalt contains no Quartz, 6
percent Orthoclase, 18 percent Albite, and 25 percent Anorthite. The SiO2
percentage is 45 percent, Alumina is 14.6 percent, and the alkali ions are
24.5 percent (MgO - 9.4, CaO - 10.7, Na2O - 2.63, K2O - 0.95).
Some metamorphic rocks, called Gneisses, have almost the same mineral and
chemical composition as granite. The major feldspar is a plagioclase but
orthoclase is present in many Gneisses. The percentage of free quartz
present ranges from 19 to 35 percent while the plagioclase percentage ranges
from 22 to 50 percent. The oxide percentages are similar to granite: SiO2 -
70 percent, Al2O3 - 15 percent, K2O - 3 or 4 percent.
Mining of Feldspar
From the description above of the mineral composition of the various rocks,
you can see that few rocks are more than 30 percent feldspar. The people
who mine feldspar require special situations or equipment to allow the
production of a nearly pure feldspar product. One approach to producing
feldspar from mine run rock is to crush the rock to a certain size and then
separate the constituents by a physical or mechanical process. As you might
imagine, this could prove to be a difficult and expensive proposition and
most mining companies avoid doing this as much as possible. The other
option is to find rock deposits that are composed solely of feldspar or that
have relatively few other minerals present. As you might expect, such
occurrences are rather rare.
There is a class of rock, called pegmatite, which is composed of very large
crystals. The composition of a pegmatite can vary but they are most usually
of a composition similar to granite. Some are composed of just feldspar and
quartz minerals. Regardless of grain size, and some pegmatites are composed
of crystals whose dimensions are measured in feet. Finding a deposit that
is composed of very large crystals makes sorting by power shovel operators
possible and effective. If the deposit is composed of only two minerals,
say a feldspar and a mica, the very different physical properties can make
separation relatively easy. An experienced shovel operator can control the
composition of the product very effectively.
According to the 1998 version of the Minerals Handbook, a government
publication, there are 9 companies mining feldspar and associated minerals
in 12 different locations. The producer of Custer feldspar is the Pacer
Corp., mining near Custer, S.D.
1998 U. S. Gov. Minerals Handbook
Company Plant location Product
APAC Arkansas Inc. Muskogee, OK Feldspar-silica mixture.
The Feldspar Corp. Monticello, GA Potash feldspar.
Do. Spruce Pine, NC Soda-potash feldspar;
feldspar-silica mixture.
Franklin Industrial Minerals Kings Mountain, NC Potash feldspar.
PW Gillibrand Co. Simi Valley, CA Feldspar-silica
mixture.
Granite Rock Co. Felton, CA Do.
KT Feldspar Corp. Spruce Pine, NC Soda-potash
feldspar.
Pacer Corp. Custer, SD Potash feldspar.
Unimin Corp. Byron, CA Do.
Do. Emmett, ID Do.
Do. Spruce Pine, NC Soda-potash feldspar.
U.S. Silica Co. Montpelier, VA Aplite.
The following are examples of analysis of feldspar available for ceramic
use.
Custer Feldspar Typical analysis
Silica (SiO2) 68.50%
Alumina (AI203) 17.00%
Iron Oxide (Fe2o3) 0.15%
Lime (CaO) 0.30%
Magnesia (MgO) Trace
Soda (Na2O) 3.00%
Potash (K20) 10.00%
Feldspar Corp. NC4 feldspar
Chemical Analysis 200 Mesh
SiO2 68.15%
Al2O3 18.85%
Fe2O3 0.07%
CaO 1.40%
MgO Trace
K2O 4.10%
Na2O 6.82%
LOI 0.09%
From these analyses and the descriptions of the products produced by the
mining companies, the feldspar offered to us in bags is sodium/potassium
feldspar. What we call Potassium Feldspar contains relatively less sodium
and what we call Soda Feldspar contains relatively less potassium but both
the kinds of feldspars contain both sodium and potassium ions.
I do not know the reason that no feldspar suppliers have sodium/calcium
feldspars for sale. It may be that the potassium feldspar deposits are
preferred because they can be mined for mineral of greater purity more
easily than deposits of sodium/calcium feldspar. It may be that there is a
commercial need for feldspar that is essentially calcium free so efforts to
mine feldspar high in sodium would also increase the amount of calcium in
the end material.
While a purely sodium feldspar mineral (Albite) exists, and so does a purely
sodium feldspathoid (Sodalite), these seems not to be mined commercially as
ceramic materials.
Here ends the discussion of soda feldspar.
Joseph Herbert
Craig Martell on fri 23 apr 04
Paul wanted to know:
>My question is: What is an appropriate test to determine the
>potash/sodium content, for a potter's perspective?
Hello Paul:
An XRF analysis will give you a percentage of everything in the
sample. Potash, Soda, alumina, silica, and any trace elements etc. You
need to provide them with a crushed, screened sample that will pass thru a
100 mesh screen. The cost is anywhere from $70 to $100 bucks, I
think. Try the geology dept at a University near you, they might have the
stuff to do the test. An XRD analysis is more expensive but it will tell
you what minerals are in the sample, plus the % analysis. An XRF is most
always the thing you want.
You might also contact the USGS Dept of Mineralogy and see if there's an
analysis already on file for the formation you are gleaning the spar from.
regards, Craig Martell Hopewell, Oregon
Paul Herman on fri 23 apr 04
Hi Craig,
Thanks for the info. After my spring show I'll get the stuff analysed.
There are two prospects, quite a few miles apart. They look the same.
One has a history of feldspar production from the 1930s. Evidently it
was a "hand-sorted product of high purity", and a few railcars full were
sent to the bay area to make toilets.
Whenever I get some results, I'll post them to the list.
Our woodfiring came out well, and Joe's doing OK.
Regards,
Paul Herman
Great Basin Pottery
Doyle, California US
http://www.greatbasinpottery.com/
Paul wanted to know:
>My question is: What is an appropriate test to determine the
>potash/sodium content, for a potter's perspective?
Hello Paul:
An XRF analysis will give you a percentage of everything in the
sample. Potash, Soda, alumina, silica, and any trace elements etc. You
need to provide them with a crushed, screened sample that will pass thru
a
100 mesh screen. The cost is anywhere from $70 to $100 bucks, I
think. Try the geology dept at a University near you, they might have
the
stuff to do the test. An XRD analysis is more expensive but it will
tell
you what minerals are in the sample, plus the % analysis. An XRF is
most
always the thing you want.
You might also contact the USGS Dept of Mineralogy and see if there's an
analysis already on file for the formation you are gleaning the spar
from.
regards, Craig Martell Hopewell, Oregon
Joseph Herbert on sat 24 apr 04
In response to Paul's query about how one might find the K/Na content of a
particular feldspar, Craig Martell suggested x-ray fluorescence analysis.
This will in fact work and does require a powdered sample and some
relatively rare, but really useful, equipment.
There are may be some other options: Optical characteristics, density
variation, or, as Mr. Martell noted, literature research.
I had in my mind a variation in some optical characteristic of orthoclase
with changing sodium content. I know there is one for the composition
variation for the plagioclase series. In consulting my optical mineralogy
book, I did not find that relationship so I will not tout it. I feel it may
exist and a real mineralogist might be able to describe such a variation.
In plagioclase the angles of the axes change with composition and I had
thought something similar happened during the substitution of sodium for
potassium in orthoclase, but I cannot supply a reference or an explanation.
Sodium and Potassium ions have different atomic weights and when they are
substituted in an unchanged crystal lattice (trying to leave aside issues of
two phases) there should be a lowering in density as the lighter sodium (23
units) substitutes for the heavier potassium (39 units). Unfortunately, the
alkali metal content of feldspar is a relatively minor percentage to start
with (like 10 %) and the substitution of 5 percent of the atoms making up
that part with some that are lighter by 16/39 doesn't make for a large
density change as the ratio of alkali metals change. I expect someone has
done work on this and that it could be uncovered by library (or internet)
research.
If the crystals you are referring to are from a commercial supplier, the
answer to your question could be just a phone call away, provided they know
the composition or the source of the material. Since Paul is writing to
clayart, that is probably not the case and he (or someone) is picking them
up somewhere. If you know where the material is found, an examination of a
geologic map of the area should give the name of the mass of rock
(formation) that is there. Once you have the name of the rock body, finding
information about the rock is much easier. If a person is picking up
samples on the surface, others have too and some of those are geologists.
Some may have been graduate students at a nearby college and may have
produced a volume of detailed information about that very rock in that very
place.
There are something like 140 minerals that are found in Franklin, New Jersey
and nowhere else. The area was an important mining district for zinc so the
study of the material had economic importance. However, the real reason
that so many minerals, some very rare, were found was that Harvard and other
eastern schools had lots of students working for professors interested in
the minerals of that part of New Jersey. These rocks were near by (compared
to Utah, for example) and easily obtainable, since the area was being
actively mined. It makes for a well studied area.
When you have the name of the rock and know the area where it originated,
the publication of the state geologic survey, the United States Geologic
survey, or any of many other publications may contain the composition of the
mineral you have. Or there may not be. If there is a local college with a
geology department, asking the question there may produce the information
you want without much work on your part at all. Someone there might just
know that fact from previous study.
Good Luck
Joseph Herbert
mailtoandrew@FSMAIL.NET on fri 30 apr 04
In the current edition of Ceramic Review, no. 207 & May/Jun 2004, theres
an article called Feldspar Facts that probaly will answer most of these
questions.
Regards,
Andrew
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