iandol on sat 28 dec 02
Dear Ron Roy,
May I wish you a Happy and Prosperous New Year for 2003=20
You seem to give a good appraisal of the general situation but I am not =
sure that the following represents what is occurring when we fire a =
glaze recipe or clay body which incorporates Nepheline Syenite
<materials we use. When compared to the feldspars we can see - it is =
short of silica. This explains the sodium leaching - glazes short of =
silica leach.>>
I agree, that by conventional definitions Nepheline Syenite will not =
contain Free Silica, more usually accepted as Quartz. But it is a Coarse =
Grained Rock and not a Mineral Species. Elementary information to hand =
tells me that it is a highly variable material which will contain =
minerals selected from Hornblende, Biotite, Nepheline and an Alkali =
Felspar with Sodium rich Plagioclase which might be Albite or =
Oligoclase. All of these appear to satisfy the rules relating to the =
bonding of silicate structures so the idea that Neph. Sy. is short on =
SiO2 seems incongruous.
But Neph. Sy. would not be used as the only source of SiO2 in one of =
your glaze recipes. In designing a glaze you would ensure that =
additional quartz would be incorporated to guarantee having sufficient =
material to make glass and to meet the Al2O3/SiO2 ratio which is most =
advantageous at the firing temperature which has been selected.
If Miners knows that Neph. Sy. will leach Alkali Metals, they, or their =
agents should be able to provide representative values, relative to =
water and other common solvents and say something about the temperature =
dependency of this phenomenon. Perhaps you can obtain this information =
from your local people.
Best regards,
Ivor Lewis. Redhill, South Australia
John Britt on sat 28 dec 02
Ivor,
By saying Nepheline Syenite is short on silica, he means that when
compared to feldspars (which NS usually is) it contains less silica.
Feldspars contain, in UMF language, 1/1/7, or 1/1/9 or 1/1/6, while
nepheline syenite is 1/1/4. So it has about half the silica of custer
feldspar.
Also free silica is not equivalent to quartz. Silica is synonymous with
quartz in recipes language. Free silica is silica that is present in a
separate crystalline state. It is not attached to the feldspar.
Happy new year to you too,
John Britt
Jim Murphy on sat 28 dec 02
In his November 27, 1998 reply to Clayart regarding
"Cust Fledspar vs Neph Sy" (see link below), Michael Banks
indicates the reason Neph Sy is a more powerful flux has less to do with its
chemistry than with its mineralogical composition. "Melting behavoir is not
strictly a function of chemistry, but is also dependant on mineral
stability."
He goes on to say the Nepheline part of Neph Sy "is a feldspathoid mineral
which only forms in an igneous rock when there is insufficient silica
available. It is said to be undersaturated in respect to silica. The
chemical bonds in the nepheline crystal lattice are therefore in a to a
degree unsatisfyed and potentially "silica hungry". It is for this reason
that nepheline syenite is significantly more reactive in ceramics compared
to it's more stable cousin feldspar. Nepheline aggressively attacks the
lattices of other silicates and quartz above 1100 degrees C, at least 2
cones lower than the point when the feldspar atomic lattice starts to
dissociate."
Perhaps the potentially "silica-hungry" characteristic of Nepheline may help
Neph Sy contribute to "better" glass formation in a glaze or body -
yes/no ????????
Jim Murphy
iandol on sun 29 dec 02
Dear John Britt,
Thanks for that information. Yes, I am aware of those ratios and I do =
believe I suggested that the shortfall in a recipe could be made up by =
adding one of the sources of silicon dioxide.
One of the problems with understanding Nepheline Syenite is that it =
tends to be regarded as a single compound. It is not. It is a mixture of =
several Minerals. Some of these are desirable, some are not. It is also =
highly variable with assays differing from source to source. Reading =
about the industrial production of Neph.Sy. suggests that it undergoes =
some sort of beneficiation to remove the undesirable materials. What is =
left is still a collection of minerals. And as Jim Murphy says in his =
quote from Michael Banks, there is the potential for many complex =
reactions to happen which will lead to early melting.
Best regards,
Ivor Lewis
| |
|
