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Nepheline Syenite
Generic
Chemistry
Chemistry available on new site, see link above
Links to Other Materials
- Soda Feldspar - Unspecified
- Cache
Miscellaneous
- Family: Feldspar
- Region: North America
- Mined At: Unspecified
- Raw Mineral: No
- Generic: Yes
Notes
Nepheline Syenite is an anhydrous sodium potassium alumino silicate. Although feldspar-like in its chemistry, mineralogically it is an igneous rock combination of nepheline, microcline, albite and minor minerals like mica, hornblende and magnetite. It is found in Canada, India, Norway and USSR.
Like feldspar, it is used as a flux in tile, sanitaryware, porcelain, vitreous and semi-vitreous bodies. Nepheline syenite contributes high alumina without associated free silica in its raw form. This makes it an excellent tile filler and melter, especially for fast firing. Nepheline syenite fluxes and forms silicates with free silica in bodies without contributing any free silica itself, thereby stabilizing the expansion curve of the fired body. Floor and wall tile bodies benefit from lower absorption and moisture expansion, better mechanical strength and lower thermal expansion than other alumina sources. Nepheline syenite is valuable in glass batches to acheive the lowest melting temperature while acting as a source of alumina.
Nepheline Syenite has been a standard in the ceramic industry for many years, and is very popular for its whiteness. For example, it is possible to make a very white vitreous medium temperature porcelain (as low as cone 4) by mixing a plastic kaolin with nepheline syenite and silica.
In pugged bodies nepheline syenite can be responsible for stiffness changes during aging. It is also more challenging to maintain stable deflocculated slurry bodies using nepheline syenite than with other feldspars.
Because of its sodium content, high nepheline syenite glazes tend to craze (because of the high thermal expansion of Na2O). However bodies fluxed with nepheline syenite do not necessarily exhibit greater expansion than other feldspar bodies.
Nepheline syenite begins melting around cone 1 and is one of the lowest melting feldspars.
Since nepheline syenite has more alumina than most feldspars, substituting it into recipes means that on one hand a lower melting temperature is achieved while on the other a more viscous melt results because of the extra alumina.
The picture of the flow test here shows that nepheline syenite by itself is barely beginning to flow and melt at cone 9. However when combined with other materials it will promote melting to a much greater degree than is suggested by its performance alone. Notice that the 400 and 270 mesh particle size versions do not melt differently at this temperature.
Nepheline syenite is not available in many parts of the world and the INSIGHT ceramic calculation software instruction manual contains a lesson on how to calculate a substitution using a soda feldspar. The chemistry of nepheline is quite different from other feldspars and this is thus well worth while.
(Richard Willis)
Plutonic rock, of anorthite, albite, nepheline; also, pyroxenes, amphiboles, olivine, melanite, cancrinite, sodalite. Formed by magma emissions high in alkalines. Popular substitute for other feldspathics at under 1100ºC firings.
A typical analysis is SiO2 60.0%, Al2O3 25.0%, Fe2O3 0.05%, TiO2 traces, CaO 0.3%, MgO 0.02%, K2O 5.0%, Na2O 10.0%
Properties
Body Maturity - Body Flux
This material is generally fluxes better than feldspar and produces whiter bodies, however it can be slightly soluble and affect body stiffness over time.
Suppliers
- Generic
Authors
- Tony Hansen (Owner)
Pictures
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Nepheline Syenite flow test
XML
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