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FeldSpars

What we use as feldspars are mixed minerals. None are a pure feldspar. What we call feldspar fits the following approximate unity formula:

1 Flux 1 Alumina 6-7 Silica

Most of the differnces in potter's real feldspars are in the distribution of fluxes.

Potash (K2O?) Feldspars include Custer, and G-200. .... this is old custer....In Custer the fluxes are .66 mols K2O?, .30 Mols Na2O? and .03 Mols CaO?. Note that this is not very high in potash but low in calcia. In F-4 the fluxes are .27 mols K2O?, .56 mols Na2O? and .16 mols CaO?. This is not very dominated by soda. F-4 has 6 mols silica versus 7 for custer, so if you are looking for low silica sources of Feldspar this would seem to help, but once you consider the calcia they are the same.

Fortunately we have other feldspar like minerals at our disposal that we can use too. Nepheline Syenite

In Neph Sy the fluxes are .22 mols K2O? .71 mols Na2O? and .06 mols CaO? the silica is much lower at 4.5 mols.

There are several lithium minerals we can use