Nepheline Syenite (commonly abbreviated neph sye) is feldspar-like in its chemistry, but has higher K2O and Na2O and less SiO2. It melts earlier than feldspars and is low in iron, making it great in glazes.
Glazes containing a high amount of neph sye tend to deflocculate in storage. Counteract this with a small amount of flocculant like Epsom Salts or vinegar.
Alternatively, use glaze chemistry to increase the clay content of the glaze
Nepheline syenite melts at lower temperatures compared to feldspars.
Nepheline Syenite in North America is typically from Spectrum, who mine the rock in Ontario, Canada.