Minspar (formerly called NC-4) is a sodium rich feldspar mined in Spruce Pines, NC. It is used in clay bodies to promote vitrification and in cone 6-10 glazes as an inexpensive glass-former. Nepheline syenite, a silica poor cousin, can be used as a substitute (replace 100g Minspar 200 with 75 g nepheline syenite + 25 g. silica).
For more information on Minspar feldspar, see the Digitalfire.com Reference Library of ceramic materials.