An inorganic fiber containing silica and magnesia as the major fiber components and which further includes intended addition of lithium oxide to improve the thermal stability of the fiber. The inorganic fiber exhibits good thermal performance at 1260°C and greater, low linear shrinkage, retains mechanical integrity after exposure to the use temperature, and exhibits low biopersistence in physiological fluids. Also provided are thermal insulation product forms prepared from a plurality of the inorganic fibers, methods of preparing the inorganic fiber and of thermally insulating articles using thermal insulation prepared from a plurality of the inorganic fibers.