An inorganic fiber containing silica and magnesia as the major fiber components which further includes an intended strontium oxide additive to improve the thermal stability of the fiber. The inorganic fiber exhibits good thermal performance at 1260 C. and greater for 24 hours or more, retains mechanical integrity after exposure to the use temperature, and exhibits low biopersistence in physiological fluids. Also provided are thermal insulation product forms, methods of preparing the inorganic fiber and of thermally insulating articles using thermal insulation prepared from a plurality of the inorganic fibers.