An inorganic fiber containing silica and magnesia as the major fiber components which further includes intended synergistic amounts of calcia and, an additional alkali metal oxide other than magnesia, such as lithium oxide, to improve the thermal performance and manufacturability of the fiber. The inorganic fiber is easier to manufacture, has a better fiber quality, exhibits good thermal performance at a use temperature of 1260° C. and greater, retains mechanical integrity after exposure to the use temperature, and exhibits low biopersistence in physiological fluids. Also provided are methods of preparing the inorganic fiber and of thermally insulating articles using thermal insulation prepared from the inorganic fibers.