An inorganic fiber containing a fiberization product of a compound comprising at least one alkaline earth silicate, at least one compound containing an element from group VII and/or IX of the periodic table, and optionally alumina and/or boria. The inclusion of a suitable amount of at least one compound containing an element from group VII and/or IX of the periodic table of elements to an alkaline-earth silicate inorganic fiber reduces fiber shrinkage, decreases biopersistence in physiological solutions, and enhances mechanical strength beyond that of alkaline earth silicate fibers without the presence of the at least one compound containing an element from group VII and/or IX. Also provided are methods of preparing the inorganic fiber and of thermally insulating articles using thermal insulation prepared from the inorganic fibers.