Inorganic fibers which have a silicon extraction of greater than about 0.02 wt % Si/day in physiological saline solutions. The fiber contains SiO.sub.2, MgO, CaO, and at least one of Al.sub.2 O.sub.3, ZrO.sub.2, TiO.sub.2, B.sub.2 O.sub.3, iron oxides, or mixtures thereof. Also disclosed are inorganic fibers which have diameters of less than 3.5 microns and which pass the ASTM E-119 two hour fire test when processed into a fiber blanket having a bulk density in the range of about 1.5 to 3 pcf.