Abstract: An improved method of making microcrystalline mineral fibers using a generally known technique of impregnating precursor organic fibers with salts of desired minerals dissolved in a solvent which is readily absorbed by the fibers, and subsequently heating and calcining the precursor fibers to leave mineral fibers comprising compounds of said impregnating salts which have diameters which in the prior art process are about half the diameter of the precursor fibers, the improvement involving controlling the diameters of the mineral fibers to reduce them greatly below the above-mentioned prior art diameters by impregnating the precursor fibers with a solution whose salt concentration is very small as compared with the high concentrations used in the prior art while taking steps to control the uniformity of the impregnation prior to calcining, the microcrystalline structures of some of the mineral fibers being improved by a final sintering step at higher temperatures than are used in the prior art.