Said fibers can additionally contain up to 4 weight percent, preferably up to 2 weight percent, and more preferably 0.5 to 1 weight percent, of additive inorganic oxides of the formula Mx Oy, where M is a cation (other than Al or Si) and x and y are integers such that when x is 1, y is 1 or 2, and when x is 2, y is 1, 3 or 5.