Pyrogenically produced silicon dioxide

Pyrogenically produced silicon dioxide (I) exhibiting a Brunauer-Emmett and Teller surface (BET) value of 200 +- 15 m 2>/g, a grindometer value of less than 30 mu m and a median value of 65-86, according to photon correlation spectroscopy, is new. Independent claims are included for: (1) the preparation of (I); and (2) a silicon rubber mass comprising 0.5-60 wt.% of the pyrogenically produced silicon dioxide and 40-99.5 wt.% of an organopolysiloxane compound of formula (Z-Si-R3-n-O-[SiR2O] x-SiR3-n-Z1). R : 1-50C alkyl, 1-50C alkoxy-, aryl-, oxime- or acetoxy residue (optionally substituted by O, S, F, Cl, Br, I or polystyrene-, polyvinylacetate-, polyacrylate-, polymethacrylate- or polyacrylnitrile residue with 40-10000 repeating units); Z : OH, Cl, Br or acetoxy-, amino-, aminoxy-, oxime-, alkoxy-amido-, alkenyloxy-, acryloxy- or phosphate residue, where the organic residue exhibits 0-20 carbon atom; Z1 : oxime-, alkoxy-, acetoxy-, amino- or amido residue; n : 1-3; and x : 100-15000.