Unlike many naturally occurring forms of crystalline silica,
fumed silica is amorphous. The very small spheroid silica particles
(on the order of 10 nm diameter) fuse irreversibly while
still semimolten, creating aggregates. When cool, these aggregates
become physically entangled to form agglomerates. Silica
produced in this way possesses remarkably high surface area,
100 to 400 m²/g as measured by the BET method developed
by Brunauer, Emmett, and Teller (Brunauer et al., 1938; Noll,
1968; Cabot Corporation, 1990).