Structure is basically the spacing between abrasive grains. An open structure would be 12 or higher while a closer structure would be 6 or so. Here again, the structure depends on a variety of factors not the least of which is how difficult the material is to grind.
Is brown aluminum oxide a possible substitute for high crystalline silica containing abrasive materials?
Yes. According to NIOSH, Brown Aluminum Oxide could be a viable substitute for consumers looking for safer alternative materials to high crystalline silica content abrasive products.