The average concentration of tin in the earth's crust is estimated to be 2-3 ppm, comparable to cerium and yttrium. Owing to the high atomic mass of tin and the high density of its important minerals, its volume concentration is very low. However, it occurs in only a small number of locations, where consequently its relative abundance is high. In general, l000-fold enrichment is necessary to give workable tin deposits, i.e., with a tin content of at least 0.2%. The question of whether a deposit can be economically extracted, for a given world market price level, depends on the mining conditions. For example, there are deposits in Bolivia containing 1 % Sn which cannot be economically extracted, whereas in South East Asia placer deposits containing 0.02% Sn are successfully mined.