The abundance of sulphur in earth's crust is about 0.6%. Free sulphur occurs in large natural deposits. In Mexico and Sicily, it is being mined and used directly without further purification. Some useful sources of sulphur are sulphide ores such as copper pyrites (CuFeS2), iron pyrites (FeS2) also known as Fool's gold, galena (PbS), zinc blende (ZnS), cinnabar (HgS) etc. When one of the oxygen atoms in the sulphate ion is replaced by sulphur, the resulting ion (S2O32-) is known as thiosulphate. The free acid, H2S2O3 is not stable but thiosulphate salts are stable.