The recognised metalloids can be obtained by chemical reduction of either their oxides or their sulfides. Simpler or more complex extraction methods may be employed depending on the starting form and economic factors. Boron is routinely obtained by reducing the trioxide with magnesium: B 2 O 3 + 3 Mg → 2 B + 3MgO; after secondary processing the resulting brown powder has a purity of up to 97%. Boron of higher purity (> 99%) is prepared by heating volatile boron compounds, such as BCl 3 or BBr 3, either in a hydrogen atmosphere (2 BX 3 + 3 H 2 → 2 B + 6 HX) or to the point of thermal decomposition. Silicon and germanium are obtained from their oxides by heating the oxide with carbon or hydrogen: SiO 2 + C → Si + CO 2 ; GeO 2 + 2 H 2 → Ge + 2 H 2 O. Arsenic is isolated from its pyrite (FeAsS) or arsenical pyrite (FeAs 2) by heating; alternatively, it can be obtained from its oxide by reduction with carbon: 2 As 2 O 3 + 3 C → 2 As + 3 CO 2. Antimony is derived from its sulfide by reduction with iron: Sb 2 S 3 → 2 Sb + 3 FeS. Tellurium is prepared from its oxide by dissolving it in aqueous NaOH, yielding tellurite, then by electrolytic reduction : TeO 2 + 2 NaOH → Na 2 TeO 3 + H 2 O; Na 2 TeO 3 + H 2 O → Te + 2 NaOH + O 2. Another option is reduction of the oxide by roasting with carbon: TeO 2 + C → Te + CO 2.