A mixture of magnesium powder and silicon dioxide (sand) is strongly heated so magnesium reduces the oxide to form elemental silicon and magnesium oxide. The crude product is washed to remove magnesium oxide, leaving black silicon powder.

1. Measure a stoichiometric mixture of finely powdered magnesium and dry silicon dioxide, using a slight excess of silicon dioxide to suppress formation of magnesium silicide.
2. In a refractory crucible with a fitted lid, gently preheat to minimize thermal shock.
3. Add the powder mixture to the hot crucible, cover, and place in a furnace or strong flame until the reaction sustains and completes.
4. Allow the crucible to cool behind a heat shield; do not open while hot.
5. Break up the solid mass and transfer it to a beaker.
6. Carefully add dilute hydrochloric acid in small portions to dissolve magnesium oxide and any unreacted magnesium; allow gases to escape safely.
7. Decant and rinse repeatedly with water until neutral.
8. Dry the remaining dark solid; this is crude elemental silicon powder.

Silicon Synthesis (From Mg and Sand) - TheChemistryShack:

Silicon and silanes from sand - Royal Society of Chemistry:

• Test electrical properties of the product by measuring resistance of a compacted pellet.

• Conduct all heating steps in a fume hood or outdoors, wear eye protection, face shield, heat-resistant gloves, and a lab coat.
• Magnesium powder is highly flammable; keep away from ignition sources and do not use water on magnesium fires (use a Class D extinguisher or dry sand).
• The reduction is very exothermic and can spatter; use a lidded refractory crucible and a heat shield, and keep observers at a safe distance.
• Mixtures that form magnesium silicide can release silane and hydrogen on contact with acids or water; add acids slowly, in a fume hood, and vent gases safely.
• Dilute hydrochloric acid is corrosive; avoid skin and eye contact and neutralize spills promptly.
• Allow the crucible to cool fully before handling to prevent burns and thermal shock fractures.
• Dispose of all chemicals according to institutional hazardous waste procedures; solid residues containing silicon and silica should be treated as chemical waste.

• What is the balanced chemical equation for the main reduction? (2Mg + SiO2 → 2MgO + Si)
• Why is silicon dioxide used in slight excess in this procedure? (To minimize formation of magnesium silicide, Mg2Si, by consuming free magnesium.)
• How could you confirm that silicon formed rather than a mixture of unreacted sand and magnesium oxide? (Test solubility of MgO in acid, examine density differences, or analyze with conductivity or simple powder X-ray patterns if available.)
• What hazards arise if magnesium silicide forms and you add acid during the workup? (Mg2Si reacts with acids to generate silane and hydrogen, which are flammable and can ignite spontaneously.)