Magnesium Silicide

Magnesium silicide, Mg₂Si, is an inorganic compound consisting of magnesium and silicon. As a powder magnesium silicide is dark blue or slightly purple in color. Silicon dioxide, SiO₂, found in sand and glass, when heated with magnesium forms magnesium oxide, and, if an excess of magnesium is used, magnesium silicide is formed. The first product in this reaction is silicon, which then reacts further with magnesium to produce Mg₂Si. The stoichiometry of these reactions are such that with a 2:1 Mg:SiO₂ molar ratio, MgO is formed:

2 Mg + SiO₂ → 2 MgO + Si

If an excess of Mg is present, Mg₂Si is formed from the reaction of the remaining magnesium with the silicon via:

2 Mg + Si → Mg₂Si

Hence, the overall reaction for the formation of magnesium silicide from a 4:1 Mg:SiO₂ molar ratio may be represented as:

4 Mg + SiO₂ → 2 MgO + Mg₂Si

These reactions proceed violently, producing a great amount of heat.

Magnesium silicide can also be formed by the reaction between magnesium hydride and silicon with the evolution of hydrogen gas at temperatures above 250°C:

2 MgH₂ + Si → Mg₂Si + 2 H₂

This reaction releases 5 wt.% hydrogen and has been considered for hydrogen storage. However, its reversibility has yet to be demonstrated.

Magnesium silicide is used to create aluminium alloys of the 6000 series, containing up to approximately 1.5% Mg₂Si. An alloy of this group can be age-hardened to form Guinier-Preston zones and a very fine precipitate, both resulting in increased strength of the alloy.

When magnesium silicide is placed into hydrochloric acid, HCl(aq), the gas silane, SiH₄, is produced. This gas is the silicon analogue of methane, CH₄, but is more reactive. Silane is pyrophoric, that is, due to the presence of oxygen, it spontaneously combusts in air:

Mg₂Si(s) + 4 HCl(aq) → SiH₄(g) + 2 MgCl₂(s)

SiH₄ + 2 O₂ → SiO₂ + 2 H₂O

These reactions are typical of a Group 2 silicide. Mg₂Si reacts similarly with sulfuric acid. Group 1 silicides are even more reactive. For example, sodium silicide, Na₂Si, reacts rapidly with water to yield sodium silicate, Na₂SiO₃, and hydrogen gas.