Magnesium silicide, Mg₂Si, is an inorganic compound consisting of magnesium and silicon. Silicon dioxide, SiO₂, found in sand and glass, when heated with magnesium forms magnesium silicide. The first product in this reaction is silicon, which then reacts further with magnesium to produce Mg₂Si:

SiO₂ + 2Mg → 2MgO + Si

Si + 2Mg → Mg₂Si

Magnesium silicide is used to create aluminum alloys of the 6xxx group, 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.^[1]

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) + 4HCl(aq) → SiH₄(g) + 2MgCl₂(s)

SiH₄ + 2O₂ → SiO₂ + 2H₂O

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.

References

[1] ASM Handbook, 10th Ed., Vol. 1, Properties and Selection: Non-ferrous Alloys and Special Purpose Materials, 1990, ASM International, Materials Park, Ohio.