Magnesium Silicide, Mg₂Si

Crude silicides are obtained by heating magnesium strongly with silica or silicates. Silicon and magnesium unite directly when they are heated in a current of hydrogen, and their freezing-point curve indicates the compound Mg₂Si.

Mg₂Si is left as brilliant slate-blue octahedra when magnesium is dissolved away from an alloy containing 25 per cent, of silicon by ethyl iodide and ether. The alloy is prepared by heating together fragments of magnesium, magnesium filings, and potassium silicofluoride. Other alloys of silicon and magnesium also contain crystals of the silicide.

It slowly decomposes water at ordinary temperatures - -evolving hydrogen. Cold hydrochloric acid attacks it violently with the evolution of hydrogen and spontaneously inflammable silicon hydride. Concentrated sulphuric or nitric acid attacks it slowly; the dilute acid acts more rapidly. It dissociates completely at 1100°-1200° C. in vacuo or in hydrogen. It precipitates many heavy metals from their salts, and even decomposes solutions of chlorides of the alkalies.