OK here is the procedure I used, but before I start I want to warn everyone that this is a dangerous reaction and should not be attempted by anyone without some serious forethought and hazard evaluation. It is not a reaction to be attempted by anyone without experience and chemistry knowledge.
I took some silicon dioxide, mixed it with magnesium turnings, heated it with a roaring blue bunsen flame until the magnesium reacted with the silicon dioxide (and the glass of the test-tube, since glass is essentially silicon dioxide too). I then allowed the resulting mixture to cool down somewhat before dumping it into 1 Molar hydrochloric acid.
The reactions are multiple. basically the test-tube contains leftover magnesium and silicon dioxide, also magnesium silicide, silicon and magnesium oxide.
the silicon dioxide and magnesium oxide both just dissolve. The magnesium reacts with the acid to form hydrogen which bubbles to the surface. The magnesium silicide reacts with the acid to form silane, which spontaneously ignites on contact with air, igniting the hydrogen. The silicon falls to the bottom, essentially pure.
I intend to try repeating the experiment with powdered magnesium which should make it faster and more spectacular. Perhaps even a fireball.
