Characteristics: Dark colored crystals (the octahedral form in which the atoms have the diamond arrangement). The amorphous form is a dark brown powder. Soluble in a mixture of nitric and hydrofluoric acids and in alkalies. Insoluble in water, nitric and hydrochloric acid. Mohs hardness 7, dielectric constant 12, coordination number 6. Si atom is surrounded by 4 oxygen atoms. In this respect it is similar to carbon, except that it does not form compounds having double or triple bonds, and is less electronegative.
Hazards: Flammable in powder form.