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Silicon is the eighth most common element in the universe by mass, but very rarely occurs as the pure element in the Earth's crust. It is most widely distributed in dusts, sands, planetoids, and planets as various forms of silicon dioxide (silica) or silicates.
This is important for Si in solar cells for instance. Silicon has an indirect band gap, which is less efficient than materials with direct band gaps. But because Si is so cheap and abundant, it is still the dominant material used in making solar cells.