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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.
Naturally occurring silicon is composed of three stable isotopes, 28 Si (92.23%), 29 Si (4.67%), and 30 Si (3.10%), with 28 Si being the most abundant. Out of these, only 29 Si is of use in NMR and EPR spectroscopy, as it is the only one with a nuclear spin (I = 1 / 2).