How does a silicon-oxygen tetrahedron function to attract other elements in its effort to form silicate minerals?

Question

It must be a bit detailed, please.

Answer 1

The silica tetrahedron is a marvelous building block for silicate minerals... a strong structure with strong charges on the edges for bonding. If you look at it as a silicon atom in the middle with four oxygen atoms bonded covalently, it's a very strong structure with a -4 net charge (the Si is +4 and each O atom is -2, so the net charge is -4, with the formula written as SiO₄^-4). One of the common minerals created from this silica tetrahedron is where every tetrahedron is bonded to another silica tetrahedron by sharing oxygen atoms between them, making the mineral quartz (formula SiO₂). Why is quartz SiO₂ and not SiO₄? Because each of the four oxygen atoms of one tetrahedron is also the oxygen atom of another tetrahedron... each oxygen is bonded to two silicon atoms (look at a diagram of this in your textbook to see). So the net number of oxygens for each silicon atom is 2, because it gets 1/2 of each of the 4 oxygen atoms, and quartz is SiO₂.

It gets more fun after that, because most other common silicate minerals involve alumina tetrahedrons as well. Aluminum atoms are nearly the same size as silicon atoms, but they have a different valence charge (+3 instead of +4), but because they're about the same size (and common in our crust) the aluminum and fit between four oxygens in just the same way that silicon does, forming a tetrahedron. Where a mineral forms with a mix of silica and alumina tetrahedrons, plus a cation or two like Na⁺¹ or Ca⁺² or K⁺¹, we get the feldspar family started.

That's just a start of course, and despite the plusses and minuses up there, it didn't really get into the chemistry of exactly how this all works. But the concept is there... the tetrahedrons are quite strong building blocks that easily bond with a variety of other elements in various combinations to make an amazing array of silicate minerals.