Oxygen and silicon are the two most common elements in Earth's crust: by mass, about 47% oxygen and 28% silicon. In Atoms, Ions and Crystals you saw minerals as stacked ions. In the crust, the most important stack starts with one small building block.
Why it works
A silicon ion, $\text{Si}^{4+}$, is tiny; an oxygen ion, $\text{O}^{2-}$, is large. The best way to pack them is four oxygens around each silicon, at the corners of a tetrahedron (a pyramid with four triangular faces). Its charge is $+4 + 4(-2) = -4$, so a lone tetrahedron is written $\text{SiO}_4^{4-}$. Something must cancel that $-4$. There are two ways: surround the tetrahedra with positive ions (such as $\text{Mg}^{2+}$ and $\text{Fe}^{2+}$), or let neighboring tetrahedra share corner oxygens. A shared oxygen belongs half to each tetrahedron, so every shared corner raises the charge per silicon by 1.
Key idea
The silica tetrahedron, one $\text{Si}^{4+}$ bonded to four $\text{O}^{2-}$, has charge $-4$. Positive ions or shared oxygens balance it. Count charge as $+4$ per silicon and $-2$ per oxygen.
Worked examples
Worked example: How many magnesium ions?
How many $\text{Mg}^{2+}$ ions balance one $\text{SiO}_4^{4-}$ tetrahedron?
- The tetrahedron carries $-4$.
- Each $\text{Mg}^{2+}$ brings $+2$: $4 \div 2 = 2$ ions.
- That is forsterite, $\text{Mg}_2\text{SiO}_4$.
Answer: Two.
Worked example: A ring of six
Six tetrahedra join in a closed ring, each sharing two corners with its neighbors. How many oxygens does the ring have, and what is its charge?
- Six separate tetrahedra would have $6 \times 4 = 24$ oxygens.
- A ring of six has six links, and each link shares one oxygen: $24 - 6 = 18$. The group is $\text{Si}_6\text{O}_{18}$.
- Charge: $6(+4) + 18(-2) = 24 - 36 = -12$. Beryl, the mineral behind emerald, is built of such rings.
Answer: 18 oxygens; charge $-12$.
Common mistake
A frequent slip is to get the tetrahedron's charge wrong: writing $-2$ (oxygen's charge alone) or $+4$ (silicon's). Add all five ions: $+4 - 8 = -4$. And remember that shared oxygens are counted once for the pair, which is why linked silicates carry less charge per silicon. Check: a fully shared framework, $\text{SiO}_2$ (quartz), comes out to $+4 - 4 = 0$, neutral, needing no other ions.