Silicates make up over 90% of the crust, but the rest include the minerals we mine most: metal ores, salt, limestone, gypsum. Like every mineral, each is built of positive and negative ions. Geologists sort them by their negative ion, because minerals that share one tend to share a structure and behave alike.
| Group | Negative ion | Examples |
|---|---|---|
| Carbonates | $\text{CO}_3^{2-}$ | calcite $\text{CaCO}_3$, dolomite $\text{CaMg(CO}_3)_2$ |
| Oxides | $\text{O}^{2-}$ (no silicon) | hematite $\text{Fe}_2\text{O}_3$, magnetite $\text{Fe}_3\text{O}_4$, corundum $\text{Al}_2\text{O}_3$ |
| Sulfides | $\text{S}^{2-}$ (sulfur, no oxygen) | pyrite $\text{FeS}_2$, galena $\text{PbS}$, sphalerite $\text{ZnS}$ |
| Sulfates | $\text{SO}_4^{2-}$ | gypsum $\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$, barite $\text{BaSO}_4$ |
| Halides | $\text{Cl}^{-}$, $\text{F}^{-}$ | halite $\text{NaCl}$, fluorite $\text{CaF}_2$ |
| Native elements | none: one element | gold Au, copper Cu, graphite and diamond C |
Why it works
The negative ion sets a mineral's chemistry. Carbonates all fizz in acid, because $\text{CO}_3^{2-}$ reacts with $\text{H}^{+}$ to release carbon dioxide gas. Sulfides form where there is little oxygen, often from hot fluids deep underground, and many are metal ores. Sulfates and halides crystallize as salty water evaporates. To write a formula, balance the charges: $\text{Fe}^{3+}$ and $\text{O}^{2-}$ need two irons for three oxygens, $\text{Fe}_2\text{O}_3$.
Key idea
Non-silicates are grouped by their negative ion: carbonates, oxides, sulfides, sulfates, halides and native elements. "-ide" means the bare element (sulfide, S); "-ate" means it comes with oxygen (sulfate, $\text{SO}_4$).
Worked examples
Worked example: Which group?
Barite is $\text{BaSO}_4$. Which group?
- Its negative ion is $\text{SO}_4^{2-}$: sulfur with oxygen.
- So barite is a sulfate.
Answer: A sulfate.
Worked example: Write a formula
Fluorite is made of $\text{Ca}^{2+}$ and $\text{F}^{-}$ ions. Write its formula.
- One $\text{Ca}^{2+}$ brings $+2$; each $\text{F}^{-}$ brings $-1$.
- Two fluorides balance one calcium: $\text{CaF}_2$.
Answer: $\text{CaF}_2$.
Common mistake
Sulfides and sulfates are easy to mix up because their names differ by one syllable. Pyrite, $\text{FeS}_2$, is a sulfide: sulfur and no oxygen, a shiny brassy ore mineral. Gypsum, $\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$, is a sulfate: each sulfur carries four oxygens, and it is soft enough to scratch with a fingernail. Check the formula for an O after the S.