What is a mineral, and how are minerals grouped?
A mineral is a natural solid with a fixed chemical make-up and atoms arranged in a regular pattern (a crystal). Rocks are mixtures of minerals.
Scientists sort minerals into groups by their chemistry (which negative ion or group they contain) and by their structure:
- Silicates: SiO₄ groups (quartz, feldspar, mica). The biggest group.
- Carbonates: CO₃ (calcite in limestone and marble).
- Oxides: metal + oxygen (hematite, magnetite: iron ores).
- Sulfides: metal + sulfur (pyrite, galena).
- Sulfates: SO₄ (gypsum).
- Halides: chlorine or fluorine (halite = rock salt, fluorite).
- Native elements: one element alone (gold, copper, diamond, graphite).
The SiO₄ tetrahedron: the brick of the crust
The two most common elements in the Earth's crust are oxygen (about 46% by mass) and silicon (about 28%). They join to make the silicon–oxygen tetrahedron, SiO₄.
A tetrahedron is a pyramid with four triangle faces. Silicon sits at the centre. Oxygen atoms sit at the four corners. The group has a charge of −4, so it must be balanced by metal ions (like Mg²⁺, Fe²⁺, Ca²⁺, Na⁺, K⁺, Al³⁺) or by sharing oxygens with other tetrahedra.
When two tetrahedra share a corner oxygen, that oxygen counts for both. This sharing is called polymerisation.
Types of silicate structures
Isolated (nesosilicates)
No shared oxygens. Si:O = 1:4. Example: olivine (Mg,Fe)₂SiO₄, garnet. Hard, grainy crystals, no good cleavage.
Single chains (inosilicates)
Each tetrahedron shares 2 oxygens. Si:O = 1:3. Example: pyroxene (augite). Two cleavages at about 90°.
Double chains
Two chains linked side by side. Si:O = 4:11. Example: amphibole (hornblende). Cleavages at about 60° and 120°.
Sheets (phyllosilicates)
Each tetrahedron shares 3 oxygens. Si:O = 2:5. Examples: mica (muscovite, biotite), talc, clay minerals. Perfect cleavage in one direction: it peels into flakes.
Frameworks (tectosilicates)
All 4 oxygens shared. Si:O = 1:2 (SiO₂). Examples: quartz and feldspar. Strong in every direction; quartz has no cleavage and is very hard (7 on the Mohs scale).
There are also ring silicates (cyclosilicates) like beryl and tourmaline, where tetrahedra join in closed rings.
How structure decides properties
- Cleavage: a mineral breaks where bonds are weakest. In mica the sheets are held by weak bonds, so it splits into sheets. In quartz bonds are equally strong everywhere, so it breaks with a curved (conchoidal) fracture.
- Melting point: isolated silicates like olivine crystallise first from hot magma (high temperature); framework quartz crystallises last (lower temperature).
- Weathering: minerals that formed at high temperature (olivine) weather fastest at the surface; quartz is the most resistant, which is why sand is mostly quartz.
- Ferromagnesian silicates (with Fe and Mg: olivine, pyroxene, amphibole, biotite) are dark and dense. Non-ferromagnesian ones (quartz, feldspar, muscovite) are light-coloured.
Building blocks of crust and life: silicon vs carbon
The crust is built from repeating silicon–oxygen units. Living things are built from repeating units too, but based on carbon: proteins are chains of amino acids, and DNA is a chain of nucleotides.
Both silicon and carbon can form four bonds, so both can make long chains and networks. Carbon bonds are more varied and can break and re-form easily in water, which suits life. Silicon–oxygen bonds are very strong and stable, which suits hard rock.
Try it: build silicates with paper
Cut 6 small paper triangles and fold each into a pyramid (or use 4 balls of clay with a toothpick in the middle). Mark the corners as oxygen. Now: (1) keep them apart; (2) join them corner to corner in a line; (3) join them into a flat sheet. Count shared corners each time and match it with the 3D scene.
Key formulas and definitions
- Silicon–oxygen tetrahedron: SiO₄ (charge −4)
- Isolated: 0 shared O, Si:O = 1:4 (olivine)
- Single chain: 2 shared O, Si:O = 1:3 (pyroxene)
- Double chain: Si:O = 4:11 (amphibole)
- Sheet: 3 shared O, Si:O = 2:5 (mica, clay)
- Framework: 4 shared O, Si:O = 1:2 (quartz SiO₂, feldspar)
- Mineral groups: silicates, carbonates, oxides, sulfides, sulfates, halides, native elements
Worked examples
1. Why does mica peel into thin sheets but quartz does not?
Mica is a sheet silicate: strong bonds inside each sheet, weak bonds between sheets, so it splits along the sheets. Quartz is a framework: every tetrahedron shares all 4 oxygens, so bonds are strong in every direction and there is no easy splitting plane.
2. Work out the Si:O ratio in a single chain.
Each tetrahedron has 1 Si. It owns 2 unshared oxygens fully and 2 shared oxygens half each: 2 + 2 × ½ = 3 oxygens. So Si:O = 1:3, written SiO₃.
3. Work out the Si:O ratio in a framework.
All 4 oxygens are shared between 2 tetrahedra, so each counts as ½: 4 × ½ = 2. Si:O = 1:2, the formula of quartz, SiO₂.
4. Sort into groups: calcite, halite, pyrite, hematite, feldspar, gold.
Calcite → carbonate; halite → halide; pyrite → sulfide; hematite → oxide; feldspar → silicate; gold → native element.
Common mistakes
- Thinking silicon and silica are the same. Silicon is an element (Si); silica is SiO₂, the compound in quartz.
- Counting a shared oxygen fully for both tetrahedra. A shared oxygen counts as ½ for each.
- Saying all silicates are hard. Talc is a sheet silicate and is the softest mineral (1 on the Mohs scale).
- Calling all shiny minerals metals. Mica is shiny but it is a silicate, not a metal.