Silicate Minerals: Earth's Most Common Minerals
One tiny building block — the silica tetrahedron — is stacked five different ways to make roughly nine of every ten minerals in Earth's crust. Learn the five ways and you can read most rocks.
Pick up almost any rock in your garden and you are holding silicates. Silicon and oxygen are the two most abundant elements in Earth's crust, and they team up into a single elegant shape — the silica tetrahedron — that geologists call the building block of most of the planet. The amazing part: by linking those tetrahedra in just five patterns, nature builds everything from the green gem olivine to the glassy quartz in sand.
The brick that built the crust
Oxygen and silicon are the two most common elements in Earth's crust — together about three quarters of it by mass. Put them together and you almost always get the same shape: one small silicon atom nestled at the centre of four larger oxygens, a pyramid called the silica tetrahedron. Roughly 90% of the crust is made of minerals built from this one unit, which is why the silicates get a whole lesson to themselves.
Sharing oxygens builds the five classes
Two tetrahedra can get rid of charge cheaply by sharing a corner — one oxygen, bonded to two silicons, counts for both. The more corners a tetrahedron shares, the more its charge is internally satisfied and the lower its oxygen-to-silicon ratio falls. The number of shared corners sorts the silicates into five families:
- 0 shared — isolated tetrahedra (O:Si = 4)
- 2 shared — single chains (O:Si = 3)
- ~2.5 shared — double chains (O:Si = 2.75)
- 3 shared — sheets (O:Si = 2.5)
- 4 shared — framework (O:Si = 2)
Let's meet each one, then put numbers on that ratio.
Isolated tetrahedra — olivine
When tetrahedra share no corners, each stands alone, bonded to cations packed around it. These nesosilicates (Greek nesos, island) include olivine — the green (Mg,Fe)₂SiO₄ gem — and garnet. Isolated tetrahedra are dense and tough, with no preferred weakness, so olivine has no cleavage and weathers slowly. It is a major mineral of the mantle and of dark igneous rocks.
Single chains — the pyroxenes
Share two corners per tetrahedron and you get a long single chain — a one-dimensional polymer. Each chain is surrounded by cations, and the bonds between chains are weaker than those within, so these pyroxenes (e.g. augite, enstatite) cleave in two directions at nearly 90°. Pyroxenes are common in basalt and gabbro.
Double chains — the amphiboles
Link two single chains side by side and you get a double chain. These amphiboles (e.g. hornblende) still cleave in two directions, but at about 60°/120° — the classic way to tell an amphibole from a pyroxene in hand specimen. Amphiboles often carry an OH group (they are hydrous), a clue we will reuse in metamorphic rocks.
Sheets — the micas
Share three corners and the tetrahedra spread into a flat sheet. Stack sheets with cations (and often water or OH) between them, and you get the micas — muscovite (clear) and biotite (black) — and the clays. Because the bonds between sheets are weak, micas peel off in perfect, paper-thin flakes: one perfect direction of cleavage. That flakiness is what makes clay soils slippery and micas sparkle.
Framework — quartz and feldspar
Share all four corners and the tetrahedra link into a full three-dimensional framework, every oxygen bridging two silicons. With no leftover charge and no weak layer, quartz (SiO₂) is hard, has no cleavage, and is the most chemically durable common mineral — which is why quartz grains survive weathering to become beach sand. Feldspar is a framework too, but with aluminium standing in for some silicon.
Putting numbers on the ratio
Here is the satisfying part. Each tetrahedron starts with 1 Si and 4 O. Every oxygen that is shared between two tetrahedra only counts as half toward each one. So the oxygen-to-silicon ratio is simply:
O:Si = (number of oxygens per tetrahedron, counting shared ones as ½) ÷ 1 Si
More sharing means a smaller ratio. Let's compute it.
- Start with 4 oxygens per tetrahedron: 2 are shared (bridging), 2 are not.
- Non-bridging oxygens count fully: 2 × 1 = 2.
- Shared oxygens count half each: 2 × ½ = 1.
- Total O per Si = 2 + 1 = 3. So O:Si = 3.
- Zero shared oxygens, so all 4 count fully: 4 × 1 = 4.
- O:Si = 4, written SiO₄ — the basic tetrahedron, as in olivine (Mg,Fe)₂SiO₄.
- All 4 oxygens are shared, so each counts as half: 4 × ½ = 2.
- O:Si = 2, written SiO₂ — exactly the formula of quartz.
Check your understanding
- Every silicate is built from the silica tetrahedron [SiO₄]⁴⁻ — one silicon surrounded by four oxygens, carrying a 4− charge.
- To balance that charge, tetrahedra bond to cations or share oxygens with each other; the pattern of sharing sorts silicates into five classes.
- The five classes are isolated (olivine), single chain (pyroxenes/augite), double chain (amphiboles/hornblende), sheet (micas/muscovite), and framework (quartz, feldspar).
- More oxygen sharing means a lower O:Si ratio: isolated = 4, single chain = 3, double chain = 2.75, sheet = 2.5, framework = 2.
- Feldspar is the most abundant crust mineral; it is a framework silicate that substitutes Al for some Si and balances the charge with K, Na, or Ca.
🎓 Go deeper: university courses & trusted references
Handpicked external material for this module — for when you want the full university treatment of minerals.
- Structure of Earth Materials (12.108) — MIT OpenCourseWare
- An Introduction to Minerals and Rocks under the Microscope — OpenLearn (The Open University)
- Department of Earth Sciences — University of Cambridge
- School of Earth Sciences — University of Bristol
- Division of Geological & Planetary Sciences — Caltech
External sites are listed for reference only. This course is independent and has no affiliation with, or endorsement from, the institutions named.