The abundance of elements in the crust, the silicon-oxygen tetrahedron as the building block of the silicate minerals, and how the degree of tetrahedral linkage (isolated, chain, sheet and framework) controls cleavage, hardness and density.

What this dot point is asking

WJEC wants you to know which elements make up the bulk of the crust, to identify the silicon-oxygen tetrahedron as the fundamental unit of the silicate minerals, and to explain how the way these tetrahedra link together controls the physical properties (cleavage, hardness and density) of the common rock-forming silicates. This is the structural reasoning that underpins mineral identification and the order of crystallisation in Bowen's reaction series.

The answer

The composition of the crust

Just eight elements make up almost 99 percent of the crust by mass. Oxygen (about 47 percent) and silicon (about 28 percent) dominate, followed by aluminium, iron, calcium, sodium, potassium and magnesium. Because oxygen and silicon are so abundant, the great majority of rock-forming minerals are silicates, built from silicon and oxygen together with the metal cations.

The silicon-oxygen tetrahedron

The building block of every silicate is the silicon-oxygen tetrahedron: one small silicon ion at the centre surrounded by four oxygen ions at the corners, written $\text{SiO}_4$. The silicon-oxygen bonds are strong and largely covalent. Tetrahedra can share corner oxygen atoms with neighbouring tetrahedra, and the degree of sharing defines the structural family of the silicate and, with it, the physical properties.

The structural families and their properties

Isolated (island) silicates

Each tetrahedron shares no oxygen with another; separate tetrahedra are held together by metal cations. Olivine is the example. The fairly uniform ionic bonding through the cations gives poor cleavage, high density (olivine is iron and magnesium rich) and an early, high-temperature crystallisation.

Single-chain silicates

Tetrahedra share two oxygen atoms to build continuous chains. The pyroxenes (such as augite) are the example, with two cleavages meeting at about 90 degrees.

Double-chain silicates

Two chains link side by side. The amphiboles (such as hornblende) are the example, with two cleavages meeting at about 120 and 60 degrees, a useful way to tell hornblende from augite.

Sheet silicates

Tetrahedra share three oxygen atoms, building continuous two-dimensional sheets. The micas (muscovite and biotite) and the clay minerals are the examples. Strong bonding within the sheets and weak bonding between them give one perfect cleavage and thin, peeling flakes.

Framework silicates

Tetrahedra share all four oxygen atoms, building a three-dimensional network. Quartz (pure $\text{SiO}_2$) and the feldspars are the examples. Quartz has uniform strong bonding in all directions, so it has no cleavage, breaks conchoidally and is hard; the feldspars include aluminium and cations and so develop two cleavages near 90 degrees.

Why this controls everything downstream

The degree of polymerisation is not a detail; it sets the order in which minerals crystallise from a melt (the isolated, iron and magnesium rich silicates first, the framework silicates last), it controls density (used to model the layered Earth) and it controls how a mineral cleaves and weathers.

Examples in context

Density and the layered Earth. The dense, iron and magnesium rich isolated silicates such as olivine dominate the mantle, while the lighter framework silicates concentrate in the crust, an idea built directly on the structure-density link. Weathering of feldspar to clay. Framework feldspar breaks down chemically to sheet-silicate clay minerals, a structural rearrangement that supplies the mud that becomes shale. Asbestos and amphibole structure. The fibrous habit of some amphiboles, prized then feared as asbestos, comes from their chain structure splitting into needles, a hazard rooted in silicate bonding.

Try this

Q1. Name the structural silicate family of quartz and state its silicon-to-oxygen ratio. [2 marks]

• Cue. Framework silicate; silicon to oxygen is 1:2 ($\text{SiO}_2$) because all four oxygen atoms are shared.

Q2. Hornblende and augite both look dark green to black. State one structural property that distinguishes them. [1 mark]

• Cue. Hornblende (amphibole, double-chain) cleaves at about 120 and 60 degrees; augite (pyroxene, single-chain) cleaves at about 90 degrees.

Q3. Explain why olivine is denser than quartz. [2 marks]

• Cue. Olivine is an isolated silicate rich in heavy iron and magnesium cations between the tetrahedra, whereas quartz is a framework of light silicon and oxygen only, so olivine has the greater density.