Minerals and rocks: the structure of the silicate minerals based on the silica tetrahedron; the silicate groups (isolated, chain, sheet and framework silicates) and how the degree of polymerisation links to composition; the physical properties (colour, lustre, hardness, cleavage, fracture, streak, density and habit) used to identify the common rock-forming minerals quartz, feldspar, mica, olivine, pyroxene and amphibole in hand specimen.

What this dot point is asking

OCR wants you to describe how silicate minerals are built from the silica tetrahedron, classify them into the isolated, chain, sheet and framework groups using how many oxygen atoms are shared, link the degree of polymerisation to composition and properties, and identify the common rock-forming minerals (quartz, feldspar, mica, olivine, pyroxene and amphibole) in hand specimen using physical properties.

The answer

The silica tetrahedron

Almost all the minerals that make up the crust are silicates, built from a single repeating unit: the silica (or silicon-oxygen) tetrahedron. One small silicon atom sits at the centre, bonded to four oxygen atoms at the corners, giving the unit a charge of $\mathrm{SiO_4^{4-}}$.

The silicate groups

The groups are defined by how many of the four corner oxygens each tetrahedron shares with its neighbours. Sharing more oxygens (more polymerisation) means a higher proportion of silicon and oxygen and a more rigid structure.

• Isolated (nesosilicates). Tetrahedra share no oxygens; they are linked only by metal cations such as $\mathrm{Fe^{2+}}$ and $\mathrm{Mg^{2+}}$. Example: olivine. Lowest silica, highest iron and magnesium.
• Single-chain (inosilicates). Each tetrahedron shares two oxygens, forming chains. Example: pyroxene (for example augite).
• Double-chain (inosilicates). Two single chains link, sharing more oxygens. Example: amphibole (for example hornblende), which also contains hydroxyl ($\mathrm{OH^-}$).
• Sheet (phyllosilicates). Each tetrahedron shares three oxygens, forming continuous sheets, which is why these minerals have perfect basal cleavage. Example: mica (biotite and muscovite).
• Framework (tectosilicates). Every tetrahedron shares all four oxygens in a three-dimensional framework. Examples: quartz and feldspar. Highest silica, lowest iron and magnesium.

Moving from isolated to framework, the silica content rises, the iron and magnesium content falls, the colour lightens, and (in a melt) the viscosity and melting behaviour change. This is the link OCR draws to igneous rocks: ultrabasic rocks are dominated by isolated and chain silicates, acid rocks by framework silicates.

Physical properties for identification

You identify minerals in hand specimen using a fixed set of properties:

• Colour. Useful but not reliable on its own (quartz can be many colours).
• Lustre. How the surface reflects light: vitreous (glassy, for example quartz), metallic (for example pyrite), or earthy.
• Hardness. Resistance to scratching on Mohs' scale, tested against a fingernail (about $2.5$), a copper coin (about $3.5$) and a steel blade or glass (about $5.5$).
• Cleavage. Tendency to split along planes of weakness; record the number of directions and the angles between them. Contrast with fracture (irregular or conchoidal breakage when there is no cleavage).
• Streak. The colour of the powdered mineral on an unglazed tile.
• Density and habit (crystal shape) complete the picture.

Examples in context

Example 1. Granite versus peridotite. Granite is dominated by framework silicates (quartz and feldspar) plus some mica, so it is pale and silica-rich. Peridotite is dominated by isolated and chain silicates (olivine and pyroxene), so it is dark and silica-poor. The mineralogy alone tells you the silica content.

Example 2. Weathering resistance. Because quartz is a framework silicate with strong bonds in all directions, it is hard and chemically stable, so it survives weathering and dominates many sandstones, while olivine (isolated, weakly bonded) breaks down first.

Try this

Q1. State the formula of the silica tetrahedron and name the group of silicates in which every tetrahedron shares all four oxygens. [2 marks]

• Cue. $\mathrm{SiO_4^{4-}}$; the framework silicates (tectosilicates), for example quartz and feldspar.

Q2. Explain why mica has a perfect cleavage in one direction. [2 marks]

• Cue. Mica is a sheet silicate; the strong bonds lie within the silicate sheets, while the bonds between the sheets are weak, so the mineral splits easily parallel to the sheets.

Q3. Give two physical properties that distinguish quartz from feldspar in hand specimen. [2 marks]

• Cue. Quartz has no cleavage and a conchoidal fracture and a hardness of $7$; feldspar has two cleavages at about $90^{\circ}$ and is slightly softer (hardness about $6$).