How are silicate minerals built, and how do we classify the major mineral groups?
Silicate minerals and mineral classification: the silicon-oxygen tetrahedron as the building block of silicates; the polymerisation series from isolated tetrahedra (olivine) through chains (pyroxenes, amphiboles) and sheets (micas, clays) to frameworks (quartz, feldspars); and the classification of non-silicate minerals into carbonates, oxides, sulphides, halides and native elements.
A focused answer to the Eduqas Geology statement on silicate structures and mineral groups. Covers the silicon-oxygen tetrahedron, the polymerisation series from isolated tetrahedra to frameworks, the silicate families (olivine, pyroxenes, amphiboles, micas, feldspars, quartz), and the classification of carbonates, oxides, sulphides, halides and native elements.
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What this dot point is asking
Eduqas wants you to use the silicon-oxygen tetrahedron as the building block of the silicate minerals, to describe the polymerisation series from isolated tetrahedra (olivine) through chains and sheets to frameworks (quartz and feldspar), and to classify the main non-silicate groups (carbonates, oxides, sulphides, halides and native elements) by the anion or element that defines them. Silicates make up the bulk of igneous and metamorphic rocks, so this structure underpins igneous classification and Bowen's reaction series.
The answer
The silicon-oxygen tetrahedron
The fundamental unit of every silicate is the silicon-oxygen tetrahedron: one small silicon ion sitting at the centre of four oxygen ions arranged at the corners of a tetrahedron, written . The silicon-oxygen bond is strong (largely covalent), so the way these tetrahedra link together controls the whole structure. Tetrahedra can stay separate or polymerise by sharing corner oxygen atoms with their neighbours.
The polymerisation series
As tetrahedra share more oxygens, the structure changes systematically. This series is the spine of silicate mineralogy and runs parallel to Bowen's reaction series.
- Isolated (independent) tetrahedra: olivine. Tetrahedra are not joined to each other; they are held together by cations (magnesium and iron). Silicon-to-oxygen ratio . Few shared bonds, so olivine is the least stable at the surface and weathers easily.
- Single chains: pyroxenes (for example augite). Tetrahedra share two oxygens to form chains, with a silicon-to-oxygen ratio of .
- Double chains: amphiboles (for example hornblende). Two chains joined side by side; these minerals contain hydroxyl (OH) and have ratios around .
- Sheets: micas (biotite, muscovite) and clays. Tetrahedra share three oxygens to make continuous sheets (ratio ). Strong bonds within the sheets but weak bonds between them give the characteristic one perfect cleavage.
- Frameworks: quartz and the feldspars. Every oxygen is shared between two tetrahedra to make a continuous three-dimensional network. Quartz () has the ratio and is the most weathering-resistant of all.
The general trend: more sharing means a lower silicon-to-oxygen ratio, more strong bonds and greater resistance to weathering, which is why quartz survives to dominate mature sandstones while olivine does not.
The silicate families to know by name
- Olivine: green, isolated tetrahedra, early-crystallising, magnesium-iron rich.
- Pyroxene (augite): dark green to black, single chains, two cleavages near 90 degrees.
- Amphibole (hornblende): black, double chains, two cleavages near 120 and 60 degrees.
- Micas (biotite black, muscovite pale): sheets, one perfect cleavage into flakes.
- Feldspars (plagioclase and potassium feldspar): framework, two cleavages near 90 degrees, the most abundant minerals in the crust.
- Quartz: framework, no cleavage, conchoidal fracture, glassy and very hard.
Classifying the non-silicate minerals
The other major mineral groups are defined by the anion (or by being a pure element):
- Carbonates (carbonate ion): calcite and dolomite; the basis of limestone and marble, and they fizz in dilute acid.
- Oxides (oxide ion): magnetite (magnetic iron oxide) and hematite (the main iron ores).
- Sulphides (sulphide ion): pyrite (fool's gold), galena (lead ore), chalcopyrite (copper ore) and sphalerite (zinc ore); important metal ores with a metallic lustre.
- Halides (halogen anion): halite (sodium chloride) and fluorite; evaporite and vein minerals.
- Native elements (a single element): gold, copper and diamond (carbon).
Examples in context
Example 1. Why quartz survives and olivine does not. In a beach sand far from its source, almost all the grains are quartz. Its framework structure resists chemical weathering, while the isolated tetrahedra of olivine and the chains of pyroxene break down quickly, so they are lost during transport.
Example 2. Ore minerals as sulphides. Many of the world's lead, zinc and copper supplies come from the sulphides galena, sphalerite and chalcopyrite, which is why the sulphide group reappears in the economic geology topic.
Try this
Q1. Name the structural class (isolated, chain, sheet or framework) of olivine, pyroxene, mica and quartz. [2 marks]
- Cue. Olivine isolated, pyroxene single chain, mica sheet, quartz framework.
Q2. Explain why framework silicates such as quartz are more resistant to weathering than isolated-tetrahedron silicates such as olivine. [3 marks]
- Cue. Frameworks share every oxygen between two tetrahedra, giving many strong silicon-oxygen bonds throughout, whereas isolated tetrahedra are linked only by weaker cation bonds, so they break down more easily.
Q3. State the defining anion (or element) of the carbonate, sulphide and halide groups, with one named mineral of each. [3 marks]
- Cue. Carbonate ion (calcite), sulphide ion (galena), halogen anion such as chloride (halite).
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20186 marksDescribe how the degree of polymerisation of the silicon-oxygen tetrahedron varies from olivine to quartz, and explain how this affects the silicon-to-oxygen ratio and the resistance of the minerals to weathering.Show worked answer →
A levels-of-response answer; move through the polymerisation series, then link it to the ratio and to weathering.
- The building block
- Every silicate is built from the silicon-oxygen tetrahedron, one silicon atom surrounded by four oxygen atoms.
- The series
- In olivine the tetrahedra are isolated (independent), joined only through cations such as magnesium and iron. In pyroxenes they form single chains, in amphiboles double chains, in micas continuous sheets, and in quartz and the feldspars a continuous three-dimensional framework where every oxygen is shared between two tetrahedra.
- The silicon-to-oxygen ratio
- As more oxygens are shared, fewer oxygens belong to each silicon, so the ratio rises from in isolated olivine towards in framework quartz ().
- Weathering resistance
- More sharing means more strong silicon-oxygen bonds holding the structure together, so framework silicates (quartz) are very resistant to weathering, while isolated-tetrahedron olivine, with the fewest shared bonds, weathers most easily. This is why quartz survives to dominate mature sandstones.
Top-band answers give the full series (isolated, single chain, double chain, sheet, framework) with examples, the change in the silicon-to-oxygen ratio, and the link to weathering resistance.
Eduqas 20224 marksName the mineral group to which each of the following belongs and give the chemical basis of the group: calcite, galena, halite and magnetite.Show worked answer →
Match each mineral to its group and the anion or element that defines it.
- Calcite
- A carbonate; the group is based on the carbonate ion (calcite is calcium carbonate, with the carbonate anion).
- Galena
- A sulphide; the group is based on the sulphide ion (galena is lead sulphide, the main lead ore).
- Halite
- A halide; the group is based on a halogen anion (halite is sodium chloride, with the chloride ion).
- Magnetite
- An oxide; the group is based on the oxide ion (magnetite is an iron oxide, strongly magnetic).
Markers reward the correct group for each, identified by its defining anion (carbonate, sulphide, halide or oxide) rather than just the chemical name.
Related dot points
- Elements, atomic structure and bonding: the major rock-forming elements; atomic structure (protons, neutrons and electrons) and isotopes; ionic, covalent and metallic bonding; how the type of bonding and the arrangement of atoms control the physical properties of minerals such as hardness and cleavage.
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- Mineral and rock tests and field skills: the practical tests used to identify minerals and rocks (hardness, acid, magnet, streak, density); the recording of field observations through field sketches, annotated specimens and sampling; and the fieldwork requirement (a minimum of four days for the A-level) and how practical and fieldwork skills are assessed within the written components.
A focused answer to the Eduqas Geology practical and fieldwork statement. Covers the practical tests for identifying minerals and rocks, recording with field sketches and sampling, the four-day fieldwork requirement, and how Eduqas assesses practical and fieldwork skills within the written components rather than a separate endorsement.
- Magma differentiation and Bowen's reaction series: the order of crystallisation of silicate minerals from a cooling magma (the discontinuous ferromagnesian branch olivine to pyroxene to amphibole to biotite, and the continuous plagioclase branch from calcium-rich to sodium-rich, then potassium feldspar, muscovite and quartz); fractional crystallisation and partial melting; and how differentiation evolves a magma from mafic to felsic.
A focused answer to the Eduqas Geology statement on magma differentiation. Covers Bowen's reaction series (the discontinuous ferromagnesian branch and the continuous plagioclase branch), fractional crystallisation and partial melting, the order of crystallisation, and how differentiation evolves a magma from mafic to felsic compositions.
- Igneous rock classification and textures: the classification of igneous rocks by silica content and composition (ultramafic peridotite, mafic basalt and gabbro, intermediate andesite and diorite, felsic rhyolite and granite) and by grain size and cooling rate (glassy, aphanitic, phaneritic, porphyritic, vesicular and pyroclastic textures); and the relationship between cooling rate and crystal size.
A focused answer to the Eduqas Geology statement on igneous rock classification. Covers the compositional series from ultramafic peridotite through mafic basalt and gabbro and intermediate andesite and diorite to felsic rhyolite and granite, the link between cooling rate and crystal size, and the named textures (glassy, aphanitic, phaneritic, porphyritic, vesicular and pyroclastic).
Sources & how we know this
- Eduqas A Level Geology Specification (A220QS) — Eduqas (2017)