Last reviewed 16 Sept 2026 · 8 min read
Geology in civil engineering
Engineering geology applies geological knowledge to the planning, design, construction and maintenance of engineering works. It helps in:
- Selecting sites for dams, tunnels, bridges, roads and buildings.
- Assessing foundation conditions, slope stability, groundwater and seismic hazards.
- Choosing construction materials — building stones, aggregates.
- Avoiding hazards such as faults, landslides, cavities and swelling clays.
What is a mineral?
A mineral is a naturally occurring, inorganic, homogeneous solid with a definite chemical composition (or range) and an ordered internal atomic structure (crystalline).
- Rocks are aggregates of one or more minerals.
- About eight elements make up most of the earth's crust by mass — oxygen and silicon being the most abundant, followed by aluminium, iron, calcium, sodium, potassium and magnesium. Hence silicate minerals dominate.
Physical properties for identification
| Property | Meaning / examples |
|---|---|
| Form (habit) | External shape — crystalline, prismatic, tabular, fibrous (asbestos), granular, massive |
| Colour | Often unreliable because impurities change colour (e.g. quartz may be colourless, white, pink, smoky) |
| Streak | Colour of the powder, obtained by rubbing on unglazed porcelain — more reliable; e.g. hematite red-brown, magnetite black, pyrite greenish black, limonite yellowish brown |
| Lustre | Appearance of the surface in reflected light — metallic (pyrite, galena), vitreous (glassy) (quartz), pearly (talc, mica), silky (asbestos), resinous, earthy/dull (kaolin) |
| Hardness | Resistance to scratching — Mohs scale |
| Cleavage | Tendency to break along definite planes of weakness related to crystal structure — perfect basal (mica), two cleavages at about 90° (feldspar), rhombohedral (calcite) |
| Fracture | Irregular breaking where there is no cleavage — conchoidal (shell-like; quartz, glass), uneven, splintery, hackly |
| Specific gravity | Most rock-forming silicates 2.6–3.0; ore minerals heavier (magnetite about 5.2, hematite about 5.3) |
| Tenacity | Brittle, malleable, sectile, flexible (chlorite), elastic (mica) |
| Special properties | Magnetism (magnetite), effervescence with dilute HCl (calcite), taste (halite), feel (talc — greasy), double refraction (Iceland spar calcite) |
Mohs scale of hardness
| Hardness | Mineral | Hardness | Mineral |
|---|---|---|---|
| 1 | Talc | 6 | Orthoclase (feldspar) |
| 2 | Gypsum | 7 | Quartz |
| 3 | Calcite | 8 | Topaz |
| 4 | Fluorite | 9 | Corundum |
| 5 | Apatite | 10 | Diamond |
- Field tests: fingernail ≈ 2.5, copper coin ≈ 3, steel knife/window glass ≈ 5.5, steel file ≈ 6.5.
- The scale is relative (ordinal) — diamond is many times harder than corundum in absolute terms.
- Mnemonic: "The Girls Can Flirt And Other Queer Things Can Do".
Silicate structures
The basic unit is the SiO₄ tetrahedron.
| Structure | Arrangement | Examples |
|---|---|---|
| Nesosilicates (isolated) | Independent tetrahedra | Olivine, garnet |
| Inosilicates — single chain | Chains of tetrahedra | Pyroxenes (augite) |
| Inosilicates — double chain | Double chains | Amphiboles (hornblende) |
| Phyllosilicates (sheet) | Sheets | Micas, clay minerals, talc, chlorite |
| Tectosilicates (framework) | 3-D framework | Quartz, feldspars |
Common rock-forming minerals
| Mineral | Composition | Key identification features |
|---|---|---|
| Quartz | SiO₂ | H = 7; no cleavage; conchoidal fracture; vitreous; colourless/white/various; very resistant to weathering |
| Orthoclase (K-feldspar) | KAlSi₃O₈ | H = 6; two cleavages at about 90°; pink/white; weathers to kaolinite |
| Plagioclase feldspar | Na–Ca aluminosilicates | H = 6–6.5; white/grey; striations (twinning lines) on cleavage faces |
| Muscovite (white mica) | K–Al mica | H = 2–2.5; perfect basal cleavage into thin elastic sheets; pearly; colourless |
| Biotite (black mica) | K–Mg–Fe mica | Similar to muscovite but dark brown/black |
| Hornblende (amphibole) | Ca–Mg–Fe–Al silicate | H = 5–6; dark green/black; two cleavages at about 56° and 124°; elongated prisms |
| Augite (pyroxene) | Ca–Mg–Fe silicate | H = 5–6; dark green/black; two cleavages at nearly 90° (87° and 93°); stubby prisms |
| Olivine | (Mg,Fe)₂SiO₄ | H = 6.5–7; olive green, granular; vitreous; in basic/ultrabasic rocks; weathers easily |
| Calcite | CaCO₃ | H = 3; rhombohedral cleavage (three directions); effervesces briskly with cold dilute HCl |
| Dolomite | CaMg(CO₃)₂ | H = 3.5–4; effervesces only when powdered or with warm acid |
| Gypsum | CaSO₄·2H₂O | H = 2 (scratched by fingernail); perfect cleavage; used in cement |
| Kaolinite | Clay mineral | Soft, earthy, white; low swelling; from weathering of feldspars |
| Montmorillonite | Clay mineral (smectite) | High swelling and shrinkage; very low permeability; in black cotton soils and bentonite |
| Illite | Clay mineral | Intermediate properties |
| Garnet | Silicate (various) | H = 6.5–7.5; red/brown; equant crystals; in metamorphic rocks; used as abrasive |
| Chlorite | Sheet silicate | Green; flexible (not elastic) flakes; in low-grade metamorphic rocks |
| Talc | Mg silicate | H = 1; greasy/soapy feel |