What is Quartz?
Quartz is crystalline silicon dioxide (SiO₂). It is the second most common mineral in the Earth’s continental crust (after feldspar) and one of the most widely distributed minerals on the planet. You encounter quartz constantly without realising it — in granite countertops, beach sand, glass, electronics, and most everyday rocks.
Key Idea: Quartz = pure SiO₂. Its incredible abundance is a consequence of silicon and oxygen being the two most common elements in the Earth’s crust.
Quartz belongs to the trigonal crystal system (often described as hexagonal) and typically forms well-developed six-sided prisms capped with rhombohedral faces. The crystals are often transparent to translucent, glassy in lustre, and colourless in their purest form — though impurities create a wide range of coloured varieties.
Physical Properties
The physical properties of quartz are consistent and make it one of the easiest minerals to identify:
| Property | Value |
|---|---|
| Hardness | 7 (Mohs scale) — scratches glass, not scratched by a knife |
| Lustre | Vitreous (glassy) |
| Colour | Colourless, white, grey; purple (amethyst), pink (rose quartz), yellow (citrine) with impurities |
| Crystal form | Hexagonal prisms with pyramidal terminations |
| Fracture | Conchoidal (smooth, curved breakage like glass) |
| Cleavage | None — fractures rather than cleaving |
| Streak | White |
| Specific gravity | 2.65 g/cm³ |
Key Idea: No cleavage + conchoidal fracture + hardness 7 + glassy lustre = almost certainly quartz. This combination is diagnostic.
Crystal Chemistry
Quartz consists of a continuous three-dimensional framework of SiO₄ tetrahedra sharing all four oxygen atoms with neighbouring tetrahedra — a framework silicate. This arrangement creates an extremely rigid, hard structure.
Every silicon atom is surrounded by four oxygens; every oxygen bridges two silicon atoms. The resulting atomic ratio is exactly 1 Si : 2 O, hence SiO₂.
Varieties of Quartz
Quartz’s susceptibility to impurities and micro-inclusions produces visually distinct varieties, all of which are chemically identical SiO₂:
- Amethyst — purple; caused by iron impurities and natural irradiation
- Rose quartz — pink; caused by titanium or manganese traces
- Smoky quartz — brown/grey; natural irradiation of aluminium-substituted sites
- Citrine — yellow-orange; iron impurities (rare in nature; most sold citrine is heat-treated amethyst)
- Milky quartz — white; fluid inclusions scattering light
- Chalcedony/Chert/Flint — cryptocrystalline varieties (microscopic crystal size) formed from silica-rich fluids
Geological Occurrence
Quartz is found in almost every geological environment:
- Igneous rocks — abundant in granites, rhyolites, and other felsic rocks as a late-crystallising mineral
- Sedimentary rocks — quartz is mechanically and chemically resistant, so it accumulates as sand and sandstone; cemented quartz grains form orthoquartzite
- Metamorphic rocks — quartz recrystallises easily; quartzite is the metamorphic equivalent of sandstone
- Hydrothermal veins — silica-rich fluids precipitate quartz in fractures; often associated with gold and other ore minerals
Key Idea: Quartz’s resistance to chemical weathering is why beaches are mostly sand (= quartz). Other minerals break down; quartz endures.
Key Terms
- Silica — SiO₂; the chemical compound forming quartz
- SiO₂ — silicon dioxide; the chemical formula of quartz
- Mohs hardness — a scale of 1–10 measuring mineral hardness by scratch resistance; quartz = 7
- Crystal system — the geometric symmetry of a crystal’s internal structure
- Conchoidal fracture — smooth, curved fracture pattern (like broken glass)
- Cleavage — tendency to split along flat planes; quartz has none
- Polymorphs — same chemistry, different crystal structure (e.g., coesite, stishovite, cristobalite are SiO₂ polymorphs)
- Rock-forming mineral — minerals that make up the bulk of common rocks
- Piezoelectricity — electric charge generated by mechanical stress; quartz crystals used in watches and electronics