Quartz
The second most abundant mineral in the Earth's continental crust, and the reference point for hardness 7 on the Mohs scale.
Quartz is silicon dioxide in its commonest crystalline form, and it is nearly everywhere: it makes up the bulk of most sand, a large fraction of granite, and a significant part of the continental crust.
Its practical importance rests on three things at once. It is hard enough to resist wear, chemically inert enough to survive weathering that destroys other minerals, and piezoelectric — it generates a voltage when squeezed, and vibrates at a very precise frequency when a voltage is applied. That last property is why nearly every clock and radio contains a quartz crystal.
Why it behaves as it does
Quartz is hard because silicon and oxygen are bonded to each other in every direction, with nothing weaker in between. Each silicon atom sits at the centre of four oxygens and each oxygen is shared between two silicons, so the whole crystal is a single continuous framework of strong covalent bonds rather than a stack of units held together by something else. Scratching it means breaking those bonds, which is why it sits at 7 on the Mohs scale and scratches window glass, steel and every common mineral below it.
The absence of a weak direction is as important as the strength of the bonds. Mica has bonds just as strong within its sheets and peels apart between them; calcite cleaves cleanly in three directions. Quartz has no cleavage at all — it fractures in curved conchoidal shells, because there is no plane through the structure that costs less to break than any other.
That combination is why quartz survives when the minerals around it do not. Granite weathering releases feldspar as clay and quartz as grains, and those grains are what sand is: the hard, chemically stable leftovers of rocks that have otherwise gone.
How to identify it
Quartz has no cleavage — it fractures conchoidally, in curved shell-like surfaces, rather than splitting along flat planes. Combined with a hardness of 7, which lets it scratch window glass and steel, and a glassy lustre, that is usually enough. Well-formed crystals are six-sided prisms ending in a six-sided point.
The commonest confusion is with calcite, which is far softer at hardness 3 and splits into rhombs, and with glass, which is softer still and often contains bubbles.
How it forms
Quartz crystallises late from cooling granitic magma, once most other minerals have taken what they need and silica is what remains. It also precipitates from hot water circulating through rock, which is what fills the veins that many well-formed crystals come from.
Name origin
From German Quarz, of uncertain earlier origin — possibly from a West Slavic word for hard.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
has variety
- Amethyst — mineral variety Wikidata
- Citrine — mineral variety · coloured yellow by iron, and most commercial material is amethyst heated to change the iron's oxidation state
- Rose quartz — mineral variety · coloured by microscopic fibrous inclusions of another mineral rather than by anything in the quartz itself
- Smoky quartz — mineral variety · coloured by natural radiation acting on aluminium substituted into the lattice — heat removes it and irradiation restores it
- Tiger's eye — mineral variety · quartz that replaced crocidolite fibre by fibre and kept the parallel structure, which is where the moving band of light comes from
- Agate — mineral variety · banded chalcedony — quartz in crystals too small to resolve, deposited in rhythm inside a cavity
- Carnelian — mineral variety · chalcedony coloured by iron oxide, and much of it heated agate — a treatment attested in the Indus Valley in the third millennium BC
- Jasper — mineral variety · chalcedony made opaque by fifteen to twenty per cent of other minerals, which are what give it its patterns
is a component of
- Granite — rock · 20–60% · by volume, alongside feldspar and lesser mica
- Soda-lime glass — material · 68–74% · as silica sand, the principal former
- Sandstone — rock · almost all sandstone grains are quartz, because quartz is what survives weathering
- Pegmatite — rock · 20–50% · one of the three essential minerals, as in the granite it derives from
- Banded iron formation — rock · 20–60% · as chert — the pale bands between the iron ones, and the material that weathering removes to leave an ore
- Greisen — rock · 30–60% · one of the two minerals the alteration leaves behind, feldspar having been destroyed
- Shale — rock · 15–40% · as silt-sized grains carried in with the mud
- Schist — rock · 20–50% · recrystallised from the silt in the parent mud, in lenses and bands between the mica
- Gneiss — rock · 20–45% · in the pale bands with the feldspar, separated from the dark minerals by metamorphic differentiation
- Flint — rock · 90–99% · silica in crystals too small to see, which is what gives it a conchoidal fracture and no grain to follow
- Porcelain — material · 20–30% · the filler that holds the shape while the feldspar is molten
- Epithermal vein — deposit type · the gangue that fills most of the vein, and the reason a quartz vein is what a prospector looks for
- Refractory brick — material · as silica brick, which is what made the Siemens-Martin open hearth possible and is destroyed by a basic slag
- Sand — material · 70–99% · almost everywhere, because quartz is what is left when everything else in a rock has weathered away — though a volcanic beach is basalt grains and a tropical one is broken shell
is a source for
- Silicon — element · reduced with carbon in an electric arc furnace
- Silicon carbide — compound · silica sand and carbon fused in an electric furnace
- Optical fibre — material · as silica, though not as sand: the glass is deposited from silicon tetrachloride vapour, because the transition metals that absorb light do not survive that route
- Borosilicate glass — material · as silica sand, still the bulk of the batch
- Fused silica — material · melted directly for the ordinary grades — and deliberately not, for optical fibre, because the transition metals that absorb light survive the melt
- Glass fibre — material · as silica sand, with limestone and clay for the rest of the E-glass batch
is an input to
- Float glass process — process · as the silica sand that forms the bulk of the batch
- Firing — process · present in most clay bodies, and the reason firing schedules slow through 573 degrees in both directions
- Acheson process — process · as silica sand, the silicon half of the charge
is used as
- Abrasive — application
- Glass and ceramics — application · as silica sand, the network former and the bulk of a glass batch
is commonly confused with
- Calcite — mineral · both common, colourless and vitreous; separated instantly by hardness and by acid
- Olivine — mineral · both glassy and hard, but olivine is green and granular where quartz is colourless and forms prisms
- Beta quartz — mineral · a hexagonal crystal outline in a volcanic rock is usually a paramorph — beta quartz habit with alpha quartz structure inside
- Topaz — mineral · topaz is harder and has one perfect cleavage; quartz has none and breaks conchoidally
is a polymorph of
- Beta quartz — mineral · same composition, hexagonal against trigonal; the two invert into one another at 573 °C and the change is instantaneous and reversible
is an alternative to
- Corundum — mineral · as blasting abrasive, and the substitution was made for the operators rather than for the work: silica sand generates respirable dust that causes silicosis, and alumina, garnet and slag replaced it wherever the regulation reached
Sources
- Material WorldOur own writing
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- COD Advisory Board / Vilnius University · CC0 — contributors place data in the public domain
- CRC Press / Taylor & Francis · Commercially published reference work, all rights reserved