Granite
A coarse-grained igneous rock of quartz and feldspar, and the archetypal building stone.
Granite is an igneous rock that cooled slowly at depth, which is why its crystals are large enough to see without magnification. It is principally quartz and feldspar with smaller amounts of mica and amphibole, and the visible speckling is those different minerals sitting side by side.
Its durability as a building stone comes directly from that composition: quartz is hard and chemically inert, and feldspar, while it does weather eventually, does so slowly in most climates.
How it forms
Granite forms from magma that cools underground over thousands to millions of years. The slow cooling is what distinguishes it from rhyolite, which has the same composition but erupted and cooled fast, producing crystals too small to see.
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.
is composed of
- Quartz — mineral · 20–60% · by volume, alongside feldspar and lesser mica
- Orthoclase — mineral · the pink or cream mineral that gives much granite its colour
- Muscovite — mineral · one of the two common micas in granite, alongside biotite
- Plagioclase — mineral · alongside the potassium feldspar and quartz
- Zircon — mineral · the classic granite accessory — a fraction of a per cent by mass, and the mineral nearly every date for a granite is measured on
- Apatite — mineral · the phosphate accessory, present in almost every granite and rarely in quantity
- Monazite — mineral · in the more evolved granites, carrying the light rare earths and the thorium that goes with them
- Uraninite — mineral · in some granites, and the reason granite terrain produces radon — uranium decays to it and the gas escapes through the fractures
- Microcline — mineral · the potassium feldspar of slowly cooled plutonic granite, ordered where a faster-cooled rock gives orthoclase instead
- Feldspar — mineral · as the mineral group — a granite is by definition feldspar-dominant, whichever species that feldspar turns out to be
- Fayalite — mineral · only in the ferroan and alkaline granites, where iron-rich olivine is stable alongside quartz; absent from ordinary granite entirely
is used in
- Construction — industry
is a source for
- Dimension stone — material · quarried in blocks for cladding, kerbs and monuments
- Pegmatite — rock · the last fraction of a granite to crystallise, not a separate magma
- Greisen — rock · the roof of a granite, chemically rebuilt by vapour escaping from the granite below it
- Sandstone — rock · the quartz grains are what survives the weathering of granite and gneiss; everything softer in the parent rock became clay and went elsewhere
- Porphyry copper deposit — deposit type · an intrusion of granitic composition, fractured and altered by fluid separating out of it as it crystallised
- Sand — material · the ultimate source of most of it — granite weathers, the feldspar becomes clay and washes out, and the quartz survives as grains
is an input to
- Quarrying — process · cut with diamond wire for dimension stone, and crushed elsewhere for aggregate
is an alternative to
- Gabbro — rock · as dimension stone, both sold as granite by the trade. Gabbro gives the dark polished finish and granite the pale speckled one, and neither is chosen for anything but appearance and price
- Marble — rock · as building and monumental stone: marble carves far more easily and weathers far worse, which is why it was the sculptor's stone and is not the outdoor one
Sources
- Material WorldOur own writing
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)