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Material World
Rock

Pegmatite

The very coarse last fraction of a granite to crystallise — crystals measured in metres, and the host of lithium, beryllium, caesium, tantalum and the rare earths.

Ore Collectible specimen

A pegmatite is a granite that crystallised last and slowest, and the result is a rock of extraordinary grain size — feldspar crystals a metre across, mica books the size of a hand, and spodumene crystals that have been measured in tens of metres. Nowhere else does ordinary rock produce crystals like that.

The coarseness is not about slow cooling alone. The residual fluid is rich in water and in elements like fluorine and boron that lower its viscosity, so atoms move through it easily and a few crystals grow very large rather than many growing small.

How it forms

Pegmatite is the end of a fractional crystallisation sequence. As a granite magma cools, quartz, feldspar and mica take the silicon, aluminium, potassium and sodium, and each rejects the ions that do not fit its structure. Lithium is too small, caesium too large, tantalum the wrong charge — so all of them stay in the shrinking pool of melt.

By the time that pool crystallises it is enriched in exactly the elements the rest of the rock refused, which is why a single rock type hosts lithium, beryllium, caesium, rubidium, niobium, tantalum and the heavy rare earths. They have nothing chemically in common except being unwelcome elsewhere.

Economic significance

Rare-element pegmatites are the hard-rock source of lithium and essentially the only source of caesium and of gem beryl. Their economics are unusual: the deposits are small, irregular and hard to model, so a pegmatite mine is a very different proposition from a porphyry copper deposit that can be planned decades ahead.

That irregularity is also why pegmatites suit artisanal mining, and why coltan from pegmatite districts has been so difficult to trace through a supply chain.

Medium confidence Weak evidence

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 · 20–50% · one of the three essential minerals, as in the granite it derives from
  • Orthoclase — mineral · 25–60% · the potassium feldspar, and the mineral that forms the largest crystals of all
  • Muscovite — mineral · 5–20% · mica books, sometimes large enough to have been mined for sheet insulation
  • Spodumene — mineral · an accessory of rare-element pegmatites rather than a defining constituent — where lithium concentrated, and it can dominate a zone entirely
  • Lepidolite — mineral · an accessory of rare-element pegmatites rather than a defining constituent — the lilac mica, in the more fluorine-rich bodies
  • Beryl — mineral · an accessory of rare-element pegmatites rather than a defining constituent — as ordinary beryl, and as emerald and aquamarine where the trace elements allow
  • Columbite — mineral · an accessory of rare-element pegmatites rather than a defining constituent — the niobium-tantalum oxide, and the reason coltan is a pegmatite product
  • Pollucite — mineral · an accessory of rare-element pegmatites rather than a defining constituent — caesium's only real host, and among the rarest of pegmatite minerals
  • Topaz — mineral · an accessory of rare-element pegmatites rather than a defining constituent — from the fluorine-rich vapours late in crystallisation
  • Monazite — mineral · an accessory of rare-element pegmatites rather than a defining constituent — carrying the light rare earths and the thorium with them
  • Xenotime — mineral · an accessory of rare-element pegmatites rather than a defining constituent — the yttrium phosphate, taking the heavy rare earths the others reject
  • Albite — mineral · as cleavelandite, the platy sodium feldspar of the replacement zones, and a signature of the more evolved bodies
  • Microcline — mineral · the potassium feldspar that forms pegmatite's largest crystals, ordered by slow cooling
  • Uraninite — mineral · in the rare-element bodies, alongside the niobium and rare earth minerals that share its incompatibility

is sourced from

  • Granite — rock · the last fraction of a granite to crystallise, not a separate magma

Sources

  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
  • Material World
    Our own writing

Questions this page answers

Where it comes from, and what it becomes

Follow Pegmatite back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Pegmatite → is sourced from (the last fraction of a granite to crystallise, not a separate magma) → Granite → is composed of (by volume, alongside feldspar and lesser mica) → Quartz
  • Pegmatite → is composed of (one of the three essential minerals, as in the granite it derives from) → Quartz
  • Pegmatite → is composed of (the potassium feldspar, and the mineral that forms the largest crystals of all) → Orthoclase
  • Pegmatite → is composed of (mica books, sometimes large enough to have been mined for sheet insulation) → Muscovite
  • Pegmatite → is composed of (an accessory of rare-element pegmatites rather than a defining constituent — where lithium concentrated, and it can dominate a zone entirely) → Spodumene
  • Pegmatite → is composed of (in the rare-element bodies, alongside the niobium and rare earth minerals that share its incompatibility) → Uraninite

These are the most distinct paths back. Pegmatite can be traced through others besides.