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Material World
Rock

Schist

Metamorphic rock rebuilt with all its mica lying the same way, so it splits into glittering sheets — the host of industrial talc, graphite and emery.

Construction material

A schist is a rock in which enough mica has grown, aligned the same way, that it splits into flakes and glitters on a fresh surface. The alignment is the definition: schistosity is a fabric produced by minerals growing under directed pressure, not by anything settling.

It sits in the middle of the metamorphic sequence. Shale becomes slate, slate becomes phyllite, phyllite becomes schist, and schist becomes gneiss — a progression of increasing temperature and pressure in which the grains get coarser at every step and the original sedimentary bedding is progressively destroyed.

How it forms

Schist forms in the roots of mountain belts, where sediment has been buried deep enough to reach a few hundred degrees under substantial directed stress. The clay minerals of the parent mud recrystallise as mica, and mica grows as flat plates that orient themselves perpendicular to the direction of squeezing.

Which other minerals appear depends on what the parent rock contained and how hot it got, and this is what makes schist useful to a geologist: garnet, staurolite, kyanite and sillimanite appear in a known sequence, so the assemblage records the conditions the rock passed through. A schist is a thermometer that survived the experiment.

Ultramafic rock takes a different path to the same fabric. Serpentinite carried further produces talc schist, which is why the world's talc comes out of rocks with no clay in their history at all.

Economic significance

Schist is rarely worth quarrying for itself. Its value is in the minerals concentrated within it: talc from ultramafic schists, flake graphite from carbon-rich metamorphosed sediments, and emery — a natural mixture of corundum and magnetite — which was the abrasive of the ancient world and remained the industrial standard until silicon carbide replaced it.

As ground to build on it is a known difficulty. A rock that is strong across its foliation and weak along it will fail in one direction only, and hillsides of schist slide along their own fabric.

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

  • Muscovite — mineral · 25–60% · the mica whose alignment defines the rock — schistosity is grains grown under directed pressure, not grains that settled
  • Quartz — mineral · 20–50% · recrystallised from the silt in the parent mud, in lenses and bands between the mica
  • Graphite — mineral · in metamorphosed carbon-rich sediments, and the source of flake graphite — the form batteries and refractories want
  • Talc — mineral · in the schists derived from ultramafic rock rather than from mud, which is where most industrial talc comes from
  • Corundum — mineral · in aluminous schists, and as emery when it occurs with magnetite — the abrasive of the ancient world
  • Rutile — mineral · a common accessory, and in some schists concentrated enough to be a titanium resource in its own right

is sourced from

  • Shale — rock · the ordinary metamorphic path — mud to slate to phyllite to schist, each step hotter and coarser than the last

is a source for

  • Gneiss — rock · the next step up in grade, where mica is consumed and the minerals separate into bands

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 Schist 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

  • Schist → is composed of (in aluminous schists, and as emery when it occurs with magnetite — the abrasive of the ancient world) → Corundum → is produced by (as synthetic sapphire, for watch glasses, LED substrates and transparent armour — grown rather than mined, and identical to the mineral) → Czochralski process → takes as input (charged to the crucible already refined; the process changes its arrangement and its purity, not its identity) → Silicon → is produced by (carbothermic reduction of silica with coke in a submerged arc furnace, which is smelting in the strict sense even though no ore is involved) → Smelting → takes as input (roasted to drive off sulfur, then reduced to copper) → Chalcopyrite
  • Schist → is sourced from (the ordinary metamorphic path — mud to slate to phyllite to schist, each step hotter and coarser than the last) → Shale → is composed of (the clay minerals that make up most of the rock, and whose flat particles give it its splitting) → Clay → is sourced from (clay is chiefly what feldspar becomes when it weathers) → Orthoclase
  • Schist → is composed of (recrystallised from the silt in the parent mud, in lenses and bands between the mica) → Quartz
  • Schist → is composed of (in metamorphosed carbon-rich sediments, and the source of flake graphite — the form batteries and refractories want) → Graphite
  • Schist → is composed of (the mica whose alignment defines the rock — schistosity is grains grown under directed pressure, not grains that settled) → Muscovite
  • Schist → is composed of (a common accessory, and in some schists concentrated enough to be a titanium resource in its own right) → Rutile

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

Downstream — what it becomes

  • Schist → is a source for (the next step up in grade, where mica is consumed and the minerals separate into bands) → Gneiss