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Material World
Mineral · (Na,Ca)Al(Si,Al)Si2O8

Plagioclase

A continuous solid solution from sodium to calcium feldspar — the commonest mineral series in the crust, and a thermometer for the magma that grew it.

Plagioclase is a series rather than a species: a continuous solid solution running from albite, the sodium end, to anorthite, the calcium end, with every composition between them occurring in nature. There is no boundary anywhere along it, which is why the names given to intermediate compositions — oligoclase, andesine, labradorite, bytownite — are conveniences rather than distinct minerals.

The substitution is unusual because it is coupled. Replacing sodium with calcium adds a positive charge, so a silicon is simultaneously replaced by an aluminium to balance it. Two substitutions happen together or neither happens, which is why the series is continuous rather than stepped.

How to identify it

Fine parallel striations on the best cleavage surface are close to diagnostic. They are twin planes, repeated on a fine scale, and no potassium feldspar shows them — which makes them the quickest way to separate plagioclase from orthoclase in a hand specimen.

Beyond that, plagioclase is usually white to grey where potassium feldspar is often pink. Labradorite, an intermediate composition, shows a spectacular blue-green iridescence caused by light scattering from submicroscopic layers within the crystal, and is prized for it.

How it forms

Plagioclase crystallises early from basaltic magma and continues throughout cooling, and its composition tracks the temperature at which it grew: calcium-rich near the start, progressively sodium-rich as the melt evolves. A single crystal frequently preserves that history as concentric zones, calcium-rich core to sodium-rich rim.

That makes plagioclase a geological thermometer. Reading the zoning in a crystal reconstructs the cooling path of the magma it came from, which is one of the more direct records available of a process nobody can observe.

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.

belongs to the group

  • Feldspar — mineral · the sodium-calcium series; itself a series with members

includes

  • Albite — mineral · the sodium end member Wikidata
  • Anorthite — mineral · the calcium end member Wikidata

contains

  • Aluminium — element
  • Calcium — element · substitutes within a solid solution — an individual specimen may hold little of it
  • Sodium — element · substitutes within a solid solution — an individual specimen may hold little of it
  • Oxygen — element
  • Silicon — element

is a component of

  • Granite — rock · alongside the potassium feldspar and quartz
  • Basalt — rock · calcium-rich plagioclase is a defining constituent of basalt
  • Gabbro — rock · 35–65% · one of the two essential minerals, generally calcium-rich labradorite rather than the sodic plagioclase of granite
  • Gneiss — rock · 10–40% · the other feldspar of the pale bands, and the one that distinguishes a gneiss of igneous parentage from one of sedimentary

is a source for

  • Clay — material · plagioclase weathers to clay as readily as potassium feldspar does

is commonly confused with

  • Orthoclase — mineral · the commonest feldspar identification problem; fine parallel striations on a cleavage face indicate plagioclase

is used as

  • Glass and ceramics — application · as feldspar concentrate rather than as a separated species — what is sold is a composition range, not a mineral

Sources

  • Material World
    Our own writing

Questions this page answers

Where it comes from, and what it becomes

Follow Plagioclase 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.

Downstream — what it becomes

  • Plagioclase → is a component of (calcium-rich plagioclase is a defining constituent of basalt) → Basalt → is an input to (for aggregate rather than dimension stone — hard, tough and angular, which is what a road base needs) → Quarrying → produces (cut rather than blasted, because a quarry that shatters its stone has destroyed what it came for) → Dimension stone → is used in (quarried rather than mined, and the distinction is legal and technical rather than merely a word) → Mining and quarrying → is associated with (steam pumping allowed mines below the water table, which is what made deep coal possible) → Industrial Revolution complete chain
  • Plagioclase → is a component of (one of the two essential minerals, generally calcium-rich labradorite rather than the sodic plagioclase of granite) → Gabbro → is an input to (for the dark polished cladding and memorials sold as black granite) → Quarrying → produces (cut rather than blasted, because a quarry that shatters its stone has destroyed what it came for) → Dimension stone → is used in (quarried rather than mined, and the distinction is legal and technical rather than merely a word) → Mining and quarrying → is associated with (steam pumping allowed mines below the water table, which is what made deep coal possible) → Industrial Revolution complete chain
  • Plagioclase → is a component of (alongside the potassium feldspar and quartz) → Granite → is a source for (quarried in blocks for cladding, kerbs and monuments) → Dimension stone → is used in (quarried rather than mined, and the distinction is legal and technical rather than merely a word) → Mining and quarrying → is associated with (steam pumping allowed mines below the water table, which is what made deep coal possible) → Industrial Revolution complete chain
  • Plagioclase → is a source for (plagioclase weathers to clay as readily as potassium feldspar does) → Clay → is an input to (supplies the silica and alumina that combine with lime in the cement kiln) → Calcination → produces (the solid residue once carbon dioxide has been driven off) → Quicklime → is a component of (as calcium oxide within the clinker phases, not as free lime) → Portland cement → is used in → Construction
  • Plagioclase → is a component of (the other feldspar of the pale bands, and the one that distinguishes a gneiss of igneous parentage from one of sedimentary) → Gneiss
  • Plagioclase → is used as (as feldspar concentrate rather than as a separated species — what is sold is a composition range, not a mineral) → Glass and ceramics

These are the most distinct paths onward. Plagioclase ends up in others besides.