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Material World
Mineral · CaAl2Si2O8

Anorthite

The calcium end of the plagioclase series — the first feldspar to crystallise from a cooling basalt, and a major constituent of the lunar highlands.

Anorthite is calcium feldspar, the far end of the plagioclase series from albite, and the highest-temperature member of it. Pure anorthite is uncommon on Earth — most calcium-rich plagioclase carries some sodium — and it is restricted to rocks that crystallised hot and fast.

Its most striking occurrence is not terrestrial. The lunar highlands are largely anorthosite, a rock made almost entirely of calcium-rich plagioclase, and their pale colour against the dark basaltic maria is visible from Earth with the naked eye. Anorthite floated to the top of the Moon's early magma ocean because it is less dense than the melt it grew in.

How it forms

Anorthite crystallises first and hottest from a basaltic melt, well above a thousand degrees, which is why it concentrates in the earliest-formed layers of large intrusions. On Earth it is found in gabbro, in anorthosite bodies, and in the calcium-rich contact zones where magma has invaded limestone.

It is the least stable plagioclase at the surface, weathering to clay more readily than the sodium-rich members, and it is correspondingly rare in sediments — the calcium end of the series does not survive transport.

Uses

Anorthite has little direct commercial use: it is less useful than potassium feldspar as a ceramic flux, because its higher melting point defeats the purpose.

Its significance is scientific. Anorthite composition in a plagioclase crystal records the temperature and water content of the magma that grew it, and anorthosite on the Moon is the principal evidence for the magma ocean model of lunar formation.

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

  • Plagioclase — mineral · the calcium end member Wikidata

contains

is a component of

  • Gabbro — rock · the calcium end of the plagioclase series, towards which gabbroic plagioclase sits — the same crystals as the group edge above, named more precisely

is used as

  • Glass and ceramics — application · the calcium end of the plagioclase series, and a constituent of the feldspar concentrates the industry actually buys

Sources

  • Material World
    Our own writing
  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
  • Crystallography Open Database
    COD Advisory Board / Vilnius University · CC0 — contributors place data in the public domain

Questions this page answers

Where it comes from, and what it becomes

Follow Anorthite 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

  • Anorthite → is a component of (the calcium end of the plagioclase series, towards which gabbroic plagioclase sits — the same crystals as the group edge above, named more precisely) → 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
  • Anorthite → is used as (the calcium end of the plagioclase series, and a constituent of the feldspar concentrates the industry actually buys) → Glass and ceramics
  • Anorthite → is a component of (the calcium end of the plagioclase series, towards which gabbroic plagioclase sits — the same crystals as the group edge above, named more precisely) → Gabbro → is an input to (for the dark polished cladding and memorials sold as black granite) → Quarrying → produces (crushed and graded from quarried rock, and the grading is the product — a concrete mix specifies a size distribution, not a stone) → Aggregate → is a component of (95 per cent of it by mass — the stone carries the load and the bitumen keeps the water out) → Asphalt concrete → is used as (as pavement rather than as structure: a surface that spreads a wheel load into the ground beneath it) → Structural engineering
  • Anorthite → is a component of (the calcium end of the plagioclase series, towards which gabbroic plagioclase sits — the same crystals as the group edge above, named more precisely) → Gabbro → is an input to (for the dark polished cladding and memorials sold as black granite) → Quarrying → produces (crushed and graded from quarried rock, and the grading is the product — a concrete mix specifies a size distribution, not a stone) → Aggregate → is a component of (70 to 80 per cent by volume — the cement is the expensive part and the stone is most of the material) → Concrete → is used in (foundations, frames, floors and civil structures) → Construction
  • Anorthite → is a component of (the calcium end of the plagioclase series, towards which gabbroic plagioclase sits — the same crystals as the group edge above, named more precisely) → 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 (cladding, flooring, kerbs and monuments) → Construction
  • Anorthite → is a component of (the calcium end of the plagioclase series, towards which gabbroic plagioclase sits — the same crystals as the group edge above, named more precisely) → 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 as (in masonry, where compression is the only load a wall is asked to carry) → Structural engineering

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