What does anorthite mean?
Anorthite is a calcium aluminium silicate mineral, CaAl₂Si₂O₈, belonging to the feldspar group — the most abundant family of minerals in the Earth's crust. It forms the calcium-rich end member of the plagioclase solid-solution series, at the opposite pole from sodium-rich albite, and typically occurs as white or greyish crystals in mafic igneous rocks such as gabbro, basalt, and dolerite. The name derives from the Greek for 'not straight', a reference to its oblique crystal form. Beyond terrestrial geology, anorthite holds special significance in planetary science: it is the dominant constituent of anorthosite, the rock composing the Moon's brilliant highland regions, making it central to theories of how the lunar crust formed from a primordial magma ocean. For geologists, identifying anorthite in a rock sample provides valuable clues about the chemistry and cooling history of the magma that produced it.
nounA white to greyish calcium-rich mineral of the feldspar group, CaAl₂Si₂O₈, occurring chiefly in igneous rocks and forming the calcium-rich end member of the plagioclase series.
- The calcium-rich end-member mineral of the plagioclase feldspar series, CaAl₂Si₂O₈, found in mafic igneous rocks such as gabbro and basalt.
"The moon's highland crust is dominated by anorthite, giving the lunar highlands their bright, pale appearance."
"Thin sections of the gabbro revealed large twinned crystals of anorthite under the microscope."
"Because anorthite weathers relatively easily, soils derived from basaltic parent rock are often rich in calcium."
The plural is rare, used when referring to multiple individual crystals or specimens; in most contexts anorthite is treated as an uncountable mineral name.
"The collection included anorthites from several different volcanic localities."
Anorthite makes up much of the Moon's bright highland crust — the same mineral scattered across Earth's igneous rocks literally glitters down on us from the lunar surface.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of anorthite
Anorthite was named in the early nineteenth century from the Greek 'an-' (not) and 'orthos' (straight, right), alluding to the fact that its crystals are oblique rather than right-angled. The Greek root 'orthos' also underlies familiar words such as orthodox, orthography, and orthopedic, making these distant etymological cousins. The mineral was first described from specimens found in Italy, and its name has remained stable in scientific usage since.
How anorthite is actually used
Anorthite is a technical term used almost exclusively in geology, mineralogy, and planetary science; it rarely appears in general conversation or informal writing.
Easily confused with anorthite
Albite is the sodium-rich opposite end of the plagioclase series, whereas anorthite is the calcium-rich end member.
Labradorite is an intermediate plagioclase mineral known for its iridescent play of colour, while anorthite is a distinct, more calcium-rich species without that optical effect.