Anorthite
Calcium endmember of plagioclase feldspar, rare on Earth but abundant on the Moon.
Anorthite is the calcium endmember of the plagioclase feldspar mineral series, with the chemical formula CaAl2Si2O8. It is found in igneous rocks and is a compositional variety of plagioclase. Pure anorthite, containing no sodium, is rare on Earth, but the term also refers to plagioclase compositions with more than 90 molecular percent of the anorthite endmember.
- chemical_formula
- CaAl2Si2O8
- type_localities
- Monte Somma and Valle di Fassa, Italy
- known_for
- Calcium-rich endmember of plagioclase series; major component of lunar highlands
Lore & Background
Anorthite is the calcium-rich endmember of the plagioclase solid solution series, the other endmember being albite (NaAlSi3O8). The composition of plagioclases is often expressed as a molar percentage of An%, where n = Ca/(Ca + Na) × 100, though exotic locales such as Lunar rocks may need to account for other cations like Fe2+ to explain differences between optically and structurally derived An% data.
Reader's Guide
Anorthite occurs in mafic igneous rock, metamorphic rocks of granulite facies, metamorphosed carbonate rocks, and corundum deposits. It is more rare in surficial rocks than it normally would be due to its high weathering potential in the Goldich dissolution series. It was also discovered in samples from comet Wild 2 and is an important constituent of Ca-Al-rich inclusions in rare varieties of chondritic meteorites.
Did You Know?
- Anorthite makes up much of the lunar highlands; the Genesis Rock from Apollo 15 is anorthosite.
- Anorthite was discovered in samples from comet Wild 2.
- It is an important constituent of Ca-Al-rich inclusions in rare varieties of chondritic meteorites.
Frequently Asked Questions
What is Anorthite?
Anorthite is the calcium-rich endmember of the plagioclase feldspar series, defined by the formula CaAl₂Si₂O₈. It sits at one extreme of a solid-solution series that stretches from sodium-rich albite to calcium-rich anorthite, and the name also covers any plagioclase containing more than 90 molecular percent of that endmember.
Where is Anorthite found on Earth and beyond?
Its type localities are Monte Somma and Valle di Fassa in Italy, though truly pure anorthite (with zero sodium) is uncommon in terrestrial rocks. Far more abundant is its presence in the lunar highlands, where it forms a major component of the Moon's crust.
Why does Anorthite matter to mineral collectors and geologists?
It anchors the calcium end of the plagioclase series, giving researchers a fixed compositional reference for classifying feldspars in igneous rocks. Its dominance in lunar highland material also makes it central to models of Moon formation and the chemistry of the lunar crust.
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