Anorthite
Calcium endmember of plagioclase feldspar, rare on Earth but abundant on the Moon.
Anorthite is the calcium-rich endmember of the plagioclase feldspar solid solution series, with the other endmember being albite. Its chemical formula is CaAl2Si2O8. While pure anorthite containing no sodium is rare on Earth, the term is also applied to plagioclase compositions where the anorthite component exceeds 90 molecular percent, with up to 10 percent albite. The composition of plagioclase minerals is typically expressed as a molar percentage, denoted An%, where n equals the ratio of calcium to the sum of calcium and sodium multiplied by 100. This calculation works well for most terrestrial rocks, but in lunar samples, other cations such as Fe2+ must be considered to reconcile differences between optically and structurally derived An% values. At standard pressure, pure anorthite melts at 1550 ± 2 °C. Anorthite occurs in mafic igneous rocks, as well as in metamorphic rocks of granulite facies, metamorphosed carbonate rocks, and corundum deposits. Its type localities are Monte Somma and Valle di Fassa, Italy, and it was first described in 1823. Due to its high susceptibility to weathering in the Goldich dissolution series, anorthite is less common in surficial rocks than its abundance might otherwise suggest. Anorthite is a major component of the lunar highlands; the Genesis Rock, collected during the Apollo 15 mission in 1971, is an anorthosite composed largely of this mineral. It has also been identified in samples from comet Wild 2 and is an important constituent of calcium-aluminum-rich inclusions found in certain rare chondritic meteorites.
- 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 feldspar solid solution series, with the other endmember being albite (NaAlSi3O8). Its chemical formula is CaAl2Si2O8. Pure anorthite, containing no sodium, is rare on Earth; the term also applies to plagioclase compositions with more than 90 molecular percent of the anorthite endmember. The composition of plagioclase is commonly expressed as a molar percentage of An%, where n = Ca/(Ca + Na) × 100, though in lunar rocks and other exotic settings, other cations such as Fe2+ must be considered to reconcile optically and structurally derived An% data. At standard pressure, pure anorthite melts at 1550 ± 2 °C. It appears as white crystals, as seen in lava from Miyake Island, Japan. Anorthite occurs in mafic igneous rocks, metamorphic rocks of granulite facies, metamorphosed carbonate rocks, and corundum deposits. Its type localities are Monte Somma and Valle di Fassa, Italy, and it was first described in 1823. Due to its high weathering potential in the Goldich dissolution series, it is less common in surficial rocks than expected. Anorthite also constitutes much of the lunar highlands; the Apollo 15 Genesis Rock is anorthosite, a rock composed largely of anorthite. It has been found in samples from comet Wild 2 and is a key component of Ca-Al-rich inclusions in rare chondritic meteorites.
Reader's Guide
Anorthite, as the calcium-rich end of the plagioclase feldspar series, is defined by its chemical formula CaAl2Si2O8. Pure anorthite, containing no sodium, is rare on Earth; the term also applies to plagioclase compositions with over 90 molecular percent of the anorthite endmember. Its composition is expressed as a molar percentage (An%), where An equals the ratio of calcium to the sum of calcium and sodium, multiplied by 100. While this formula works terrestrially, lunar rocks may require accounting for other cations, such as iron, to reconcile differences between optically and structurally derived An% values. At standard pressure, pure anorthite melts at 1550 °C. It occurs in mafic igneous rocks, granulite-facies metamorphic rocks, metamorphosed carbonate rocks, and corundum deposits. Its type localities are Monte Somma and Valle di Fassa, Italy, and it was first described in 1823. Anorthite has a high weathering potential in the Goldich dissolution series, making it rarer in surficial rocks than expected. It constitutes much of the lunar highlands; the Apollo 15 Genesis Rock is anorthosite, a rock composed largely of anorthite. The mineral has also been found in samples from comet Wild 2 and is a key component of calcium-aluminum-rich inclusions in certain 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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