Anorthite
Calcium endmember of plagioclase feldspar, rare on Earth but abundant on the Moon.
Last updated
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.
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.
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. 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.
More in Minerals & Crystals
Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Anorthite (CC BY-SA 4.0).
- Word definitions: the Codexery glossary, each quoted from its Wikipedia article.
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced