Anorthite is the calcic end member of the plagioclase feldspar series.
In thin section, plagioclase feldspar grains often look like they’re sprinkled with dirt (PPL) or tiny confetti (XPL). You can find the explanation on the plagioclase feldspar page.
Anorthite weathers quite readily, so it’s not terribly abundant on Earth. On the moon, however, weathering forces do not pertain; anorthite remains anorthite and constitutes much of the lunar highlands. Apollo 15 astronauts David Scott and Jim Irwin collected a hand sample with an intriguing “little white corner.” The little white corner turned out to be anorthite, and the rock was interpreted to be a piece of the moon’s original crust. Scroll down to the bottom of the page for more, including thin section photos. Thin sections! Of the moon!
3D frameworks of linked tetrahedra
|Hardness||6 to 6.5|
|Cleavage||Perfect (001), good (010),
|XPL||1st order grays
Polysynthetic/albite twinning (“zebra stripes”)
Twins may be wider than the twins in albite,
|after Perkins, 309|
Anorthite in Hand Sample
Anorthite in Thin Section
Thin Section GigaPans
Anorthite in plane polarized light. Magnification: 40x To view this mineral in crossed polars, click here: https://youtu.be/7_4mxDpByKM To explore this slide…
Anorthite in cross polarized light. Magnification: 40x To view this mineral in plane polars, click here: https://youtu.be/OVtdk-ScaZY To explore this slide i…
Anorthite on the Moon
Apollo 15 astronauts David Scott and Jim Irwin collected a hand sample with an intriguing “little white corner” which turned out to be anorthite. Possibly more excited than anyone has ever been about finding a plagioclase feldspar, they bagged it and brought it home. Lunar Sample 15415 was interpreted to be original moon crust and nicknamed Genesis Rock.
NASA’s Astromaterials Acquisition and Curation Office has a synthesis of the analyses performed on 15415. It’s worth a read for the information, and especially for the short transcript at the beginning which captures Scott and Irwin’s giddiness at spotting the rock.