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!

Physical Properties
Chemical formula CaAl2Si2O8
Class Tectosilicate
3D frameworks of linked tetrahedra
Crystal system Triclinic
Habit Prismatic
Cleavable masses
Irregular grains
Color White
Hardness 6 to 6.5
Specific gravity 2.76
Cleavage Perfect (001), good (010),
poor {110}
Fracture Uneven
Luster Vitreous
Transparency Translucent
Streak White
Optical Properties
PPL Colorless
Low relief
XPL 1st order grays
Polysynthetic/albite twinning (“zebra stripes”)
δ 0.013
Twinning Polysynthetic/albite

Twins may be wider than the twins in albite,
but x-ray diffraction or other tests are
required to differentiate them for certain.

after Perkins, 309

Anorthite in Hand Sample

Anorthite is the light mineral in this hand sample.
Scanning electron micrograph of anorthite

Anorthite in Thin Section

Thin Section GigaPans

Anorthite in plane polars
Anorthite in crossed polars

Anorthite, PPL

Anorthite, XPL

Further Reading

Anorthite at
Anorthite at

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.

The Lunar Science and Planetary Institute hosts dozens of images of 15415 (and other lunar samples!), including thin sections, stereo views, and in situ photographs.