Anorthosite
Anorthosite covers extended portions of the Moon (the white areas) but is pretty uncommon on Earth. In its purest form, it is composed almost exclusively of plagioclase, with very little quartz (< 5%) and less than 10% mafic minerals (often pyroxene). Locations where it can be found in the U.S. include the Adirondacks Highlands, NY (click here to see page in new tab), Minnesota (both Archean and Proterozoic), and the Laramie Mountains, Wyoming.
Click here to see where is anorthosite on the QAPF diagram (opens in new tab)
1 Hand Samples
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Figure 1-1. Hand sample of Archean anorthosite, Minnesota.
Off-white, grey and blue crystals are plagioclase and red mineral is hematite. Width of sample as seen on picture: 8 cm.
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Figure 1-2. Close-up of the hand sample of fig. 1-1.
Nice subhedral crystal of plagioclase at left. Red hematite crystals. Width of sample as seen on picture: 3 mm.
Figure 1-3. Close-up of a hematite crytal.
Euhedral crystal of hematite (Fe2O3) from sample of fig. 1-1. Width of sample: 0.25 mm.
Figure 1-4. Hand sample of anorthosite from the Adirondack Highlands, New York.
This sample is almost exclusively composed of plagioclase. Width of sample as seen in picture: 9cm.
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Figure 1-5. Hand sample of anorthosite with blue crystals (Adirondack Highlands, New York).
Blue crystals of andesine and labradorite, black crystals of pyroxene. Width of sample as seen on picture: 6 cm.
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Figure 1-6. Hand sample of anorthosite from the Laramie Mountains, Wyoming.
Mostly off-white plagioclase with some blue labradorite. A 5 mm-wide euhedral labradorite crystal can be seen at top left. Width of sample as seen on picture: 6 cm.
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Figure 1-7. Hand sample of anorthosite with labradorite.
Also from the Laramie Mountains, Wyoming. This sample has more labradorite (blue). Mostly off-white plagioclase with some blue labradorite. Width of sample as seen on picture: 8.5 cm.
Next: Anorthosite thin sections