anorthite

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A white to gray variety of plagioclase feldspar that contains a high proportion of calcium to sodium and occurs in alkaline igneous rocks such as gabbro and anorthosite.

from The Century Dictionary.

• noun A triclinic lime feldspar, found in small transparent crystals on Mount Vesuvius, and existing also as a constituent of some rocks. See feldspar.

from the GNU version of the Collaborative International Dictionary of English.

• noun A mineral of the feldspar family, commonly occurring in small glassy crystals, also a constituent of some igneous rocks. It is a lime feldspar. See feldspar.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun mineralogy A plagioclase feldspar, the final member of the Albite-Anorthite solid solution series. Anorthite is very rare on Earth, but abundant in moon rocks. This is because Anorthite weathers rapidly to clay minerals under atmospheric conditions. Good crystals have been found in lava-digested limestone blocks erupted from Mount Vesuvius.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun rare plagioclastic feldspar occurring in many igneous rocks