Anorthite crystals (white) in lava from Miyake Island, Japan (size: 2.4 × 1.7 × 1.7 cm)
Anorthite is the calcium-rich endmember of the plagioclase solid solution series, the other endmember being albite (NaAlSi3O8). Anorthite also refers to plagioclase compositions with more than 90 molecular percent of the anorthite endmember. The composition of plagioclases is often expressed as a molar percentage of An%, or (for a specific quantity) Ann, where n = Ca/(Ca + Na) × 100.[8] This equation predominantly works in a terrestrial context; exotic locales and in particular Lunar rocks may need to account for other cations, such as Fe2+, to explain differences between optically and structurally derived An% data observed in Lunar anorthites.[9]
^Deer, W.A., Howie, R.A. and Zussman, J. (1966). An Introduction to the Rock Forming Minerals. London: Longman. p. 336. ISBN0-582-44210-9.{{cite book}}: CS1 maint: multiple names: authors list (link)
^Wenk, H. -R.; Wilde, W. R. (1 August 1973). "Chemical anomalies of Lunar plagioclase, described by substitution vectors and their relation to optical and structural properties". Contributions to Mineralogy and Petrology. 41 (2): 89–104. doi:10.1007/BF00375035.