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The composition of a plagioclase feldspar is typically denoted by its overall fraction of anorthite (%An) or albite (%Ab). There are several named plagioclase feldspars that fall between albite and anorthite in the series. The following table shows their compositions in terms of constituent anorthite and albite percentages. [19] [20]
Its chemical formula is (Ca, Na)(Al, Si) 4 O 8, where Ca/(Ca + Na) (% anorthite) is between 30 and 50%. The formula may be written as Na 0.7-0.5 Ca 0.3-0.5 Al 1.3-1.5 Si 2.7-2.5 O 8. [2] The plagioclase feldspars are a continuous solid solution series and as such the accurate identification of individual members requires detailed optical study ...
Bytownite is a calcium rich member of the plagioclase solid solution series of feldspar minerals with composition between anorthite and labradorite.It is usually defined as having between 70 and 90%An (formula: (Ca 0.7−0.9 Na 0.3−0.1)[Al(Al,Si)Si 2 O 8]).
Anorthite (an = not, ortho = straight) is the calcium endmember of the plagioclase feldspar mineral series. The chemical formula of pure anorthite is CaAl 2 Si 2 O 8. Anorthite is found in mafic igneous rocks. Anorthite is rare on the Earth [6] but abundant on the Moon. [7]
Feldspar (/ ˈ f ɛ l (d) ˌ s p ɑːr / FEL(D)-spar; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. [3] The most common members of the feldspar group are the plagioclase (sodium-calcium) feldspars and the alkali (potassium ...
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula Na Al Si 3 O 8. It is a tectosilicate. Its color is usually pure white, hence its name from Latin, albus. [5]
Oligoclase is a rock-forming mineral belonging to the plagioclase feldspars. In chemical composition and in its crystallographic and physical characters it is intermediate between albite (Na Al Si 3 O 8) and anorthite (CaAl 2 Si 2 O 8). [1] The albite:anorthite molar ratio of oligoclase ranges from 90:10 to 70:30.
Anorthosite (/ ə ˈ n ɔːr θ ə s aɪ t /) is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most commonly present.