The primordial Lunar crust is mostly calcic plagioclase. However, why did the Lunar Magma Ocean produce so much anorthite, why not a feldspar of a different composition, like albite or orthoclase for instance?
1 Answer
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A base-level justification is that calcium is much more abundant than sodium or potassium - about 10 times in bulk earth, and double in the crust. Since the moon is thought to have formed as a result of a collision with another Mars-sized object 4.5 billion years ago, the bulk composition of the Moon would have been influenced by the Earth's composition, particularly because the Earth hadn't differentiated very much (i.e., become chemically layered) at the time of the impact.
A more important reason involves the process of differentiation itself. The Moon has been geologically dead for about 3 billion years now, meaning that it can't sustain volcanic systems and so its geology is effectively set in stone (ha). When it first formed, the moon would have undergone some differentiation (i.e., heavy elements like Fe and Ni concentrate in the core), leaving essentially a magma ocean of roughly basaltic composition. Calcic plagioclase - anorthite - is a common liquidus mineral (i.e., early to crystallize) in magma of this composition, and it predominates because Ca becomes stable in the solid phase much earlier than either K or Na do (a concept called 'compatibility').
The thing about anorthite is it is less dense than your typical basaltic magma (~2.7 g/cm3 compared to ~3.3 g/cm3 for basalt), and so it floats while the ferromagnesian minerals (olivine, pyroxene) sink! This is why so much of the lunar surface is covered in anorthosite. The other rock on the surface of the moon is basalt, erupted from the residual basaltic magma at depth onto the maria during its active phase 3 billion years ago.
