# Is a controlled fractional crystallization process feasible on the Moon?

I am looking for a way to separate metal oxides on the Moon. Please bear with me - it's for an ultra-hard science fiction project. Processes typical on Earth are not effective there. Heat is cheap on the Moon, there is strong constant sun that is easily concentrated. Heat also dissipates slowly thanks to the vacuum, and reflective insulation can be used to slow heat loss much more.

So, I wondered if a form of controlled fractional crystallization could be used to get pure olivine from a molten melt of rock, and then the olivine could be split into forsterite and fayalite the same way. The forsterite might make decent clear glass, or if not, the fayalite can be processed with hydrogen to get iron, water, and silica, and the silica can be used to make glass. It might be necessary to apply centrifugal force to get the crystals to separate in the Moon's low gravity. If the technique works, it would seem the same process could be used to isolate other metal oxides through other paths.

Hydrogen and carbon atoms are present on the level of around 100 ppm. Am I correct in thinking that mineral separation would occur this way and could be controlled to get good purity? I know there are engineering issues, but in terms of the geochemistry, is it correct that crystallization and settling would occur in this way?

(Note: I heavily edited this after my continued searching on the topic finally got me to 'fractional crystallization', and then I came across a few things that made me rethink the whole thing.)

• mmm, ... interesting question. I'm not sure whether a geologist or a metallurgist would be the best to answer the question. – Fred Apr 13 '17 at 17:50
• @Fred i asked a related question on Chemistry.SE - chemistry.stackexchange.com/q/72483/9853. I thought some people here would be more likely to know how such separation works in nature, and thus what factors influence it. – kim holder Apr 13 '17 at 18:13
• looking forward to your book – Jack R. Woods Apr 19 '17 at 1:32

https://chemistry.stackexchange.com/q/67403/8083

https://chemistry.stackexchange.com/q/72483/8083

The forsterite might make decent clear glass

Probably not, see above.

fayalite can be processed with hydrogen to get iron, water, and silica, and the silica can be used to make glass

Again, probably not. To make the fayalite component react fully in reasonable time you are going to need to mill the olivine to very fine dust. Then, separating the silica from this fine dust in solid form is almost impossible. This will require heavy liquids (like polytungstate) which are not available on the moon. Doing this in liquid form and centrifuging will require heating, which will inevitable cause reaction of forsterite and silica to form enstatite: $\ce{SiO2 + Mg2SiO4 = 2MgSiO3}$, which is useless for glass making.

Is a controlled fractional crystallization process feasible on the Moon?

Basically yes. This would be similar to a distillation process that slowly over repeated melting and solidification processes will remove Mg-rich solid from Fe-rich liquid. The problem is that even very low amounts (1% and lower) of Fe in forsterite will colour it green, which is undesirable for your glass. And that's without even considering the forsterite is not a good glass material (see links above). Seperating the Mg out will become harder and harder as you go along in your process.

Good glass material has alkalis in it. Fortunately, some lunar rocks have Na and K in them (e.g. the KREEP basalts). It has about 1 % of potassium, which could be enriched in fractional crystallisation. The problem is that you will also enrich the glass in Th, making it slightly radioactive and possibly browning with time.

• This is a completely molten process, nothing is separated or reacted in solid form. The energy required comes from sunlight, which is constant for 2 weeks every month. I am not concerned about the energy budget. The question is if the purity needed can be gotten from a molten process. I edited the question as it doesn't state that the process is molten. In geology fractional crystallization is about magma bodies, so it is sort of implied. So the question would seem to be whether the transition elements can be purified out by the process enough for clear glass. – kim holder Apr 17 '17 at 14:39
• (By the way, the sun is concentrated directly for heating, this isn't energy from solar cells.) – kim holder Apr 17 '17 at 14:43