Lewis Structure of Fe4(SiO4)3

I'm a first year university student and am working on a paper regarding factors that affect how far an asteroid will penetrate into the Earth's atmosphere before exploding.


One of the factors I came across is the composition of the asteroid, and hence, the boiling points of the substances within the asteroid. I am currently comparing 2 types of asteroids in this manner: Carbonaceous asteroids (mostly composed of organic compounds), and silicaceous asteroids (composed mainly of magnesium- and iron-silicates such as $\ce{Fe4(SiO4)3}$). I have been trying to draw the Lewis structure for $\ce{Fe4(SiO4)3}$ for what seems like ages but I don't seem to be getting anywhere.


I feel like I'm missing something but I'm not sure what it is.


So now I have two questions in particular:


1. Is a structure for $\ce{Fe4(SiO4)3}$ even possible and if so how would you go about drawing it?
2. Is there an easier way for me to determine (and prove) which compound will have the lower boiling point?




There are several mistakes in your question:



> 
> Carbonaceous asteroids (mostly composed of organic compounds)
> 
> 
> 


No. They are composed mostly of silicates and oxides. They usually have a few percent of carbonaceous material (not necessarily organic, by the way).



> 
> $\ce{Fe4(SiO4)3}$
> 
> 
> 


This thing doesn't exist. The formula implies that all iron is oxidised: $\ce{Fe\_4^3+(SiO4)\_3^4-}$. Asteroids are not oxidised enough for this to happen. Even if they were for some reason, the stable oxidised minerals would be magnetite or hematite ($\ce{Fe3O4}$ or $\ce{Fe2O3}$). The compound you're looking for is fayalite: $\ce{Fe^2+\_2SiO4}$, which never occurs in this pure form in asteroids but rather as in [solid solution](https://en.wikipedia.org/wiki/Solid_solution) with forsterite ($\ce{Mg2SiO4}$) to form [olivine](https://www.mindat.org/min-2983.html).



> 
> I have been trying to draw the Lewis structure for Fe4(SiO4)3 for what seems like ages but I don't seem to be getting anywhere.
> 
> 
> 


That's because you're trying to draw something that doesn't exist, and if it did, it wouldn't be a molecule drawable as a Lewis structure. It would be a crystalline solid with a defined repeating structure.



> 
> Is there an easier way for me to determine (and prove) which compound will have the lower boiling point?
> 
> 
> 


Any organic material will boil first. It's pretty intuitive: boiling or melting organic material is easier than boiling rocks. Note that in the case of pure carbon as graphite or diamond this will not happen, as these are the some of the materials with the highest boiling points known. For the Mg and Fe silicates, a rule of thumb is that the Mg-bearing compounds are more refractory. $\ce{Mg2SiO4}$ melts at 1890 °C whereas $\ce{Fe2SiO4}$ melts at slightly above 1200 °C. Intermediate solid solutions (olivine) will (*partially*) melt at intermediate temperatures.


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