[nikita]

i wasnt too sure as to where to post this question as it does deal with chemistry, but also geology.  so here it is in chemistry- other.

in the pyroxene silicate mineral series, there is a solid solution between enstatite (MgSiO₃) and ferrosilite (FeSiO₃).  I know the rules for solid solution- same size, charge, temp/pressure and availability of the ions.  So, i know Mg and Fe are similar in size and charge and can substitute for one another in the solution in a constant temperature/pressure situation.  why?  why would Mg go into the site, rather than Fe?  can it really be the availability of the ions or is there something obvious im missing here?  i feel like the reason cant be that obvious.  but, i cant think of any other reason why.  the size difference in the ionic radius is so small (Fe's is 8% bigger and its under the 15% limit) it should not make a difference.  furthermore, i guess i want to know why a solid solution is occurring at all.  why not some other mineral with the same size or charge or why dont all minerals have solid solutions.  any help is appreciated, even and especially if its telling me what to read or teach myself, lol.

[Borek]

Not that I know anything about solid solutions, but first of all - I have no idea what you are asking about...

why would Mg go into the site, rather than Fe?

Are you asking on what are "rules of selection" that make paricular ion occupy particular position? Like pointing a finger to the atom and asking "why we have here Mg and no Fe"? I don't think there are any rules here - if atoms (ions) are similar in charge/size and there are no specific interactions between them and already solid matrix - it is just random.

[nikita]

yes, Borek, thats what i was asking that day.  i realised that what you said is true-  its just the availability of the ions during formation.  whatever is there is going to go in.  i guess now im wondering why there isnt just one pyroxene.  like simply the Mg one.  i think it might have something to do with the miscibility gap, but i realise that i am asking more of a foundation of solid solution question- not just why there are 2 pyroxenes, but what causes solid solution in the first place.  im still in the process of figuring this out, but thanks for your reply.