I was hoping for some explanation with these problems for geochemistry. We\'ve b

Question

I was hoping for some explanation with these problems for geochemistry. We've been covering stuff like Pauling's and Goldschmidt's rules. Please don't get too complex, if possible.

1) The size mismatch that results from trying to substitute a cation of a different size for another in a mineral structure is associated with an energy change for the structure. Do you think that energy is positive (destabilizing the structure) or negative (stabilizing the structure) and why?

2)Do you think it is possible to make Ca+2 substitute for Mg+2 in the olivine structure (Mg+2, Fe+2)2SiO4? How about Al+3?

Thanks

Explanation / Answer

1) Energy is positive because this mismatch in size leads to change in contacts in crystal lattice of the mineral. Hence, this destabilizes the crystal lattice.

2)Al+3 and Mg +2 have similar sizes, Whereas Ca+2 is much bigger. Hence, Al+3 could be used by balancing charge from replacements.

But using Ca+2 would destabilize the system to a larger extent

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