Several Sulfates, Anhydrite and Barite

If we just consider the packing of the anions, we are concerned with three factors  

How to pack anions that are tetrahedral with negative charges at their comers in a way that will minimize repulsions between the anions. 

How to pack the anions to maximize the attractions between the cations and the negative charges at the comers of the anions. 

Examine the following ways of arranging adjacent sulfate ions and determine which is energetically the most favorable. 

Detennine how many calcium ions are coordinated to each oxygen. 

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