Determine Coordination Number of Cations in Chemical Equation

[Mitchtwitchita]

Can anybody please tell me if there's a way to determine the coordination number of a cation in relation to an anion just from the chemical equation (without atomic radii, tables, etc.)? For example, the C.N. of Ca2+, Al3+ and Si4+ in Ca3Al2Si3O12 (grossular garnet).

 

[GCT]

I'm pretty certain that you need to know the structure for this one.