Thus, silicon usually has a coordination number of 4. Coordination numbers are used to produce a structural formula. Remember that the subscripts give the number of ions in one formula unit and the numbers above the chemical symbols given the coordination numbers. They have identical chemical compositions but differ in structure and hence in physical properties. An increase in pressure favors a larger coordination number.
Temperature has the reverse effect.
This is an octahedral structure. In aragonite each calcium ion is surrounded by 9 nearest neighbor oxygens. Consider the two minerals Forsterite and Fayalite. The rest of the chemical formulas are identical. When such similarities occur the ions may substitute for each other forming a solid solution series .