Calculate the bond energies for the average Ca-O bonds in each phase. b. Based on this, which phase forms at low T&P and which at higher (Give a full explanation as to why and how your argument connects to part a)? c. What is the coordination number of Ca in calcite? And in aragonite? How does this answer relate to a&b?
Aragonite (a common biological product) and calcite (a common soil and sediment
mineral) are polymorphs of the formula CaCO3. The former has an average Ca-O bond
length of ≈ 2.5Å and the later has one of ≈3.2Å. Let’s assume the same born constant of 9
for both phases and a Madelung constant for Aragonite of 2.5 and for Calcite of 1.74.
a. Calculate the bond energies for the average Ca-O bonds in each phase.
b. Based on this, which phase forms at low T&P and which at higher (Give a full
explanation as to why and how your argument connects to part a)?
c. What is the coordination number of Ca in calcite? And in aragonite? How does
this answer relate to a&b?
a. Bond energy of the average Ca-O bonds in calcite phase= (Madelung constant×Born constant) / r where r is the distance between the bonds that is bond length= (1.74×9)/3.2 kJ= 4.89kJ
Bond energy of the average Ca-O bonds in aragonite phase= (Madelung constant×Born constant) / r where r is the distance between the bonds that is the bond length =(2.5×9)/2.5kJ= 9kJ.
b. Aragonite is formed at a high temperature and pressure condition, whereas calcite is formed at low temperature and pressure condition.
It can be noted that the bond length is inversely proportional to bond energy. Therefore, from the previous part (part a), it can be found that as the bond length of Ca-O is more than the bond length of Ca-O in aragonite, therefore, the bond energy for the phase calcite is found to be lower than the bond energy of the phase aragonite, ensuring that calcite forms at low temperature and pressure conditions and aragonite forms at high temperature and pressure conditions.
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