Question

Question: The most common feldspars are those that contain potassium, sodium, and calcium cations. They are called, respectively, orthoclasen$\left({\mathrm{KAlSi}}_3{\mathrm{O}}_8\right)$, albite$\left({\mathrm{NaAlSi}}_3{\mathrm{O}}_8\right)$, and anorthite$\left({\mathrm{CaAl}}_2{\mathrm{Si}}_2{\mathrm{O}}_8\right)$. The solid solubility of orthoclase in albite is limited, and its solubility in anorthite is almost negligible. Albite and anorthite, however, are completely miscible at high temperatures and show complete solid solution. Offer an explanation for these observations, based on the tabulated radii of the K⁺, Na⁺, and Ca²⁺ions from Appendix F.

Short answer

The solid solubility of orthoclase in albite is limited and the solubility of orthoclase in anorthite is almost negligible but Albite and anorthite are completely miscible at high temperatures and show complete solid solution because the compound with small ions are less soluble then the compound with large ions. The smaller ions have strong attractive forces so the ions are closer to each other.

Step-by-step solution

Step 1:

The solid solubility of orthoclase in albite is limited because the density of both K and Na are nearly same which is K=0.971 and Na=0.862 and also the electronegativities are nearly same. The Ionic radius of K is 1.33 and Na is 0.98 so the compound with small ions are less soluble then the compound with large ions.

Step 2:

The solubility of orthoclase in anorthite is almost negligible because the density of both K and Ca atoms are far different which is K=0.971 and Ca=1.55 according to Appendix F. The Ionic radius of K is 1.33 and Ca is 0.99 so the compound with small ions are less soluble then the compound with large ions.

Step 3:

Albite and anorthite are completely miscible at high temperatures and show complete solid solution because the ionic radius of Na is 0.98 and Ca is 0.99 and the density of Na is 0.971 and Ca is 1.55 so the compound with small ions are less soluble then the compound with large ions.