5) Sodium (Z 11) and calcium (Z- 20 are group l (alkali and group 2 (alkaline) e

Question

5) Sodium (Z 11) and calcium (Z- 20 are group l (alkali and group 2 (alkaline) elements in the periodic table, with strong geochemical and mineralogical affinities. A. Describe, on the basis of the information given, the atomic structures of the two elements and their contrasting properties in the atomic and ionic state (10 marks). B. The plagioclase feldspar system has a Na (albite) and a Ca (anorthite) end-member, with the general idealized formula I(AlxSiy)40s. What are the stoichiometric formulas for the two end-member feldspar compositions and the values ofx and y in each case? (6 marks) C. Albite and anorthite are end-members of a perfect solid solution. Is this an example of a simple or a coupled (complex) solid-solution? Provide an explanation for your answer (10 marks).

Explanation / Answer

Question 5

A) Calcium (z=20) has the electron configuration of [Ar]4s2 which give 2 electrons in the outer shell. Whereas sodium (z=11) has the electron configuration of [Ne] 3s1 which give 1 electron in the valence shell. The calcium belongs to group 2 elements which have +2 oxidation state. The sodium which belongs to the group 1 have +1 oxidation state

B) The structural formula of plagioclase is Y(Si,Al)4O8,

Albite : NaAlSi3O8., X= 1/4, Y= 3/4

Anorthite: CaAl2Si2O8., X= 1/2, Y= 1/2

C) If an ion of different charge is substituted coupled substitution occurs. Hence another substitution is needed in order to maintain charge balance. In the solid solution between albite and anorthite, the local charge balance is maintained by the coupled substitution Na+ + Si4+ = Ca2+ + Al3+. Therefore this is an example of a coupled solid-solution.

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