Mineral Examples for Mg, Fe, Ca, Na, K and Al: 18-Geol-A1 Mineralogy and Petrology, May 2017, Q2

Question 2: Mineral Examples for Mg, Fe, Ca, Na, K and Al (Part 1 – 10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Each element listed is the essential (structure-defining) cation of at least one common rock-forming silicate; one representative mineral is given for each below.

One representative silicate mineral per essential element
Element	Mineral	Formula	Structural role
Mg	Forsterite (Mg-olivine)	$\text{Mg}_2\text{SiO}_4$	Divalent cation in the octahedral (M) site of a nesosilicate.
Fe	Fayalite (Fe-olivine)	$\text{Fe}_2\text{SiO}_4$	Divalent $\text{Fe}^{2+}$ substitutes freely for Mg in the same M site (complete olivine solid solution).
Ca	Anorthite (Ca-plagioclase)	$\text{CaAl}_2\text{Si}_2\text{O}_8$	Large divalent cation charge-balances the extra $\text{Al}^{3+}$ that substitutes for $\text{Si}^{4+}$ in the tectosilicate framework.
Na	Albite (Na-plagioclase)	$\text{NaAlSi}_3\text{O}_8$	Monovalent cation in the same framework cavity, forming a continuous plagioclase solid solution with anorthite.
K	Orthoclase (K-feldspar)	$\text{KAlSi}_3\text{O}_8$	Larger monovalent cation occupies the same tectosilicate framework site, too large to substitute readily into plagioclase.
Al	Kyanite	$\text{Al}_2\text{SiO}_5$	$\text{Al}$ occupies octahedral sites essential to the mineral (one of the three $\text{Al}_2\text{SiO}_5$ polymorphs with andalusite and sillimanite), rather than substituting into a tetrahedral Si site.

Topic. Cation substitution in rock-forming silicates.

Key relations. Ionic radius and charge govern substitution: $\text{Mg}^{2+}$↔$\text{Fe}^{2+}$ (similar radius/charge, complete solid solution); $\text{Ca}^{2+}\text{Al}^{3+}$↔$\text{Na}^+\text{Si}^{4+}$ coupled substitution (plagioclase); $\text{K}^+$ too large for the plagioclase site.

Why this works. Goldschmidt's rules predict which cations substitute for which in a silicate lattice on the basis of ionic radius and charge; this is why Mg and Fe form one continuous olivine/pyroxene solid solution while Ca-Na substitution in feldspar requires a coupled, charge-balancing Al-Si exchange, and why K, being both larger and lower-charged, is essentially excluded from plagioclase and instead forms its own separate feldspar and mica structures.

Common pitfall. Listing an oxide (e.g. corundum for Al) instead of a silicate when the question specifically asks for silicate mineral examples.

Source. Klein & Dutrow, Manual of Mineral Science, 23rd ed., ch.11–12 (silicate mineral chemistry, cation substitution).

Question 2 of 13: Mineral Examples for Mg, Fe, Ca, Na, K and Al

Question 2: Mineral Examples for Mg, Fe, Ca, Na, K and Al (Part 1 – 10 marks)