18-Geol-A1 Mineralogy and Petrology · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exam — 18-Geol-A1 Mineralogy and Petrology. 3 hours, closed book, no calculator permitted. Two parts, twelve ten-mark short-answer questions in total: Part 1 (Q1–5) requires all five questions (50 marks); Part 2 (Q6–12) is printed as "answer 5 of the 7" on one page and "answer 5 of the 5" on another (the paper's own instructions disagree on the count) — every question in both parts is solved in full below so this set also serves as a complete study reference. This sitting is treated as undated because the paper is internally inconsistent about its own date: the first-page footer reads "May 2018" (matching the 18-Geol-A1 code, in use from December 2018 onward) while a later page's footer reads "19-Geol-A1 / May 2019". No exam date is asserted.
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate/oxide structural classification, mineral chemistry, solid solution and exsolution, crystal systems); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, Bowen's reaction series, tectonic settings of magmatism and melting, ophiolites, LIPs, anatexis, contact/thermal metamorphism).
There is no numeric given data anywhere in this qualitative/descriptive paper.
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.
A solid solution is a single, homogeneous crystal structure that accommodates a continuous range of chemical composition by substituting one ion (or coupled group of ions) for another of similar ionic radius and charge, without changing the underlying structure type. Olivine is a classic example: Mg$^{2+}$ and Fe$^{2+}$ have almost identical radius and identical charge, so they substitute freely at the same crystallographic site, giving complete solid solution across the whole series from forsterite, $\text{Mg}_2\text{SiO}_4$, to fayalite, $\text{Fe}_2\text{SiO}_4$ — any intermediate composition $(\text{Mg,Fe})_2\text{SiO}_4$ is a single mineral (olivine), not a mixture of two.
Exsolution is the subsolidus (all-solid) process by which a homogeneous high-temperature solid solution becomes unstable on cooling — once its bulk composition crosses a temperature-composition solvus — and unmixes into two chemically distinct, crystallographically intergrown phases while remaining entirely solid throughout (no melt is involved, unlike liquid immiscibility). Example: alkali feldspar. At high magmatic temperature, $(\text{K,Na})\text{AlSi}_3\text{O}_8$ exists as one homogeneous solid solution. On slow cooling it crosses the alkali-feldspar solvus and exsolves into K-rich lamellae (orthoclase/microcline, $\text{KAlSi}_3\text{O}_8$) intergrown with Na-rich lamellae (albite, $\text{NaAlSi}_3\text{O}_8$), producing the characteristic banded texture called perthite (or, when the Na-rich phase is the host, antiperthite).
Substitution in a solid solution is governed by Goldschmidt's rules: two ions readily substitute for each other when their ionic radii differ by less than about 15% and their charges either match or are compensated by a coupled substitution elsewhere in the structure (as in plagioclase, where $\text{Na}^+\text{Si}^{4+} \leftrightarrow \text{Ca}^{2+}\text{Al}^{3+}$ balances charge across two sites at once). K$^+$ and Na$^+$ satisfy the size condition loosely at high temperature, when thermal expansion widens the alkali site enough to tolerate both, but the size mismatch becomes intolerable as the structure contracts on cooling — which is precisely why the same pair that mixes freely near the solidus is forced to exsolve well below it.
