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.
Both groups are common mafic rock-forming silicates, but they belong to different silicate structural classes, and that single structural difference controls every other property that separates them.
| Group | Structure class | Formula | Cleavage | Example |
|---|---|---|---|---|
| Olivine | Nesosilicate — isolated $\text{SiO}_4^{4-}$ tetrahedra, linked only through interstitial Mg–Fe cations, no shared oxygens | $(\text{Mg,Fe})_2\text{SiO}_4$ | None (conchoidal fracture) | Forsterite (Mg-rich end-member) |
| Pyroxene | Inosilicate — single chain, each tetrahedron sharing 2 of 4 oxygens with its neighbours | $(\text{Mg,Fe,Ca})(\text{Si,Al})_2\text{O}_6$, e.g. enstatite $\text{Mg}_2\text{Si}_2\text{O}_6$ | Two directions at $\approx 87\,{}^{\circ}/93\,{}^{\circ}$ | Enstatite (Mg-rich orthopyroxene) |
Because olivine's tetrahedra are fully isolated and bonded in every direction only by ionic Mg–Fe–O bonds of similar strength, there is no structurally weak plane and hence no cleavage — it breaks with conchoidal fracture instead. Pyroxene's tetrahedra are linked into a covalently-bonded chain, so the only weak bonds run between adjacent chains; that gives two cleavage directions at roughly $87\,{}^{\circ}/93\,{}^{\circ}$, the classic "near-90°" pyroxene cleavage. The chain structure also tolerates a wider range of cation and Al-for-Si substitution (Ca, Al enter pyroxene readily) than olivine's simple Mg–Fe solid solution.
Olivine and pyroxene occur together in peridotite — the dominant upper-mantle rock type. A typical fertile mantle lherzolite is olivine + orthopyroxene + clinopyroxene + a Al-phase (spinel or garnet, depending on depth); a residual, more depleted harzburgite is olivine + orthopyroxene with pyroxene diminished. Both minerals also co-crystallize early (highest on Bowen's discontinuous branch) in a mafic magma, so they are likewise common phenocryst or cumulate phases together in olivine-bearing basalt and gabbro.
