18-Geol-A1 Mineralogy and Petrology · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A1 Mineralogy and Petrology, 2017-May. Closed book; no calculator permitted. Part 1 requires all five 10-mark short-answer questions (50 marks); Part 2 lists eight questions with instructions to answer "5 of the 7" (a source discrepancy noted on the exam page itself).
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate structural classification, mineral chemistry/formulas); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation and mixing, metamorphic agents/facies, volcanic processes, phase equilibria and AFM projections, magma viscosity, layered intrusions, tectonic melting mechanisms).
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.
All three groups are built from the $\text{SiO}_4^{4-}$ tetrahedron, but they occupy three different rungs of the silicate structural classification, distinguished by how many of a tetrahedron's four oxygens are shared with a neighbouring tetrahedron. This degree of polymerization is what directly controls each group's diagnostic cleavage and fracture.
Olivine (example: forsterite, $\text{Mg}_2\text{SiO}_4$) is a nesosilicate: individual $\text{SiO}_4$ tetrahedra share none of their oxygens with each other, each held to its neighbours only by ionic bonds through interstitial divalent cations ($\text{Mg}^{2+}$, $\text{Fe}^{2+}$). With no chain or sheet of covalent Si–O bonds to define a plane of weakness, olivine has no true cleavage and instead breaks with a curved, glassy conchoidal fracture.
Pyroxenes (example: enstatite, $\text{MgSiO}_3$) are single-chain inosilicates: tetrahedra share 2 of their 4 oxygens, linking into one continuous chain, repeat unit $(\text{SiO}_3)_n$. The chains are bonded to their neighbours only by weaker ionic bonds, giving two cleavage directions at roughly 87°/93°.
Amphiboles (example: hornblende, $\text{Ca}_2(\text{Mg,Fe,Al})_5(\text{Si,Al})_8\text{O}_{22}(\text{OH})_2$) are double-chain inosilicates: two single chains cross-link, sharing 3 of 4 oxygens on the linking tetrahedra, repeat unit $\text{Si}_4\text{O}_{11}$, and carry an essential $\text{OH}^-/\text{F}^-$ site that olivine and pyroxene lack. The wider double-chain repeat gives two cleavage directions at roughly 56°/124° — the standard hand-specimen test that separates amphibole from pyroxene.
