18-Geol-A1 Mineralogy and Petrology · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 18-Geol-A1 Mineralogy and Petrology, 2018-Dec. Closed book; no calculator permitted.
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate/oxide structural classification, mineral chemistry and formulas, crystal systems); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, Bowen's reaction series, metamorphic agents/facies, volcanic and pyroclastic processes, partial melting); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (carbonate mineral diagnostics).
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 inosilicates (chain silicates) built from the same $\text{SiO}_4^{4-}$ tetrahedron, but they differ in how many tetrahedra link side by side into the repeating chain — a single chain in pyroxene versus a double, cross-linked chain in amphibole — and this one structural difference controls their diagnostic cleavage, composition and habit.
Pyroxenes (example: augite, $(\text{Ca,Mg,Fe,Ti,Al})_2(\text{Si,Al})_2\text{O}_6$) are single-chain inosilicates: each tetrahedron shares 2 of its 4 oxygens with its neighbours, giving repeat unit $(\text{SiO}_3)_n$ (Si:O = 1:3). Only ionic bonding through the interlayer cations links adjacent chains, giving two cleavage directions at roughly 87°/93° (near-orthogonal) and a stubby, blocky, 8-sided prismatic habit.
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 are cross-linked so that alternating tetrahedra share 3 of 4 oxygens instead of 2, giving repeat unit $\text{Si}_4\text{O}_{11}$ (Si:O = 4:11) and an essential $\text{OH}^-/\text{F}^-$ site that pyroxene lacks. The wider double-chain repeat gives two cleavage directions at roughly 56°/124° and a longer, more bladed prismatic habit than pyroxene.
