18-Geol-A1 Mineralogy and Petrology · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A1 Mineralogy and Petrology, 2016-Dec. Closed book; no calculator permitted. Part 1 requires all six 10-mark short-answer questions (60 marks); Part 2 instructs "answer 4 of the 7 ten-mark questions" (40 marks).
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (mineral/silicate structural classification, ore mineralogy); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, phase diagrams, metamorphic reactions and facies, AFM projections, volcanic processes, ophiolites and oceanic crust, subduction-zone/rift/hotspot melting); Nesse, Introduction to Optical Mineralogy, 4th ed. (index-mineral optics).
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 same fundamental unit, the $\text{SiO}_4^{4-}$ tetrahedron, but they belong to three different silicate structural classes distinguished by how many of a tetrahedron's four oxygens are shared with neighbouring tetrahedra. That linkage geometry — not composition — is what sets each group's diagnostic physical properties, above all its cleavage.
Pyroxenes (example: enstatite, $\text{MgSiO}_3$, or the Ca-Mg-Fe clinopyroxene augite) are single-chain inosilicates: each tetrahedron shares 2 of its 4 oxygens with its neighbours, linking into one continuous chain of repeat unit $(\text{SiO}_3)_n$ (Si:O = 1:3). Adjacent chains are held together only by the weaker ionic bonds of the interlayer cation sites, producing two prismatic cleavages at roughly 87°/93° (near-orthogonal, reflecting the compact single-chain repeat).
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 $\text{OH}^-$ site absent from pyroxene. The wider double-chain repeat gives two cleavage directions at roughly 56°/124° — this cleavage-angle contrast with pyroxene (near-90° vs. 56°/124°) is the standard hand-specimen test separating the two inosilicate groups.
Micas (example: muscovite, $\text{KAl}_2(\text{AlSi}_3\text{O}_{10})(\text{OH})_2$) are phyllosilicates: each tetrahedron shares 3 of its 4 oxygens with neighbours, but the sharing extends in two directions at once, building a continuous 2-D sheet of repeat unit $(\text{Si}_2\text{O}_5)_n$ (Si:O = 2:5), with sheets stacked and weakly bonded through interlayer K$^+$. Because the strong covalent Si–O bonding is confined to the sheet plane while the interlayer bond is very weak, mica shows one perfect cleavage parallel to the sheet, splitting into thin, flexible, elastic flakes — qualitatively different from the two-direction prismatic cleavage of either inosilicate.
