18-Geol-A1 Mineralogy and Petrology · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A1 Mineralogy and Petrology, 2016-May. Closed book; no calculator permitted. Part 1 requires all six 10-mark short-answer questions (60 marks); Part 2 instructs "four of the seven 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, cumulates, metamorphic reactions and facies, AFM projections, volcanic processes, oceanic crust petrogenesis); Nesse, Introduction to Optical Mineralogy, 4th ed. (index-mineral optics); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sedimentary/pyroclastic textural context).
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 building block, 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 — and this linkage geometry is what actually controls their diagnostic physical properties, above all cleavage.
Pyroxenes (example: augite, $(\text{Ca,Mg,Fe,Ti,Al})_2(\text{Si,Al})_2\text{O}_6$) are single-chain inosilicates: tetrahedra share 2 of their 4 oxygens, linking into one continuous chain with repeat unit $(\text{SiO}_3)_n$, a Si:O ratio of 1:3. Chains are held to their neighbours only by the (weaker) ionic bonds of the interlayer cations, giving two cleavage directions 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, with alternating tetrahedra sharing 3 of 4 oxygens instead of 2, giving repeat unit $\text{Si}_4\text{O}_{11}$ (Si:O = 4:11) and incorporating an essential $\text{OH}^-$/$\text{F}^-$ site absent from pyroxene. The wider double-chain repeat gives two cleavage directions at roughly 56°/124° — this cleavage-angle difference (near-90° vs. 56°/124°) is the standard hand-specimen/thin-section test separating the two inosilicate groups.
Micas (example: biotite, $\text{K}(\text{Mg,Fe})_3(\text{AlSi}_3\text{O}_{10})(\text{OH})_2$) are phyllosilicates: each tetrahedron shares 3 of its 4 oxygens with neighbours, extending the linkage indefinitely in two directions to build a continuous 2-D sheet, repeat unit $(\text{Si}_2\text{O}_5)_n$ (Si:O = 2:5), with sheets stacked and weakly bonded by interlayer K+. Because the strong Si–O bonding is confined to the sheet plane and 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.
