18-Geol-A1 Mineralogy and Petrology · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A1 Mineralogy and Petrology, 2017-Dec. Closed book; no calculator permitted. Part 1 requires all five 10-mark short-answer questions (50 marks); Part 2's page-1 header says "5 of the 8" while the page-3 instructions say "5 of the 7" and list exactly 7 questions (a source discrepancy noted on the exam page itself).
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate/sulfide/carbonate structural classification, mineral chemistry and formulas); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, metamorphic agents/facies, volcanic processes, layered intrusions and cumulates, partial melting, ophiolites); 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 built from the $\text{SiO}_4^{4-}$ tetrahedron, but they sit at opposite ends of the silicate structural classification: garnet is an isolated-tetrahedra structure while feldspar is a fully polymerized framework structure, and that single structural difference controls almost everything else that distinguishes them in hand specimen.
| Property | Garnet (e.g. almandine) | Feldspar (e.g. orthoclase) |
|---|---|---|
| Structural class | Nesosilicate — isolated $\text{SiO}_4$ tetrahedra, 0 of 4 O shared | Tectosilicate — fully 3-D framework, all 4 O shared with neighbouring tetrahedra |
| Formula | $\text{Fe}_3\text{Al}_2(\text{SiO}_4)_3$ (almandine) | $\text{KAlSi}_3\text{O}_8$ (orthoclase) |
| Charge balance | Divalent + trivalent cations ($X_3Y_2$) sit interstitially between isolated tetrahedra | $\text{Al}^{3+}$ substitutes directly for $\text{Si}^{4+}$ in the framework, requiring a charge-balancing large cation ($\text{K}^+$, $\text{Na}^+$, $\text{Ca}^{2+}$) in a framework cavity |
| Cleavage / fracture | No cleavage — isometric packing gives conchoidal fracture | Two good cleavages at ≈90° (planes of weaker Al-Si-O framework linkage) |
| Crystal system | Isometric (cubic) | Monoclinic (orthoclase) or triclinic (plagioclase) |
| Density | Relatively high (≈3.5–4.3 g/cm³) — close-packed isolated tetrahedra with dense interstitial cations | Lower (≈2.5–2.8 g/cm³) — open, low-density framework |
