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.
Sanidine, orthoclase and microcline are the three principal K-feldspar polymorphs: all three share the identical bulk formula $\text{KAlSi}_3\text{O}_8$, and all three form by the same crystallization process, but they differ in Al–Si ordering within the tetrahedral framework, which in turn is controlled by the temperature (and cooling rate) at which the structure last equilibrated.
| Mineral | Crystal system | Al–Si order | Typical occurrence |
|---|---|---|---|
| Sanidine | Monoclinic | Highly disordered (stable at highest T) | Rapidly quenched high-T volcanic rocks (rhyolite/trachyte phenocrysts) — quenching freezes in the high-T disordered structure. |
| Orthoclase | Monoclinic | Partially ordered (intermediate T) | Slowly cooled plutonic rocks (granite, syenite), where cooling is slow enough to allow partial but not complete Al–Si ordering. |
| Microcline | Triclinic | Fully ordered (stable at lowest T) | Very slow subsolidus cooling or low-T hydrothermal growth; the symmetry drop to triclinic produces its diagnostic cross-hatched ("tartan") twinning. |
All three are true polymorphs of one compound: identical chemistry ($\text{KAlSi}_3\text{O}_8$, tectosilicate framework), near-identical physical properties (hardness ≈6, density ≈2.56–2.63 g/cm³), and the same two prismatic cleavages at (or very near) 90° ({001} and {010}) — the near-orthogonal angle is itself diagnostic of the K-feldspar (orthoclase-type) group as opposed to plagioclase, whose cleavages meet at a slightly oblique angle. All three can also host exsolved albite lamellae (perthite) on slow cooling.
