18-Geol-A1 Mineralogy and Petrology · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 18-Geol-A1 Mineralogy and Petrology, 2019-Dec. Closed book; no calculator permitted.
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate structural classification, mineral chemistry and substitution, crystal systems, sulfide/carbonate ore mineralogy); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, Bowen's reaction series, tectonic settings of magmatism, metamorphic/metasomatic processes, volcanic and pyroclastic processes, plate-tectonic cycle).
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.
A typical felsic rock (granite/rhyolite) is dominated by quartz, alkali feldspar and plagioclase, with a subordinate sheet-silicate mica; each has a specific, reliable diagnostic separating it from the others.
| Mineral | Diagnostic hand-lens / microscope feature |
|---|---|
| Quartz ($\text{SiO}_2$) | No cleavage — conchoidal fracture; vitreous, glassy luster; colourless/gray and typically anhedral, filling the interstices between earlier-formed feldspar grains, because it is the last mineral to crystallize (Bowen's series terminus). Hardness 7 (scratches glass/steel). |
| K-feldspar (orthoclase/microcline, $\text{KAlSi}_3\text{O}_8$) | Two cleavages at exactly $90\,{}^{\circ}$; commonly pink to flesh-coloured in hand specimen (distinguishing colour from white plagioclase); microcline specifically shows diagnostic cross-hatched "tartan" twinning under cross-polarized light in thin section. |
| Plagioclase (e.g. oligoclase/albite, $(\text{Na,Ca})\text{Al}(\text{Si,Al})\text{Si}_2\text{O}_8$) | Two cleavages near (but not exactly) $90\,{}^{\circ}$, like K-feldspar, but distinguished by fine, parallel polysynthetic (albite) twin striations visible on the cleavage face with a hand lens, or as alternating light/dark twin lamellae under cross-polarized light. |
| Muscovite ($\text{KAl}_2(\text{AlSi}_3\text{O}_{10})(\text{OH})_2$) | Perfect one-directional (basal) cleavage into thin, flexible, elastic sheets — unmistakable even without magnification; colourless and non-pleochroic in thin section, but shows very high-order (pink/green) birefringence colours under cross-polarized light. |
In practice these four minerals are identified in a fixed logical order: first separate the sheet silicate (muscovite) by its unmistakable elastic cleavage, then separate quartz from the two feldspars by the presence or absence of cleavage, and finally separate K-feldspar from plagioclase — the hardest step, since both share near-identical cleavage — using colour in hand specimen or twin striations/cross-hatching under the microscope. This same four-mineral assemblage (quartz + alkali feldspar + plagioclase + a mica) is exactly what the modal QAPF classification diagram uses to distinguish granite from granodiorite and syenite, so correctly identifying and roughly estimating the modal proportion of each is also the first step toward formally classifying the rock.
