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.
All three minerals below are common, ore-forming sulfides, but each has one unambiguous, low-effort diagnostic property that separates it from the other common brassy or metallic sulfides.
| Mineral | Formula | Diagnostic property |
|---|---|---|
| Pyrite | $\text{FeS}_2$ | Brass-yellow ("fool's gold"), but hardness 6–6.5 — cannot be scratched with a knife (unlike chalcopyrite or gold); typically forms cubic or pyritohedral crystals with fine striated faces, and shows no cleavage. |
| Galena | $\text{PbS}$ | Perfect cubic cleavage in three directions at $90\,{}^{\circ}$, breaking into small cubes; lead-gray colour, metallic luster, and unusually high density (specific gravity ≈7.6) for its softness (H = 2.5) — heavier in the hand than almost any other common mineral of similar size. |
| Sphalerite | $\text{ZnS}$ | Perfect dodecahedral cleavage in six directions (unique among common sulfides); resinous, non-metallic to submetallic luster; colour is highly variable (pale yellow to black) depending on Fe content, but streak is always lighter than the mineral's body colour. |
All three form in similar hydrothermal ore environments (veins, skarns, and volcanogenic massive sulfide deposits), so composition and paragenetic setting alone are unreliable identification criteria; the physical-property tests above (hardness for pyrite, cleavage geometry and density for galena, cleavage-direction count and streak-versus-colour contrast for sphalerite) are chosen specifically because each measures a property fixed by crystal structure rather than trace chemistry, and so gives a reproducible result regardless of which deposit the specimen came from. A student who memorizes "brassy and hard = pyrite, heavy and cubic = galena, resinous and six-cleavage = sphalerite" can correctly separate the three common sulfides from a hand sample with no other equipment beyond a knife blade and a streak plate.
