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.
All three carbonate minerals below share the same rhombohedral cleavage and the $(\text{CO}_3)^{2-}$ anion group, so they are distinguished from one another primarily by their reaction to dilute hydrochloric acid and by density/hardness, not by cleavage habit.
| Mineral | Formula | Diagnostic property |
|---|---|---|
| Calcite | $\text{CaCO}_3$ | Effervesces vigorously in cold, dilute HCl — the single fastest and strongest acid reaction of the three, immediately diagnostic in the field. |
| Dolomite | $\text{CaMg}(\text{CO}_3)_2$ | Effervesces only weakly, or only when powdered or in warm HCl — the key test separating it from calcite in hand specimen, since the two share nearly identical cleavage and appearance; crystal faces are often visibly curved ("saddle-shaped"). |
| Siderite | $\text{FeCO}_3$ | Does not effervesce in cold dilute HCl (reacts only slowly when powdered/warmed); distinguished from calcite/dolomite by its higher density, slightly greater hardness, and a tan-to-brown limonitic weathering stain from its iron content. |
