18-Geol-A1 Mineralogy and Petrology · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exam — 18-Geol-A1 Mineralogy and Petrology. 3 hours, closed book, no calculator permitted. Two parts, twelve ten-mark short-answer questions in total: Part 1 (Q1–5) requires all five questions (50 marks); Part 2 (Q6–12) is printed as "answer 5 of the 7" on one page and "answer 5 of the 5" on another (the paper's own instructions disagree on the count) — every question in both parts is solved in full below so this set also serves as a complete study reference. This sitting is treated as undated because the paper is internally inconsistent about its own date: the first-page footer reads "May 2018" (matching the 18-Geol-A1 code, in use from December 2018 onward) while a later page's footer reads "19-Geol-A1 / May 2019". No exam date is asserted.
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate/oxide structural classification, mineral chemistry, solid solution and exsolution, crystal systems); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, Bowen's reaction series, tectonic settings of magmatism and melting, ophiolites, LIPs, anatexis, contact/thermal metamorphism).
There is no numeric given data anywhere in this qualitative/descriptive paper.
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.
Two of the most common Fe-oxide minerals — magnetite and hematite — are easily confused by hand-specimen colour alone, but two simple field tests, targeting two genuinely different physical properties, separate them unambiguously.
| Mineral | Formula | Diagnostic property |
|---|---|---|
| Magnetite | $\text{Fe}_3\text{O}_4$ | Strongly magnetic (attracted to a hand magnet; natural lodestone specimens can themselves act as a magnet); black streak; isometric, octahedral crystal habit. |
| Hematite | $\text{Fe}_2\text{O}_3$ | Non-magnetic (or at most very weakly so); diagnostic reddish-brown ("blood-red" to "Indian red") streak regardless of hand-specimen colour, which itself ranges from black/steel-gray (specular) to dull red-earthy; rhombohedral crystal habit. |
Streak is decisive because it reveals a mineral's true colour in fine powder form, removing the surface-oxidation and grain-size effects that make hand-specimen colour unreliable for both minerals; magnetism is decisive because it directly probes structure — magnetite's inverse-spinel structure hosts both $\text{Fe}^{2+}$ and $\text{Fe}^{3+}$, giving it strong ferrimagnetism, while hematite's corundum-type structure hosts only $\text{Fe}^{3+}$ and is only weakly (canted-antiferromagnetically) magnetic, not attracted to an ordinary hand magnet. Used together, the two tests are unambiguous even when hand-specimen colour overlaps.
The two minerals also differ economically and petrogenetically: magnetite is a common accessory phase across nearly every igneous and metamorphic rock type and forms directly by magmatic crystallization or metamorphic recrystallization, whereas much economic hematite instead forms as a low-temperature, near-surface oxidation/weathering product (including the specular hematite coatings on many banded iron formations) or by diagenetic replacement, so the presence of abundant primary hematite versus magnetite in a rock is itself a rough indicator of its oxidation state and formation environment.
