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.
Bowen's reaction series predicts the crystallization order of a cooling mafic magma: a discontinuous branch (olivine → orthopyroxene → clinopyroxene → amphibole → biotite, each phase reacting with the melt to form the next as $T$ falls) runs in parallel with a continuous branch (Ca-rich → Na-rich plagioclase). Olivine and Cr-spinel sit at the very top of the discontinuous branch — they crystallize first, at the highest liquidus temperature, from a mafic/ultramafic parent melt.
Olivine (and, where present, Cr-spinel and early orthopyroxene) crystallizes first from a cooling mafic parent magma and is also denser than the coexisting liquid, so individual crystals sink and accumulate on the floor of the magma chamber by gravitational settling, building a crystal-mush layer (the cumulus pile) with residual liquid trapped between grains. Because settling continues to draw the same early, dense, discontinuous-branch phases out of the liquid, the accumulated pile is progressively enriched in exactly the minerals Bowen's series predicts crystallize earliest — producing monomineralic to bimineralic ultramafic cumulate rock (dunite: essentially pure cumulus olivine; peridotite: olivine + pyroxene; pyroxenite: cumulus pyroxene) even though the parent magma itself was only basaltic in bulk composition and never itself had an ultramafic liquid composition. The residual liquid left behind, now depleted in the early-settled components, continues down the reaction series (pyroxene, then amphibole) to crystallize progressively less mafic rock higher in the intrusion.
