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.
Bowen's reaction series is a schematic model, based on Bowen's melting-crystallization experiments, of the order in which minerals crystallize from a cooling basaltic magma. It has two parallel branches: a discontinuous branch, on which each mafic mineral reacts with the remaining melt to be replaced by the next as temperature falls (olivine → orthopyroxene → clinopyroxene → amphibole → biotite), and a continuous branch, on which a single mineral, plagioclase, progressively changes composition without a discrete reaction, from Ca-rich to Na-rich. Both branches converge at low temperature on the last, most felsic phases: K-feldspar, muscovite and quartz.
The series is, at any instant, a snapshot of which minerals are in equilibrium with a melt of a given composition and temperature; because melt composition evolves continuously toward the residual, low-temperature end as earlier (higher-temperature) minerals crystallize and are removed, the series simultaneously predicts the sequence of mineral assemblages and, if crystals are ever separated from melt, the sequence of derivative rock compositions. A parent basaltic magma that crystallizes completely in place, with no crystal–melt separation, solidifies as basalt/gabbro, carrying the whole assemblage. But if early, dense minerals (olivine, then pyroxene) are physically removed — by gravitational settling into a cumulate pile in a layered mafic intrusion — the residual liquid is left depleted in Mg/Fe/Ca and enriched in Si/Na/K/Al, continuing to fractionate down the series toward andesitic/dioritic and, ultimately, granodioritic/granitic compositions. This process, fractional crystallization, is the single mechanism by which Bowen's series explains how the full compositional spectrum of igneous rock — ultramafic through felsic — can descend from one basaltic parent magma.
