18-Geol-A1 Mineralogy and Petrology · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 18-Geol-A1 Mineralogy and Petrology, 2018-Dec. Closed book; no calculator permitted.
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate/oxide structural classification, mineral chemistry and formulas, crystal systems); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, Bowen's reaction series, metamorphic agents/facies, volcanic and pyroclastic processes, partial melting); 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 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, as it continues to crystallize. 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 stable in equilibrium with a melt of a given composition and temperature; because melt composition itself evolves continuously toward the residual, low-temperature end as the earlier (higher-temperature) minerals crystallize and are removed, the series simultaneously predicts the sequence of mineral assemblages and, if crystals are ever physically separated from melt, the sequence of derivative rock compositions. A parent basaltic magma that crystallizes and cools completely in place, with no separation of crystals from melt, solidifies as basalt/gabbro, carrying the whole discontinuous-plus-continuous mineral assemblage. But if early, dense minerals (olivine, then pyroxene) are physically removed from the liquid — by gravitational settling into a cumulate pile, as in a layered mafic intrusion — the residual liquid is left depleted in Mg/Fe/Ca and enriched in Si/Na/K/Al, and it continues fractionating down the series to progressively more evolved compositions: from basaltic, through andesitic/dioritic (amphibole–intermediate plagioclase dominant) and on to granodioritic/granitic (biotite, K-feldspar, muscovite, quartz) rock, if fractionation proceeds far enough. This process, fractional crystallization, is the single mechanism by which Bowen's series explains how the entire compositional spectrum of igneous rock — ultramafic through felsic — can descend from one basaltic parent magma.
