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.
Metasomatism is allochemical metamorphism: a chemically active fluid phase actively adds and/or removes components across a rock's boundary, changing its bulk composition, in contrast to ordinary (isochemical) metamorphism, where bulk composition is essentially conserved and only mineralogy/texture re-equilibrate.
A skarn is the coarse-grained, Ca–Fe–Mg–Mn calc-silicate rock (typically garnet–pyroxene–wollastonite–epidote) produced by metasomatism at, most commonly, an igneous–carbonate contact. A hot, chemically active fluid derived largely from the cooling intrusion carries Si, Fe, Al and Mg outward into the adjacent limestone/dolostone, while $\text{Ca}$ and $\text{CO}_2$ diffuse back inward — a bidirectional exchange that produces two mirror-image metasomatic zones straddling the same contact: exoskarn on the country-rock (carbonate) side, and endoskarn within the margin of the intrusion itself.
Skarn is simply the diagnostic rock-type expression of metasomatism at a reactive (carbonate) contact: the sustained, open-system fluid flow required for skarn formation is exactly what defines the process as metasomatic rather than isochemical — without cross-boundary mass transfer, the same intrusion-into-limestone contact would instead produce only a thermally-recrystallized, compositionally unchanged marble or hornfels.
