18-Geol-A1 Mineralogy and Petrology · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A1 Mineralogy and Petrology, 2016-Dec. Closed book; no calculator permitted. Part 1 requires all six 10-mark short-answer questions (60 marks); Part 2 instructs "answer 4 of the 7 ten-mark questions" (40 marks).
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (mineral/silicate structural classification, ore mineralogy); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, phase diagrams, metamorphic reactions and facies, AFM projections, volcanic processes, ophiolites and oceanic crust, subduction-zone/rift/hotspot melting); Nesse, Introduction to Optical Mineralogy, 4th ed. (index-mineral optics).
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.
Metamorphism is isochemical: mineralogy and texture change in response to heat, pressure and/or differential stress, but the rock's bulk chemical composition (apart from volatile loss/gain in dehydration or decarbonation reactions) is essentially preserved. Metasomatism is allochemical: the rock's bulk composition itself is changed by fluid-mediated addition and removal of chemical components, independent of any dehydration/decarbonation the rock might also undergo.
Hornfels is the fine-grained, non-foliated rock produced by contact (thermal) metamorphism of the country rock: it recrystallizes to new mineral assemblages appropriate to the local temperature, but its bulk composition stays essentially that of the original protolith.
Skarn is a coarse-grained, Ca–Fe–Mg–Mn calc-silicate rock (garnet, pyroxene, wollastonite, epidote) produced where a hot, chemically active fluid exchanges components between an intrusion and an adjacent carbonate country rock — Si, Fe, Al and Mg typically migrating outward from the pluton and Ca migrating inward.
Both processes occur together at the same intrusion–carbonate contact and are driven by the same heat source; they are distinguished only by whether mass was exchanged (metasomatism → skarn, zoned outward from endoskarn through exoskarn) or not (isochemical contact metamorphism → hornfels, typically further from the contact or where the protolith was not a reactive carbonate). A single contact aureole commonly grades outward: endoskarn (in the pluton) → exoskarn (replacing the carbonate nearest the contact) → marble/hornfels (isochemically recrystallized rock further out).
