18-Geol-A1 Mineralogy and Petrology · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A1 Mineralogy and Petrology, 2016-May. Closed book; no calculator permitted. Part 1 requires all six 10-mark short-answer questions (60 marks); Part 2 instructs "four of the seven 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, cumulates, metamorphic reactions and facies, AFM projections, volcanic processes, oceanic crust petrogenesis); Nesse, Introduction to Optical Mineralogy, 4th ed. (index-mineral optics); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sedimentary/pyroclastic textural context).
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 (mass transfer in solution), 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 (aside from any volatiles driven off).
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 (limestone/dolostone) country rock — typically Si, Fe, Al and Mg diffusing/advecting outward from the pluton and Ca diffusing inward, so both sides of the contact gain material they did not originally contain.
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 developed 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 carbonate/pelite further from the contact, beyond significant fluid-driven mass transfer).
