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.
All seven reaction types below are solved so this set is a complete study reference; the exam asks for any four.
A reaction among fixed-composition (pure) phases that, by the condensed (isobaric) phase rule $F = C - P + 1$, has zero degrees of freedom at fixed pressure and so occurs abruptly at one specific temperature (or along one curve on a full P–T diagram) rather than over a range. Example: the polymorphic transitions among the $\text{Al}_2\text{SiO}_5$ triple point minerals — kyanite ↔ andalusite ↔ sillimanite — each boundary is a sharp univariant curve on a P–T diagram, so crossing it converts the rock essentially instantaneously (in P–T terms) from one polymorph to the other.
A reaction involving one or more phases of variable (solid-solution) composition, which therefore proceeds continuously over a range of P–T (or bulk composition) rather than at a single point, because the extra compositional degree(s) of freedom raise F above zero (F ≥ 2 in the general Gibbs form). Example: the continuous Fe–Mg exchange reaction between garnet and biotite as grade increases — garnet progressively becomes more Mg-rich (and biotite more Fe-rich) over a wide T interval rather than snapping to a new assemblage at one temperature, which is exactly what makes this exchange usable as a continuous geothermometer.
A reaction that converts one solid phase (or assemblage) directly into another with no fluid phase involved (no $\text{H}_2\text{O}$ or $\text{CO}_2$ produced or consumed), proceeding by solid-state diffusion/recrystallization alone. Example: the $\text{Al}_2\text{SiO}_5$ polymorphic transformations of part (a) are themselves solid–solid reactions (kyanite recrystallizing directly to sillimanite involves no volatile species at all).
A reaction that releases structurally-bound $\text{H}_2\text{O}$ as metamorphic grade increases, progressively converting hydrous mineral phases to less-hydrous or anhydrous ones. Example: the classic upper-amphibolite-facies breakdown of muscovite + quartz to K-feldspar + an $\text{Al}_2\text{SiO}_5$ polymorph + water, $\text{KAl}_2(\text{AlSi}_3\text{O}_{10})(\text{OH})_2 + \text{SiO}_2 \rightarrow \text{KAlSi}_3\text{O}_8 + \text{Al}_2\text{SiO}_5 + \text{H}_2\text{O}$, which marks the muscovite-out isograd and, by releasing free water (and locally triggering partial melting), is a major control on migmatization at the highest metamorphic grades.
A reaction that releases $\text{CO}_2$ as grade increases, breaking down carbonate minerals in an impure (siliceous) carbonate protolith. Example: calcite + quartz reacting to wollastonite plus $\text{CO}_2$, $\text{CaCO}_3 + \text{SiO}_2 \rightarrow \text{CaSiO}_3 + \text{CO}_2$, the classic contact-metamorphic reaction that produces wollastonite skarn/hornfels in a siliceous limestone aureole.
A reaction in which one or more elements change oxidation state, commonly buffered by coexisting Fe-oxide/Fe-silicate phases that fix the oxygen fugacity of the system. Example: the magnetite–hematite ($\text{Fe}_3\text{O}_4 \leftrightarrow \text{Fe}_2\text{O}_3$) buffer reaction in a metamorphosed banded iron formation, where the proportion of magnetite to hematite records the ambient $f_{\text{O}_2}$ during metamorphism.
A reaction that redistributes cations (most commonly Fe2+ and Mg2+) between two coexisting mineral phases without destroying or creating any mineral or changing the modal mineralogy — only the compositions of the existing phases shift. Example: the Fe–Mg exchange between coexisting garnet and biotite already introduced in part (b); because the equilibrium partitioning of Fe/Mg between the two minerals is temperature-dependent, this exchange is the basis of the garnet–biotite exchange geothermometer.
