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18-Geol-A1 Mineralogy and Petrology · May 2016

Question 12 of 13: AFM Diagrams — Barrovian Index Mineral Progression

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

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 12: AFM Diagrams — Barrovian Index Mineral Progression (Part 2 – 10 marks)

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.

An AFM diagram is a ternary projection (Thompson, 1957) tailored to pelitic (metapelite) bulk compositions, with apices A = $\text{Al}_2\text{O}_3 - (\text{K}_2\text{O}+\text{Na}_2\text{O})$ (excess alumina beyond that tied up in feldspar/mica), F = $\text{FeO}+\text{MnO}$ and M = $\text{MgO}$, all projected geometrically from the phases muscovite, quartz and $\text{H}_2\text{O}$ (assumed present in excess) so that a single triangle can represent the Fe–Mg–Al mineral chemistry of a pelite independent of its (variable) K/Na and Si content.

Barrovian progression on the AFM diagram

As grade increases along the Barrovian (medium-P/T) path, the index minerals appear at their characteristic AFM positions and progressively restructure the tie-line network:

  1. Chlorite zone Chlorite (an F–M phase near the F–M edge) is the stable Fe–Mg phase, coexisting with muscovite.
  2. Biotite zone Biotite (plotting closer to the A apex than chlorite, reflecting its K/Al content) appears; chlorite + biotite + muscovite form a three-phase field with tie-lines linking chlorite to biotite.
  3. Garnet zone Almandine-rich garnet (plotting near the F apex, essentially pure FeO end-member for a first approximation) appears via a discontinuous reaction consuming chlorite; a garnet–biotite tie-line now cuts across the diagram, and garnet grows at chlorite's expense as grade rises further (garnet's field expands, chlorite's shrinks).
  4. Staurolite zone Staurolite (a high-A, Fe-rich phase) appears from a reaction consuming chlorite + garnet (or chlorite + muscovite), adding a new tie-line triangle staurolite–biotite–garnet and marking the point where chlorite is finally eliminated from Fe-rich bulk compositions.
  5. Kyanite zone Staurolite breaks down (staurolite + muscovite + quartz → kyanite + biotite + H2O-type reaction) and kyanite (plotting at the pure-A apex) appears, with a kyanite–biotite–garnet tie-triangle replacing the staurolite-bearing one.
  6. Sillimanite zone Kyanite inverts (solid–solid, per Question 8a) to sillimanite, which occupies the same A-apex position; the highest-grade Barrovian assemblage is sillimanite + K-feldspar (once muscovite finally breaks down) + garnet + biotite.
A F M Ky/Sil St Grt Bt Chl increasing grade
Schematic AFM projection: successive Barrovian index minerals (Chl→Bt→Grt→St→Ky→Sil, blue tie-lines) plot at progressively more A- and F-rich positions with increasing grade (red dashed arrow), each new mineral's appearance restructuring the local tie-line network.