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.
A cumulate is an igneous rock formed by the mechanical accumulation of crystals (the cumulus phase, e.g. settled olivine or chromite) from a magma, with the pore space between the accumulated grains occupied by trapped liquid (intercumulus liquid) that subsequently crystallizes in place, either as overgrowths on the cumulus grains or as distinct intercumulus minerals. The three recognized categories, defined by how much of that trapped liquid remains (as a proportion of total rock volume) once crystallization is complete, are:
| Category | Approx. intercumulus liquid | Texture |
|---|---|---|
| Orthocumulate | Highest (≈7–25%) | Cumulus grains are a relatively minor framework; abundant well-formed intercumulus minerals/overgrowths crystallized from a large trapped-liquid fraction. |
| Mesocumulate | Intermediate (≈1–7%) | Transitional texture between the other two end-members. |
| Adcumulate | Lowest (≈0–1%) | Cumulus grains are in near-total mutual contact (tightly packed, often with straight/annealed grain boundaries); little to no trapped-liquid component, produced by efficient expulsion/compaction of intercumulus liquid or extensive synneusis-driven overgrowth during burial compaction. |
