18-Geol-A1 Mineralogy and Petrology · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exam — 18-Geol-A1 Mineralogy and Petrology. 3 hours, closed book, no calculator permitted. Two parts, twelve ten-mark short-answer questions in total: Part 1 (Q1–5) requires all five questions (50 marks); Part 2 (Q6–12) is printed as "answer 5 of the 7" on one page and "answer 5 of the 5" on another (the paper's own instructions disagree on the count) — every question in both parts is solved in full below so this set also serves as a complete study reference. This sitting is treated as undated because the paper is internally inconsistent about its own date: the first-page footer reads "May 2018" (matching the 18-Geol-A1 code, in use from December 2018 onward) while a later page's footer reads "19-Geol-A1 / May 2019". No exam date is asserted.
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate/oxide structural classification, mineral chemistry, solid solution and exsolution, crystal systems); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, Bowen's reaction series, tectonic settings of magmatism and melting, ophiolites, LIPs, anatexis, contact/thermal metamorphism).
There is no numeric given data anywhere in this qualitative/descriptive paper.
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.
The seven crystal systems classify every crystalline mineral by the symmetry and relative lengths/angles of its crystallographic axes; two contrasting examples — one at the high-symmetry end, one at the lowest — are described below.
| System | Axial lengths / angles |
|---|---|
| Hexagonal | 4 axes: 3 equal-length horizontal axes ($a_1=a_2=a_3$) at $120\,{}^{\circ}$ to each other, plus 1 vertical axis $c\ne a$ at $90\,{}^{\circ}$ to the others |
| Triclinic | 3 unequal axes ($a\ne b\ne c$); all three interaxial angles unequal and $\ne 90\,{}^{\circ}$ — the lowest-symmetry system, with no restriction on either axial length or angle |
The hexagonal system has a single 6-fold (or 6/m) principal symmetry axis along $c$, with three equal horizontal axes at $120\,{}^{\circ}$ producing 6-sided prismatic crystal forms. Example: beryl ($\text{Be}_3\text{Al}_2\text{Si}_6\text{O}_{18}$), which forms classic hexagonal prisms terminated by a basal pinacoid, with no cleavage (a cyclosilicate, ring silicate).
The triclinic system has the lowest possible symmetry of any crystal system — only a centre of symmetry (or none at all) — because none of its three axes are equal and none of its three interaxial angles are constrained to $90\,{}^{\circ}$. Example: plagioclase feldspar (e.g. albite, $\text{NaAlSi}_3\text{O}_8$, or anorthite, $\text{CaAl}_2\text{Si}_2\text{O}_8$), which shows two cleavages very close to but not exactly $90\,{}^{\circ}$ (the small deviation is diagnostic against monoclinic K-feldspar) and characteristic polysynthetic albite twinning.
Ranking all seven systems by symmetry — isometric, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic, triclinic, from highest to lowest — hexagonal and triclinic sit at opposite ends of that spectrum: hexagonal's four-axis frame with three equal, symmetric horizontal axes gives its minerals the highest rotational symmetry (6-fold) available outside the isometric system, while triclinic's total lack of axial or angular constraint leaves its minerals with essentially none.
