18-Geol-A1 Mineralogy and Petrology · December 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A1 Mineralogy and Petrology, 2013-Dec. Closed book; approved calculator permitted; 3 hours. Most questions call for a paragraph/essay answer, and point form is acceptable where appropriate. Question 1 instructs "fully explain 5 of the following 8" and Question 3 instructs "answer any THREE of the following 6".
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (mineral identification, crystal symmetry, silicate structural classification); Nesse, Introduction to Optical Mineralogy, 4th ed. (relief, interference figures, optic sign); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (phase diagrams, CIPW norm, magma viscosity, metamorphic P-T paths, subduction-zone magmatism); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sedimentary rock classification).
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 mineral is defined by five joint criteria: it is (1) naturally occurring (formed by geological processes, excluding synthetic/lab-grown or biologically manufactured analogues), (2) generally inorganic, (3) a solid at standard conditions (a few authorities admit native mercury as an exception), (4) has a definite, though commonly variable, chemical composition that can be expressed as a formula (fixed for a stoichiometric mineral such as quartz $\text{SiO}_2$, or a solid-solution range for a mineral such as plagioclase or olivine), and (5) has an ordered, crystalline internal atomic arrangement — a repeating three-dimensional lattice, which is what distinguishes a mineral from an amorphous solid such as volcanic glass or opal (a "mineraloid").
The inosilicates (chain silicates) are built by linking $\text{SiO}_4^{4-}$ tetrahedra corner-to-corner into continuous chains, sharing two of the four oxygens per tetrahedron in a single chain, or, in the double-chain sub-group, alternating tetrahedra that share three oxygens with two additional sharings across to a parallel chain.
Single-chain inosilicates are the pyroxene group (e.g. augite, diopside), with unit composition $(\text{SiO}_3)_n$, i.e. a Si:O ratio of 1:3. Double-chain inosilicates are the amphibole group (e.g. hornblende, tremolite), with unit composition $\text{Si}_4\text{O}_{11}$, a Si:O ratio of 4:11. The physical property consistent across both groups is two directions of prismatic cleavage parallel to the chains, reflecting the strong Si–O bonds within a chain versus the weaker ionic bonds holding chains together across the structure: pyroxenes cleave at approximately 87°/93° (near-orthogonal, reflecting the single-chain repeat), while amphiboles cleave at approximately 56°/124° (reflecting the wider double-chain repeat) — this cleavage-angle difference is the standard hand-specimen/thin-section test separating the two groups.
A crystal's external form and internal lattice can possess the following symmetry elements:
The particular combination of these elements present in a crystal places it into one of the 32 crystallographic point groups, which group further into the six (or, treating trigonal separately from hexagonal, seven) crystal systems — triclinic (lowest symmetry, at most a centre), monoclinic (one 2-fold axis and/or one mirror), orthorhombic (three mutually perpendicular 2-folds and/or mirrors), tetragonal (one 4-fold axis), hexagonal/trigonal (one 6-fold or 3-fold axis), and isometric/cubic (four 3-fold axes along the cube diagonals, the highest symmetry) — each defined by a characteristic minimum symmetry content and, correspondingly, a characteristic unit-cell shape (the relative lengths/angles of the crystallographic axes a, b, c and α, β, γ).
Relief is the apparent contrast, or degree to which a mineral grain "stands out" from its surroundings (the mounting medium, typically Canada balsam, n ≈ 1.54, or an adjacent grain) when viewed in plane-polarized light in thin section. It is caused by the difference in refractive index between the mineral and the medium: a large index contrast bends light strongly at the grain boundary, producing a thick, dark outline, a rough-looking surface texture, and a prominent, strongly-moving Becke line (a bright rim that migrates toward the higher-index medium as the microscope stage is lowered) — described as high relief (e.g. olivine, garnet, zircon against balsam). A small index contrast gives low relief, a smooth appearance, and a faint, barely-moving Becke line (e.g. quartz or feldspar against balsam, both close to n = 1.54). Relief is further qualified as positive (mineral index > medium index) or negative (mineral index < medium index), distinguished by the direction the Becke line moves.
The CIPW norm (Cross, Iddings, Pirsson & Washington, 1902) is a standardized calculation that converts a whole-rock chemical (oxide wt.%) analysis into a hypothetical set of normative anhydrous standard minerals (quartz, orthoclase, albite, anorthite, diopside, hypersthene, olivine, magnetite, ilmenite, apatite, etc.), assuming they crystallized in a fixed order under idealized, water-free, low-pressure equilibrium — it is not the rock's actual (modal) mineralogy. It is used to classify and compare igneous rocks on a common, purely chemical basis, which is essential for glassy or very fine-grained volcanic rocks whose true crystalline mineralogy cannot be optically resolved, and it lets petrologists plot chemically-defined rocks on classification diagrams (e.g. the normative QAPF-equivalent or the Ab-An-Or ternary) that would otherwise require a modal point count.
Basalt is a mafic volcanic rock (45–52 wt.% $\text{SiO}_2$), dominated by calcic plagioclase (labradorite–bytownite, An₅₀–₁₀₀) and clinopyroxene (augite), commonly with olivine, and little to no free quartz or alkali feldspar; it is dark-coloured, has relatively low viscosity, and is typical of mid-ocean-ridge, oceanic-island (hotspot) and back-arc settings. Andesite is an intermediate volcanic rock (52–63 wt.% $\text{SiO}_2$), with more sodic plagioclase (andesine–oligoclase) plus hornblende and/or biotite in addition to (or instead of) pyroxene, minor quartz, and a lighter overall colour; its higher silica content gives a more polymerized, higher-viscosity melt. Andesite is the characteristic rock of continental volcanic (subduction) arcs, forming by a combination of flux melting of the mantle wedge, fractional crystallization, and crustal assimilation of a basaltic parent as it rises through thicker continental crust — the compositional shift from basalt to andesite broadly tracks increasing silica polymerization, decreasing Mg/Fe content, and a shift from anhydrous to hydrous ferromagnesian minerals.
The Gibbs phase rule, $F = C - P + 2$, relates the number of degrees of freedom F (independent intensive variables, e.g. T, P, composition, that can be changed without changing the number of phases present) to the number of independently variable components C and the number of phases P coexisting at equilibrium. For petrologic T–X diagrams determined at a fixed total pressure (such as the Di–An diagram in Question 2b), pressure is not a free variable and the condensed (isobaric) phase rule applies instead: $F = C - P + 1$. This is why, on a binary T–X diagram, a field with a single phase (liquid alone) is divariant (F = 2, both T and X can vary independently), a two-phase liquidus/solidus curve is univariant (F = 1, fixing T fixes the coexisting compositions), and the eutectic point — three phases (liquid + two solids) coexisting in a two-component system — is invariant (F = 2−3+1 = 0), which is exactly why the eutectic must occur at one single, fixed temperature and composition rather than along a curve.
A mudrock is a fine-grained clastic (siliciclastic) sedimentary rock composed predominantly of silt- and clay-sized particles (< 0.0625 mm, per the Wentworth scale), dominated by clay minerals (illite, smectite, kaolinite) together with fine quartz, feldspar and mica. The group is subdivided by fissility: shale splits readily along thin, closely-spaced bedding-parallel partings (due to the parallel alignment of platy clay minerals), while mudstone is texturally identical but blocky/non-fissile. Mudrocks form by settling of the finest suspended sediment in low-energy environments — offshore marine shelf/basin, lake bottoms, floodplains, and distal deltaic/turbidite settings — where current or wave energy is too low to keep silt and clay in suspension or to winnow them away, and they are collectively the single most abundant sedimentary rock type by volume in the geologic record.
