Term
| What chemical data are used to describe igneous rocks? |
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Definition
| Weight % of oxides, mole %, or cation % |
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Term
| Which element has two cations that must be treated differently, and what are they? |
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Definition
| Iron: Ferric (Fe3+) and ferrous (Fe2+) |
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Term
| What are the IUGS classification criteria for igneous rocks? |
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Definition
- Q (quartz), A (alkali feldspar), P (plagioclase), F (foids), M (mafic minerals; olivine, pyroxene, amphibole, hornblende)
- If M < 90%, use QAPF diagram; if M > 90%, use M diagrams
- Igneous rocks within the QAPF diagram will plot within either QAP or FAP; foids/quartz cannot coexist
- Modal mineralogy is needed
- Intrusive/extrusive data is needed |
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Term
| What is the step-by-step procedure by which igneous rocks are classified? |
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Definition
- Obtain modal abundance of Q, A, P, F, M, and accessory minerals
- If M < 90%, ignore abundance of M minerals and normalise QAPF abundances
- Determine whether the rock is phaneritic (plutonic) or aphanitic (volcanic) in order to use the appropriate diagram
- Plot the values of QAP or FAP on the correct diagram and read the rock name from the field in which they intersect
- If the rock contains M > 90%, normalise the abundances of the M minerals and plot in the appropriate diagram |
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Term
| How are pyroclastic rocks classified? |
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Definition
| By bulk chemistry and the type and size of fragments (pyroclasts) |
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Term
| What is the nucleation theory of crystal growth? |
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Definition
- crystal growth occurs in response to undercooling (ie T < liquidus)
- first step is to form a thermodynamically stable nucleus
- for every degree of undercooling, there is a critical radius at which increasing the radius of the nucleus will lead to thermodynamically favourable crystal growth |
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Term
| What is the difference between homogeneous and heterogeneous nucleation? |
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Definition
Homogeneous nucleation: random fluctuations of matter in the melt cause clusters of crystalline components to form nuclei; these then redissolve into the melt
Heterogeneous nucleation: crystal nucleation occurs on pre-existing particles or surfaces, due to a lowering of the surface energy term. *Most common in nature |
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Term
| What is the diffusion theory of crystal growth? |
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Definition
- transport of material to/from a growing crystal is controlled by diffusion, ie random (thermal) motion of atoms or molecules
- usually seen as a net flux of atoms in a concentration gradient (chemical diffusion) |
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Term
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Definition
| DT=D0exp(-EA/RT), where D is the diffusion coefficient, E is the activation energy, R is the thermodynamic gas constant, and T is temperature
Although the thermodynamic driving force for crystallisation increases linearly with decreasing T, diffusion coefficients decrease exponentially with decreasing T |
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Term
| What is the crystal growth process? |
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Definition
1. Diffuse components for the crystal to the growth surface or particle
2. Agglomerate the crystal components at the surface or particle (ie, reaction at melt/crystal interface)
3. grow a new surface by attaching agglomerated material to the surface or particle (at edges, steps, or on a screw dislocation)
4. diffuse unwanted components away from surface
**The slowest of these mechanisms will determine the crystal growth mechanism and control growth morphology |
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Term
| What is the crystal morphology for small degrees of undercooling? |
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Definition
| Agglomerate crystal growth dominates the total morphology, leading to equant, coarse-grained crystals |
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Term
| What is the crystal morphology for large degrees of undercooling? |
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Definition
| Diffusion crystal growth (ie components are diffused to the growth surface/particle and then unwanted components are diffused away from the surface). This results in a lack of planar faces developing (spinifex, hopper, or straw textures). |
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Term
| What other factors contribute to the growth rate of crystals |
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Definition
- Growth rates are fastest for faces with low atomic density, usually corresponding to edges where melt volume is highest
- Growth rates are promotes on narrow crystals |
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Term
| What factors determine a rock's texture? |
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Definition
| Nucleation and growth rates |
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Term
What do the numbers 1 through 4 represent on the following diagram?
[image] |
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Definition
- Glass, no crystals
- Abundant, small, euhedral crystals
- Acicular, skeletal crystals
- Rare, large, euhedral crystals
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Term
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Definition
| Distinctly bimodal grain size distribution within a single rock (ex. large biotite crystals within a fine-grained matrix) |
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Term
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Definition
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Term
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Definition
| Interstitial, fine-grained groundmass |
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Term
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Definition
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Term
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Definition
| Crystals nucleate on pre-existing crystals, surfaces, particles |
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Term
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Definition
| Spinifex/straw/hopper growth; results when diffusion rates for materials required to grow the crystal are slower than the rate of growth- moderate-large degrees of undercooling |
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Term
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Definition
| large crystal enclosing other smaller crystals of another mineral |
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Term
| Graphic, granophyric, micropegmatitic, sympletic textures |
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Definition
| Intergrowth of two or more minerals, usually quartz and K-feldspar; results from rapid degassing of a granitic magma causing a rise in the liquidus of the melt |
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Term
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Definition
| Colour variations in a crystal, extinction position in a crystal, or mean atomic number variations (when viewed under an SEM) |
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Term
| How can zoning be induced? |
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Definition
Constant-P cooling:
- Crystal/melt equilibrium NOT maintained: rim of new composition is added around old composition; solid composition evolves down temperature faster than liquid composition; diffusion slower than crystal growth |
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Term
| What is the difference between normal, oscillotory, and reverse zoning? |
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Definition
- Normal zoning: mineral composition changes as a function of the expected down-temperature changes in the melt composition (ex plagioclase becomes more albitic)
- Oscillotory zoning: Short-term mineral compositional changes that result from transient changes in melt composition over the entire crystallisation history; may result from magma mixing
- Reverse zoning: only occurs when growth of the mineral is associated with INCREASING temperature |
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Term
| How can zoning be induced without a change in temperature? |
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Definition
| Changes in the pressure of water or any other parameters that cause solidus-liquidus relationships to shift |
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Term
| What is the difference between mesocumulate, orthocumulate, and adcumulate? |
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Definition
- Orthocumulate: interstitial liquid crystallises in place, leaving space between larger phenocrysts; phenocrysts comprise 75-85% of the rock
- Adcumulate: interstitial liquid is entirely used to form larger phenocrysts, leaving no matrix
- Mesocumulate: intermediate cumulate texture between orthocumulate and adcumulate |
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Term
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Definition
| Matrix material coarse-grained enough to see the birefringence of the grains under a standard microscope |
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Term
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Definition
| Matrix material not coarse-grained enough to see birefringence under a microscope |
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Term
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Definition
| Interstitial glass is a major component of the rock |
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Term
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Definition
| Microlites that are significantly larger than the overall groundmass grain size but still microscopic |
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Term
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Definition
| Glass is abundant and visibly encloses microlites or microphenocrysts as inclusions |
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Term
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Definition
| Glass dominates the rock and contains tiny microlites |
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Term
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Definition
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Term
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Definition
| >80% glass, with a rhyolitic to dacitic composition |
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Term
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Definition
| Trapped bubbles of vapour or gas |
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Term
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Definition
| Vesicles filled with late-stage minerals |
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Term
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Definition
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Term
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Definition
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Term
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Definition
| Fragments in pyroclastic rocks |
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Term
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Definition
| Structures in pyroclastic rocks (bends, folds) resulting from compression and deformation of ash-rich rocks after settling |
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Term
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Definition
| Squashed fragments resulting from crushing of pyroclasts after degassing |
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Term
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Definition
| Spheroid balls of ash formed during ash fall in moist air; ash clings to nucleus |
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Term
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Definition
| Consolidated deposits of lapilli, resulting from welding of fragments |
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Term
| What is the difference between a eutectic and a minimum point? |
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Definition
| The liquid phase will not migrate to the minimum unless it has the exact composition of the minimum or if fractional crystallisation, whereas a eutectic will be reached in both equilibrium and fractional crystallisation. A minimum is also not a point but rather a small gap between the two two-phase 'bunny ears'. |
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Term
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Definition
| For solids, the process by which a homogeneous mineral composition separates into two physically distinct minerals, each enriched in one of the two components in the system; the process occurs at the critical temperature |
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Term
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Definition
| The thin, intergrown mineral layers or lenses in a mineral grain that has experienced exsolution |
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Term
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Definition
| The temperature at which phase separation (ex. exsolution or unmixing occurs); marks the minimum temperature at which the homogeneous one-phase solid is stable; unique intersection with the solvus for each bulk feldspar composition |
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Term
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Definition
| a rock (or mineral) that has crystallised at conditions under which a single feldspar crystallises and then unmixes to two feldspars due to the presence of a minimum occurring above a solvus; usually water-poor or dry system at high temperatures |
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Term
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Definition
| A rock or mineral that has crystallised at conditions under which two feldspars crystallise simultaneously due to the presence of a eutectic intersecting a solvus; usually water-rich system at low temperatures |
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Term
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Definition
| the lowest possible temperature at which a liquid can exist in a binary system at which point only two phases coexist in equilibrium (a solid and a liquid) and these two phases have the same composition |
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