Shared Flashcard Set

Details

Igneous Petrology
Definitions and processes
52
Geology
Undergraduate 3
10/05/2013

Additional Geology Flashcards

 


 

Cards

Term
What chemical data are used to describe igneous rocks?
Definition
Weight % of oxides, mole %, or cation %
Term
Which element has two cations that must be treated differently, and what are they?
Definition
Iron: Ferric (Fe3+) and ferrous (Fe2+)
Term
What are the IUGS classification criteria for igneous rocks?
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
Term
What is the step-by-step procedure by which igneous rocks are classified?
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
Term
How are pyroclastic rocks classified?
Definition
By bulk chemistry and the type and size of fragments (pyroclasts)
Term
What is the nucleation theory of crystal growth?
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
Term
What is the difference between homogeneous and heterogeneous nucleation?
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
Term
What is the diffusion theory of crystal growth?
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)
Term
What is Fick's law?
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
Term
What is the crystal growth process?
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
Term
What is the crystal morphology for small degrees of undercooling?
Definition
Agglomerate crystal growth dominates the total morphology, leading to equant, coarse-grained crystals
Term
What is the crystal morphology for large degrees of undercooling?
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).
Term
What other factors contribute to the growth rate of crystals
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
Term
What factors determine a rock's texture?
Definition
Nucleation and growth rates
Term

What do the numbers 1 through 4 represent on the following diagram? 

 

[image]

Definition
  1. Glass, no crystals
  2. Abundant, small, euhedral crystals
  3. Acicular, skeletal crystals
  4. Rare, large, euhedral crystals
Term
Porphyritic
Definition
Distinctly bimodal grain size distribution within a single rock (ex. large biotite crystals within a fine-grained matrix)
Term
Phenocrysts
Definition
Crystals
Term
Matrix
Definition
Interstitial, fine-grained groundmass
Term
Vitrophyric
Definition
Glassy matrix
Term
Epitaxial
Definition
Crystals nucleate on pre-existing crystals, surfaces, particles
Term
Dendritic
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
Term
Poikilitic
Definition
large crystal enclosing other smaller crystals of another mineral
Term
Graphic, granophyric, micropegmatitic, sympletic textures
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
Term
What indicates zoning?
Definition
Colour variations in a crystal, extinction position in a crystal, or mean atomic number variations (when viewed under an SEM)
Term
How can zoning be induced?
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
Term
What is the difference between normal, oscillotory, and reverse zoning?
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
Term
How can zoning be induced without a change in temperature?
Definition
Changes in the pressure of water or any other parameters that cause solidus-liquidus relationships to shift
Term
What is the difference between mesocumulate, orthocumulate, and adcumulate?
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
Term
Microlites
Definition
Matrix material coarse-grained enough to see the birefringence of the grains under a standard microscope
Term
Crystallites
Definition
Matrix material not coarse-grained enough to see birefringence under a microscope
Term
Intersertal
Definition
Interstitial glass is a major component of the rock
Term
Microphenocrysts
Definition
Microlites that are significantly larger than the overall groundmass grain size but still microscopic
Term
Hyalo-ophitic
Definition
Glass is abundant and visibly encloses microlites or microphenocrysts as inclusions
Term
Hyalopilitic
Definition
Glass dominates the rock and contains tiny microlites
Term
Holohyaline
Definition
Glassy texture
Term
Obsidian
Definition
>80% glass, with a rhyolitic to dacitic composition
Term
Vesicles
Definition
Trapped bubbles of vapour or gas
Term
Amygdules
Definition
Vesicles filled with late-stage minerals
Term
Scoria
Definition
mafic vesicular rock
Term
Pumice
Definition
Felsic vesicular rock
Term
Tephra
Definition
Fragments in pyroclastic rocks
Term
Eutaxitic
Definition
Structures in pyroclastic rocks (bends, folds) resulting from compression and deformation of ash-rich rocks after settling
Term
Filamme
Definition
Squashed fragments resulting from crushing of pyroclasts after degassing
Term
Accretionary lapilli
Definition
Spheroid balls of ash formed during ash fall in moist air; ash clings to nucleus
Term
Pisolitic
Definition
Consolidated deposits of lapilli, resulting from welding of fragments
Term
What is the difference between a eutectic and a minimum point?
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'.
Term
Exsolution
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
Term
Exsolution lamellae
Definition
The thin, intergrown mineral layers or lenses in a mineral grain that has experienced exsolution
Term
Critical temperature
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
Term
Hypersolvus composition
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
Term
Subsolvus composition
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
Term
Minimum
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
Supporting users have an ad free experience!