ES 4901 Petrology
Homework 3
Chapter 3 Igneous Textures and Petrography
A.
Memorization for Match the Terms later
Abbreviations Tx = texture T = Temperature, xtals = crystals rxn = reaction
M = melting
1.
Surface Energy
excess energy at the surface of a material
2.
Instability
separation of clusters of compatible ions due to high surface charge.
3.
Nucleation
forming a critical-sized embryonic crystal
4.
Undercooling
cooling of a melt below the crystallization T of some mineral
5.
Crystal Growth
addition of ions to an existing crystal
6.
Diffusion
the spread of something more widely
7.
Euhedral Crystal
well-formed, sharp, easily recognized faces. Opposite: Anhedral
8.
Epitaxis
preferred nucleation of one mineral on another pre-existing mineral
9.
Porphyritic
having distinctly different crystal sizes (Igneous term)
10. Phenocrysts
the large xtals in a porphyry
11. Poikilitic
phenocrysts contain inclusion of minerals they surrounded
12. Interstitial
filling the spaces between
13. Ophitic Tx
clinopyroxenes (Augite, etc.) surround Plagioclase laths
14. Granophyric Tx
angular intergrowth, ex. K-spar and Quartz form simul. low H2O melt
15. Graphic Tx
coarse Granophyric Tx, looks like square writing
16. Resorption
dissolution of a mineral back into the melt.
17. Decompression M
a decrease in pressure. *(see below)
18. Dewatering
raises melting point; forming minerals now more undercooled
19. Trachytic Tx
microphenocrysts aligned due to flow
20. Felty (pilotaxitic) Tx
microphenocrysts randomly oriented
21. Intergranular Tx
Pyroxene and Plagioclase xtals similar in size
* Pressure affects melting points. Most substances expand as they melt. An increase of pressure
retards the melting of these substances. A decrease in pressure allows expansion and melting.
22. Orthocumulate
melt between settled crystals isolated, remains, forms interstitial xtals
23. Adcumulate
melt between settled crystals not isolated, squashed out
24. Polysynthetic twins
multiple twins aligned in parallel; loosely “Albite twins”
25. Carlsbad twin
two crystals, one rotates parallel c-axis (perpendicular to a-axis)
26. Sericite
a fine grain potassium mica, can be an alteration product of K-spars
27. Myrmekitic
an intergrowth of quartz in a single crystal of Plagioclase
B. Mixed Format Abbreviations s = slide
1. Name two hydrous minerals
2. What does glass (no crystals) look like under crossed polars?
3. Explain the role of water in the formation of large crystal in a Pegmatite s9
lower viscosity, easier diffusion, better growth
4. Explain why a melt at some constant temperature can produce different minerals that
nucleate and grow at different rates. s10
5. Explain the consequences of rapid dewatering of a magma. s12
Loss water, raise melting po int, currently forming undercooled, to n ucl, aphanitics
6. Which grows faster s13 & 14
a. a plane with a high density of lattice points, smaller interplanar distance, and low
surface energy, and a larger pocket of undepleted melt
b. a plane with a low density of lattice points, larger interplanar distance, and high
surface energy, and a smaller pocket of undepleted melt
7. For a growing crystal, which area usually has a greater volume of undepleted melt from
which it can obtain components? s14
a. an edge between two planar sides
b. a planar side
8. In Bowen’s Reaction Series, which Plagioclase end member forms at a higher
temperature? Choose [ Anorthite CaAl (AlSi2O8 ) OR Albite NaSi (AlSi2O8 ) ]
9. Igneous Plagioclase crystals may show a CaAl - rich composition in the center, and a
NaSi - rich composition on the outside. Slide 18. The reason is Plagioclase does not reequilibrate with the melt when the cooling melt changes composition, which would
require substitution of Al for Si in one position. This is difficult because:
a. Al-O and Si-O bonds are very strong
b. Al+3 is a very slow diffuser
c. both of the above
10. Oscillatory Zoning. Electron Microprobe scans of compositionally zoned Plagioclase
crystals from center to rim show zones of decreasing Anorthite
CaAl (AlSi2O8 ) , followed by a sharp increase in Anorthite. Slide 19.
Give one possible explanation for this.