Transcript Introduction to Environmental Geochemistry

Petrology Lecture 3
Igneous Rock Textures
GLY 4310 - Spring, 2012
1
Primary
• Form during solidification
• They result from interactions between
mineral crystals and melt
2
Secondary
• Develop by alteration of the rock after
crystallization
3
Nucleation
• Clusters of a few tens of ions are essentially
all surface
• Ratio of surface area/volume is fantastically
high
• Ions on the surface have unbalanced
charges because they are not surrounded
completely by other ions, and are easily
disrupted
• Nucleation usually requires undercooling 4
Growth
• Involves the addition of ions to the
nucleated cluster
• Some crystals have preferred directions of
growth
5
Rate of Diffusion
• Controls movement of ions in many
magmas
• Determines the rate of dissipation of the
heat of crystallization
6
Cooling Rate
• Slow cooling allows system to maintain
thermodynamic equilibrium
• Rapid cooling contributes to a nonequilibrium system
7
Nucleation
vs. Growth
8
Blue Glassy Pahoehoe
• Large embayed olivine
phenocryst with
smaller plagioclase
laths and clusters of
feathery augite
nucleating on
plagioclase.
Magnification ca. 400
X.
© John Winter and Prentice Hall.
9
Blue Glassy Pahoehoe
• Feathey quenced
augite crystal
nucleating on
plagioclase and
growing in a semiradiating form
outwards
• Mag. 2000x
© John Winter and Prentice Hall.
10
Available Liquid
a
b
• The volume of liquid
available to the edge of a
crystal is larger than to a
face, and a corner has
even greater available
liquid. (left)
• The end of a slender
crystal will have the
largest available liquid.
(right)
© Chapman and Hall. London.
11
Zoned Hornblende
© John Winter
and Prentice Hall.
• Field of view 1 mm
12
Zoned
Plagioclase
© John Winter and
Prentice Hall.
• Carlsbad twin
• Field of view o.3 mm
13
Grain Shape
•
•
•
•
Mineral Term
Euhedral
Subhedral
Anhedral
Rock Term
Idiomorphic
Hypidomorphic
Xenomorphic
14
Euhedral
Crystal
© John Winter and
Prentice Hall.
• Euhedral early pyroxene with late interstitial
plagioclase
• Field of view 5 mm
15
Dimension Relationships
•
•
•
•
Mineral term
Equant
Prismatic
Tabular
Rock term
Massive
Lineated
Foliated
16
Ophitic
Texture
© John Winter and
Prentice Hall.
• Pyroxene envelops plagioclase laths
• Field of view 1 mm
17
Granophyric
Texture
© John Winter and
Prentice Hall.
• Quartz-alkali fldspar intergrowth
• Field of view 1 mm
18
Graphic
Texture
© John Winter and
Prentice Hall.
• Single crystal of cuniform quartz intergrown
with alkali feldspar
19
Pyroxene Replacing Olivine
• Left – Olivine mantled by pyroxene, ppl
• Right – CN – Olivine is extinct, Opx stands
out
• © John Winter and Prentice Hall.
20
Dehydration
Rim
© John Winter and
Prentice Hall.
• Hornblende phenocryst dehrates to Fe-oxides
plus pyroxene due to pressure release on
eruption
21
• Width 1 mm
Embayed
Texture
© John Winter and
Prentice Hall.
• Field of view 0.3 mm
• Partially resorbed olivine phenocryst
22
Sieve
Texture
© John Winter and
Prentice Hall.
• Plagioclase phenocrysts
• Field of view 1 mm
23
Trachytic
Texture
• Sub-parallel alkali feldspar laths form
sheaves and swirls around earliercrystallised minerals
• CN, medium power
24
Pilotaxic or Felty Texture
© John Winter
and Prentice
Hall.
• Microphenocrysts are randomly aligned
25
Flow
Banding
© John Winter and
Prentice Hall.
• Andesite, Mt. Rainier
• Long-handled hammer for scale
26
Intergranular
Texture
© John Winter and Prentice
Hall.
• Columbia River Basalt Group
• Width 1 mm
27
Carlsbad
Twin
© John Winter and
Prentice Hall.
• Form as the result of mistakes during
growth
• Field of view ≈ 1 mm
28
Albite
Twinning
© John Winter and Prentice
Hall.
• Also thought to be form as the result of
mistakes during growth
• Field of view ≈ 1 mm
29
Tartan Twinning
• Microcline
• Field of view ≈ 1 mm
© John Winter and
Prentice Hall.
30
Deformational
Albite
Twinning
© John Winter and
Prentice Hall.
• Typically occurs in nearly pure Ab
• Note that twins “pinch-out” at the edge
• Width 1 mm
31
Exsolution Textures
• Perthite - The host is K-spar, with albite
lamellae appearing as a coherent
intergrowth
 Coherent means the exsolved phase lattices
have a specific relationship to the host lattice.
• Antiperthite - The host is albite, with K-spar
lamellae showing a coherent intergrowth
32
Types of Perthite
• In perthite, intergrowths may sometimes be seen
by the unaided eye
• In microperthite, however, they are
distinguishable only microscopically
• In cryptoperthite the crystals are so small that the
separation can be detected only by X-ray
diffraction
• Perthite was originally thought to be a single
mineral, described at a locality near Perth,
Ontario, from which its name is derived
33
Bronzite
Photomicrograph
• Bronzite crystal from an
ultramafic rock
• Thin lamellae of a calciumrich species, probably
pigeonite, have separated
from the bronzite, and the
host (grayish) thus has a very
low calcium content
(magnified about 40×)
34
Augite Pigeonite
• Complex
separation of
augite from an
inverted
pigeonite
(magnified about
70.4×)
35
Ocelli
• Liquid immiscibility can produce spherical
to ovoid inclusions, ranging in size from
mm's to a few cm's
• Intermixing of magmas may form ocelli by
the suspension of blobs of one magma in
another
36
Post-Solidification Processes
• Autometamorphic
• Deuteric
• Diagenetic
37
Deuteric Reactions
• Uralization
 Symplectite
•
•
•
•
•
Biotitization
Chloritization
Seritization
Saussuritization
Serpentization
38
Uralite
Pyx
Hbl
© John Winter and
Prentice Hall
• Pyroxene largely replaced by hornblende
• Width 1 mm
39
Chloritization
© John Winter and Prentice
Hall
• Chlorite (light) replaces biotite (dark) at the
rim and along cleavages
• Width 0.3 mm
40