1 petrology lecture 3 igneous rock textures gly 4310 - spring, 2016
TRANSCRIPT
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Petrology Lecture 3
Igneous Rock Textures
GLY 4310 - Spring, 2016
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Primary
• Form during solidification• They result from interactions between
mineral crystals and melt
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Secondary
• Develop by alteration of the rock after crystallization
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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
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Growth
• Involves the addition of ions to the nucleated cluster
• Some crystals have preferred directions of growth
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Rate of Diffusion
• Controls movement of ions in many magmas
• Determines the rate of dissipation of the heat of crystallization
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Cooling Rate
• Slow cooling allows system to maintain thermodynamic equilibrium
• Rapid cooling contributes to a non-equilibrium system
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Nucleation vs. Growth
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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.
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Blue Glassy Pahoehoe
• Feathey quenced augite crystal nucleating on plagioclase and growing in a semi-radiating form outwards
• Mag. 2000x
© John Winter and Prentice Hall.
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Available Liquid
• 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)
ba
© Chapman and Hall. London.
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Zoned Hornblende
• Field of view 1 mm
© John Winter and Prentice Hall.
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Zoned Plagioclase
• Carlsbad twin• Field of view 0.3 mm
© John Winter and Prentice Hall.
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Grain Shape
• Mineral Term Rock Term• Euhedral Idiomorphic• Subhedral Hypidomorphic• Anhedral Xenomorphic
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Euhedral Crystal
• Euhedral early pyroxene with late interstitial plagioclase
• Field of view 5 mm
© John Winter and Prentice Hall.
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Dimension Relationships
• Mineral term Rock term• Equant Massive• Prismatic Lineated• Tabular Foliated
Poikilitic Texture
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Ophitic Texture
• Pyroxene envelops plagioclase laths• Field of view 1 mm
© John Winter and Prentice Hall.
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Granophyric Texture
• Quartz-alkali fldspar intergrowth• Field of view 1 mm
© John Winter and Prentice Hall.
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Graphic Texture
• Single crystal of cuneiform quartz intergrown with alkali feldspar
© John Winter and Prentice Hall.
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Pyroxene Replacing Olivine
• Left – Olivine mantled by pyroxene, ppl• Right – CN – Olivine is extinct, Opx stands
out• © John Winter and Prentice Hall.
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Dehydration Rim
• Hornblende phenocryst dehrates to Fe-oxides plus pyroxene due to pressure release on eruption
• Width 1 mm
© John Winter and Prentice Hall.
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Embayed Texture
• Field of view 0.3 mm• Partially resorbed olivine phenocryst
© John Winter and Prentice Hall.
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Sieve Texture
• Plagioclase phenocrysts• Field of view 1 mm
© John Winter and Prentice Hall.
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Trachytic Texture
• Sub-parallel alkali feldspar laths form sheaves and swirls around earlier-crystallised minerals
• CN, medium power
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Pilotaxic or Felty Texture
• Microphenocrysts are randomly aligned
© John Winter and Prentice Hall.
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Flow Banding
• Andesite, Mt. Rainier• Long-handled hammer for scale
© John Winter and Prentice Hall.
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Intergranular Texture
• Columbia River Basalt Group• Width 1 mm
© John Winter and Prentice Hall.
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Carlsbad Twin
• Form as the result of mistakes during growth
• Field of view ≈ 1 mm
© John Winter and Prentice Hall.
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Albite Twinning
• Also thought to be form as the result of mistakes during growth
• Field of view ≈ 1 mm
© John Winter and Prentice Hall.
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Tartan Twinning
• Microcline• Field of view ≈ 1 mm
© John Winter and Prentice Hall.
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Deformational Albite
Twinning
• Typically occurs in nearly pure Ab• Note that twins “pinch-out” at the edge• Width 1 mm
© John Winter and Prentice Hall.
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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
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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
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Bronzite Photomicrograph
• Bronzite crystal from an ultramafic rock
• Thin lamellae of a calcium-rich species, probably pigeonite, have separated from the bronzite, and the host (grayish) thus has a very low calcium content (magnified about 40×)
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Augite - Pigeonite
• Complex separation of augite from an inverted pigeonite (magnified about 70.4×)
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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
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Post-Solidification Processes
• Autometamorphic• Deuteric• Diagenetic
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Deuteric Reactions
• Uralization Symplectite
• Biotitization• Chloritization• Seritization• Saussuritization• Serpentization
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Uralite
• Pyroxene largely replaced by hornblende• Width 1 mm
Pyx
Hbl© John Winter and Prentice Hall
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Chloritization
• Chlorite (light) replaces biotite (dark) at the rim and along cleavages
• Width 0.3 mm
© John Winter and Prentice Hall
Undulatory extinction
• Quartz grain in orthogneiss showing undulatory extinction 42