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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