1 Petrology Lecture 3 Igneous Rock Textures GLY Spring, 2016

2 Primary Form during solidification They result from interactions between mineral crystals and melt

3 Secondary Develop by alteration of the rock after crystallization

4 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

5 Growth Involves the addition of ions to the nucleated cluster Some crystals have preferred directions of growth

6 Rate of Diffusion Controls movement of ions in many magmas Determines the rate of dissipation of the heat of crystallization

7 Cooling Rate Slow cooling allows system to maintain thermodynamic equilibrium Rapid cooling contributes to a non- equilibrium system

8 Nucleation vs. Growth

9 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.

10 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.

11 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) b a © Chapman and Hall. London.

12 Zoned Hornblende Field of view 1 mm © John Winter and Prentice Hall.

13 Zoned Plagioclase Carlsbad twin Field of view 0.3 mm © John Winter and Prentice Hall.

14 Grain Shape Mineral TermRock Term EuhedralIdiomorphic SubhedralHypidomorphic AnhedralXenomorphic

15 Euhedral Crystal Euhedral early pyroxene with late interstitial plagioclase Field of view 5 mm © John Winter and Prentice Hall.

16 Dimension Relationships Mineral termRock term EquantMassive PrismaticLineated TabularFoliated

Poikilitic Texture 17

18 Ophitic Texture Pyroxene envelops plagioclase laths Field of view 1 mm © John Winter and Prentice Hall.

19 Granophyric Texture Quartz-alkali fldspar intergrowth Field of view 1 mm © John Winter and Prentice Hall.

20 Graphic Texture Single crystal of cuneiform quartz intergrown with alkali feldspar © John Winter and Prentice Hall.

21 Pyroxene Replacing Olivine Left – Olivine mantled by pyroxene, ppl Right – CN – Olivine is extinct, Opx stands out © John Winter and Prentice Hall.

22 Dehydration Rim Hornblende phenocryst dehrates to Fe-oxides plus pyroxene due to pressure release on eruption Width 1 mm © John Winter and Prentice Hall.

23 Embayed Texture Field of view 0.3 mm Partially resorbed olivine phenocryst © John Winter and Prentice Hall.

24 Sieve Texture Plagioclase phenocrysts Field of view 1 mm © John Winter and Prentice Hall.

25 Trachytic Texture Sub-parallel alkali feldspar laths form sheaves and swirls around earlier- crystallised minerals CN, medium power

26 Pilotaxic or Felty Texture Microphenocrysts are randomly aligned © John Winter and Prentice Hall.

27 Flow Banding Andesite, Mt. Rainier Long-handled hammer for scale © John Winter and Prentice Hall.

28 Intergranular Texture Columbia River Basalt Group Width 1 mm © John Winter and Prentice Hall.

29 Carlsbad Twin Form as the result of mistakes during growth Field of view ≈ 1 mm © John Winter and Prentice Hall.

30 Albite Twinning Also thought to be form as the result of mistakes during growth Field of view ≈ 1 mm © John Winter and Prentice Hall.

31 Tartan Twinning Microcline Field of view ≈ 1 mm © John Winter and Prentice Hall.

32 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.

33 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

34 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

35 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×)

36 Augite - Pigeonite Complex separation of augite from an inverted pigeonite (magnified about 70.4×)

37 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

38 Post-Solidification Processes Autometamorphic Deuteric Diagenetic

39 Deuteric Reactions Uralization  Symplectite Biotitization Chloritization Seritization Saussuritization Serpentization

40 Uralite Pyroxene largely replaced by hornblende Width 1 mm Pyx Hbl © John Winter and Prentice Hall

41 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