Petrology Lecture 3 Igneous Rock Textures GLY 4310

Petrology Lecture 3 Igneous Rock Textures GLY 4310 - Spring, 2019 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. 2000 x © 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 0. 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

Poikilitic Texture 17

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

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

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

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

Dehydration Rim © John Winter and Prentice Hall. • Hornblende phenocryst dehydrates to Feoxides plus pyroxene due to pressure release on eruption 22 • Width 1 mm

Embayed Texture © John Winter and Prentice Hall. • Field of view 0. 3 mm • Partially resorbed olivine phenocryst 23

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

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

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

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

Cluster Texture • When crystals remain in suspension, a number of cluster textures are possible. § Synneusis - Clustering of suspended phenocrysts - it may result in growth (primary) twin formation, since this may be the lowest energy method by which crystals of the same mineral may adhere. § Cumulophyric - Composed of multiple-grain clusters of adhering phenocrysts § Glomeroporphyritic - Composed of multiple-grain clusters of adhering phenocrysts of the same mineral 28

Intergranular Texture © John Winter and Prentice Hall. • Columbia River Basalt Group • Width 1 mm 29

Carlsbad Twin © John Winter and Prentice Hall. • Form as the result of mistakes during growth • Field of view ≈ 1 mm 30

Albite Twinning © John Winter and Prentice Hall. • Also thought to be form as the result of mistakes during growth • Field of view ≈ 1 mm 31

Tartan Twinning • Microcline • Field of view ≈ 1 mm © John Winter and Prentice Hall. 32

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 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 calciumrich 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 Pyx Hbl © John Winter and Prentice Hall • Pyroxene largely replaced by hornblende • Width 1 mm 41

Chloritization © John Winter and Prentice Hall • Chlorite (light) replaces biotite (dark) at the rim and along cleavages • Width 0. 3 mm 42

Undulatory extinction • Quartz grain in orthogneiss showing undulatory extinction 43