ign petrography_2 (1).pptx

Texture by Grain Size In a rock with a 3 cm grained to the right to describe the texture: In a rock with a 0.03 - 0.3 porphyritic texture, we microphenocrysts mm...

Texture by Grain Size In a rock with a 3 cm grained to the right to describe the texture: In a rock with a 0.03 - 0.3 porphyritic texture, we microphenocrysts mm use the above table to 0.3 - 5 mm phenocrysts define the grain size of the groundmass or > 5 mm megaphenocrysts matrix, and this table to describe the Seriate texture - the texture of an igneous rock, phenocrysts: typically porphyritic, in which the sizes of the grains vary gradually or in a continuous series. Crystallinity Another aspect of texture, particularly in medium to coarse grained rocks is referred to as fabric. Fabric refers to the mutual relationship between the grains. Three types of fabric are commonly referred to: If most of the grains are euhedral - that is they are bounded by well-formed crystal faces. The fabric is said to be idomorphic granular or automorphic. If most of the grains are subhedral - that is they bounded by only a few well-formed crystal faces, the fabric is said to be hypidiomorphic granular. If most of the grains are anhedral - that is they are generally not bounded by crystal faces, the fabric is said to be allotriomorphic granular or xenomorphic.  Grain Shape If the grains have particularly descriptive shapes, then it is essential to describe the individual grains. Some common grain shapes/habit are: Tabular - a term used to describe grains with rectangular tablet shapes. Equant - a term used to describe grains that have all of their boundaries of approximately equal length. Prismatic - elongated crystals that are thicker than needles or the acicular crystals Fibrous - a term used to describe grains that occur as long fibers. Acicular - a term used to describe grains that occur as long, slender crystals Texture by Grain Shape If most of the minerals of the rock is euhedral, the texture is panidiomorphic-granular of automorphic-granular (also lamprophyric) If most of the minerals are anhedral, the texture is allotriomorphic- grannular or xenomorphic-granular (also aplitic, sugary or saccharoidal) Combination of the two above, texture is hypidiomorphic-granular or hypautomorphic-granular (also  Special Textures If an igneous rock is mostly composed of equant crystals, then the texture of the rock is granular. Intergranular texture - a texture in which the angular interstices between plagioclase grains are occupied by grains of ferromagnesium minerals such as olivine, pyroxene, or iron titanium oxides Intersertal texture - a texture similar to intergranular texture except that the interstices between plagioclase grains are occupied by glass or cryptocrystalline material also of chlorite, chlorophaeite, nontronite Porphyritic texture – large crystals in a fine groundmass or matrix.  Glomeroporphyritic - if phenocrysts are found to occur as clusters of crystals, then the rock should be described as Glomeroporphyritic or cumulophyric Vitrophyric - if the matrix is glass Felsophyric – if matrix is dense intergrowth of feldspar and quartz (felsite) Orthophyric - if feldspars in the matrix have stumpy rectangular Feldspar groundmass textures Felty or felted - feldspar microlites interwoven in irregular fashion Pilotaxitic or Trachytic texture - a texture wherein plagioclase grains show a preferred orientation due to flowage, and the interstices between plagioclase grains are occupied by glass or cryptocrystalline material. Spinifex Texture Usually easy to recognize in hand samples. They are sub-parallel to dendritic growth of olivine or clinopyroxene crystals in some quenched ultramafic rocks generated from lava flows.  Ophitic texture - laths of plagioclase in a coarse grained matrix of pyroxene crystals, wherein the plagioclase is totally surrounded by pyroxene grains. This texture is common in diabases and gabbros. Subophitic texture - similar to ophitic texture wherein the plagioclase grains are not completely enclosed in a matrix of pyroxene grains. Poikilitic texture - smaller grains of one mineral are completely enclosed in large, optically continuous grains of another mineral.  Hyaloophitic texture - a texture similar to ophitic texture except that glass completely surrounds the plagioclase laths. Hyalopilitic texture - a texture wherein microlites of plagioclase are more abundant than groundmass, and the groundmass consists of glass which occupies the tiny interstices between plagioclase grains Rapakivi Texture It is an overgrowth of plagioclase minerals on alkali feldspar mineral. This texture is caused by epitaxial (heterogeneous) nucleation where new nuclei is formed on a pre-existing crystal. In most cases, this can be easily recognizable in hand samples. Vesicular - if the rock contains numerous holes that were once occupied by a gas phase, then this term is added to the textural description of the rock. Amygdular- if vesicles have been filled with material (usually calcite, chalcedony, or quartz, then the term amygdular should be added to the textural description of the rock. An amygdule is defined as a filled vesicle. Miarolitic cavities – contains large subhedral to euhedral crystals projecting into the cavity Diktytaxitic – minute, angular cavities like interstitial (but are not!) and may be filled with minerals of same size as Miarolitic cavities are typically associated with granitic pegmatites and are formed due to the entrapment of mineral-rich fluids which have segregated by vesiculation of granitic magma during its final stage of crystallization (Kurosawa et al., 2010). The term miarolitic comes from the Italian miarole in reference to the mineral-rich pegmatite region of Baveno and Cuasso al Monte in northern Italy. Geodes are rounded vesicles or any opening, lined with crystals from hydrothermal solutions Cumulate texture – Cumulate rocks are the typical product of precipitation of solid crystals from a fractionating magma chamber. These accumulations typically occur on the floor of the magma chamber, although they are possible on the roofs if anorthite plagioclase is able to float free of a denser mafic melt. Terminology Cumulates are named according to their dominant mineralogy and the percentage of crystals to their groundmass(Hall, 1996). Adcumulates are rocks containing ~100-93% accumulated magmatic crystals in a fine grained groundmass. Mesocumulates are rocks with between 93-85% accumulated minerals in a groundmass. Orthocumulates are rocks containing between 85-75% accumulated minerals in groundmass. Cumulate rocks are typically named according to the cumulate minerals in order of abundance, and then cumulate type (adcumulate, mesocumulate, orthocumulate), and then accessory or minor phases. For example: a layer with 50% plagioclase, 40% pyroxene, 5% olivine and 5% groundmass (in essence a gabbro) would be termed a plagioclase- pyroxene Adcumulate with accessory olivine. Spherulitic - a texture commonly found in glassy rhyolites wherein spherical intergrowths of radiating quartz and feldspar replace glass as a result of devitrification. Axiolites – elongated spherulite in which there is an aggregation of minute acicular crystals arranged at right angles to a central axis rather than from a point. Highly magnified view (note scale) shows axiolitic texture of feldspar and cristoba along the walls of a large shard representing th walls of several bubbles. VARIOLITIC Mm to cm –scale , light-colored, radial aggregates of plagioclase and pyroxenes in mafic volcanic rocks. Denotes rapid cooling (quenching) in aquaeous setting such as in pillow basalts. Should not be confused with spherulites which are devitrification features. BOSTONITIC Common in felsic intrusives, composed of subparallel to radial, divergent plagioclase microlaths in a finer matrix Orbicular - a texture usually restricted to coarser grained rocks that consists of concentrically banded spheres wherein the bands consist of alternating light colored and dark colored minerals. Coronas or reaction rims - often times reaction rims or coronas surround individual crystals as a result of the crystal becoming unstable and reacting with its surrounding crystals or melt. If such rims are present on crystals they should be noted in the textural description. Kelyphitic rims are post magmatic and characterized by fibrous concentric shells around a mineral (fibrous mineral aggregate than a single mineral phase – Obata Masaaki, Kyoto Univ) A reaction rim is a mantle of one mineral completely or partially surrounding a different existing mineral that demonstrably originates due to reaction and replacement of the original phase. Reaction rims indicate that mineral with the rim was out of equilibrium with its surroundings. Textural evidence suggestive of reaction: (1) an embayed or rounded mantled mineral (2) a composite rim which changes in mineralogy outwards. The term corona texture is preferred where evidence for reaction is not present. Rims may also form by overgrowth by preferential nucleation of the mantling phase on the substrate.  Symplectite is a more general textural term that also refers to fine-grained, but usually coarser-grained than typical kelyphites, vermicular intergrowth of minerals, which may or may not show radial structure like kelyphites (a) kelyphite from a Norwegian garnet peridotite (BSE image) and (b) pyroxene– spinel symplectite from Horoman peridotite (cross-polarized light) that show similarity in microstructure, despite of a large difference in scale between the two. (a) modified from Obata & Ozawa (2011); (b) modified from Obata (2007) Graphic - a texture consisting of intergrowths of quartz and alkali feldspar wherein the orientation of the quartz grains resembles cuneiform writing. This texture is most commonly observed in pegmatites. Myrmekitic texture - an intergrowth of quartz and plagioclase that shows small wormlike bodies of quartz enclosed in plagioclase. This texture is found in granites. Perthitic texture - exsolution lamellae of albite occurring in orthoclase or microcline.Patchy zoning - This sometimes occurs in plagioclase crystals where irregularly shaped patches of the crystal show different compositions as evidenced by going extinct at angles different from other zones in the crystal. Oscillatory zoning - This sometimes occurs in plagioclase grains wherein concentric zones around the grain show thin zones of different composition as evidenced by extinction phenomena. Sieve Texture The following image shows a mineral (olivine) that has been formed in two different chemical conditions. The larger euhedral crystal is the initial crystal. After most of the initial crystal has formed, the mineral has started to remelt producing a slightly different composition of olivine at the corners (top right and bottom left). This process is known as resorption. An electron microprobe may be used to analyze the chemical differences between them. This is a type of disequilibrium texture  To see sample thin sections of minerals and textures: https://www3.nd.edu/~asimonet/CE30540/SP2017/ Lecture_2_Igneous_textures_SP2017.pdf http://leggeo.unc.edu/Petunia/IgMetAtlas/minerals/ minerals.html For a comprehensive discussion on igneous and metamorphic petrology : http://www.brocku.ca/earthsciences/people/gfinn/ petrology http://www.tulane.edu/~sanelson/eens212/intro&textures.htm http://www.studyblue.com https://wwwf.imperial.ac.uk http://www.alexstrekeisen.it www.geology.sdsu.edu