Igneous Rock Textures Explained PDF

GEOL 102 PETROLOGY Textures of Igneous Rocks 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Igneous Texture Degree of crystallinity of mineral groups comprising igneous rocks, depending on cooling rate, temperature, and pressure.  D...

GEOL 102 PETROLOGY Textures of Igneous Rocks 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Igneous Texture Degree of crystallinity of mineral groups comprising igneous rocks, depending on cooling rate, temperature, and pressure.  Degree of Crystallinity Holocrystalline Hypocrystalline Holohyaline All components are All components are partially crystalline, All components are crystalline. partially glassy. glassy.  Crystal Faces Euhedral Subhedral Anhedral Minerals with complete crystal Minerals with partially faces and are not impinged complete crystal faces by Minerals lack any upon by other crystals. which at least one crystal face observable crystal Developed in early phases of is impinged upon by adjacent faces. crystallization. material. Factors in Crystallization How does crystals form from a liquid phase?  Factors in Crystallization 1.Crystal Nucleation Formation of new crystals large enough to persist and grow into larger crystals. Crystal Nucleation Rate Number of new seed crystals that develop per volume per unit time (nuclei/cm3/s).  Factors in Crystallization 1.Crystal Nucleation Crystal Nucleation Rate Rate of Availability Ease of Ion Undercooling of Ions Migration  Factors in Crystallization 1.Crystal Nucleation Undercooling Occurs when liquids are cooled to sub-liquidus temperatures  Factors in Crystallization 1.Crystal Nucleation Nucleation Rate is low near Liquidus Temperature, and increases at Sub-liquidus (Undercooling) Nucleation Rate is ZERO at very large degrees of Undercooling due to increased magma viscosity, which implies near-Solidus condition.  Factors in Crystallization 2. Crystal Growth Rate Measure of increase in crystal radius over time (cm/s) Rate of Availability Magma Undercooling of Ions Viscosity  Factors in Crystallization 2. Crystal Growth Rate Measure of increase in crystal radius over time (cm/s) At Liquidus temperature, small number of large crystals forms, ultimately producing phaneritic, porphyritic or pegmatitic texture of euhedral to subhedral crystals.  Factors in Crystallization 2. Crystal Growth Rate Measure of increase in crystal radius over time (cm/s) At Sub-Liquidus temperature, a large number of small crystals are formed, resulting to Aphanitic texture.  Factors in Crystallization 2. Crystal Growth Rate Measure of increase in crystal radius over time (cm/s) If temperature decrease is very rapid, nucleation growth rate is prevented, forming non-crystalline texture.  Factors in Crystallization 3. Ion Availability Availability of ions that can fill specific ionic sites in the crystal lattice structure. Readily-available ions enhances crystal growth  Factors in Crystallization 4. Viscosity Resistance of fluid to shear stress; depends on SiO2 Viscosity affects ion diffusion from magma to sites. Low viscosity = high diffusion rate = increased crystal growth rate  Factors in Crystallization 5. Network Formers vs. Network Modifiers Network Formers increase molecular linkages, increasing viscosity. Si, O, Al. Network Modifiers decrease molecular linkages, reducing viscosity. Fe, Mg  Factors in Crystallization 6. Heat Temperature is inversely proportional to viscosity and molecular bonding. Increase Temperature = decrease bonding = reducing viscosity  Factors in Crystallization 7. Gases H2O, CO2, SO2; minor gases include N, H, S, F, Ar, Co, Cl Pressurized dissolved gases are network modifiers. High PH2O decreases viscosity of silicic magmas, increasing diffusion rate and crystal growth rate, forming small numbers of large crystals.  Factors in Crystallization 2. Nucleation Rate Undercooling Occurs when liquids are cooled to sub-liquidus temperatures Crystalline Igneous Texture  Crystalline Texture Pegmatitic Large crystals > 30mm diameter Early-formed, euhedral crystals surrounded by later-formed subhedral crystals.  Crystalline Texture Phaneritic Crystals 30mm – 1mm diameter Early-formed, euhedral crystals surrounded by later-formed subhedral to anhedral crystals  Crystalline Texture Phaneritic Finely phaneritic is 1mm – 3mm Medium phaneritic is 3mm – 10mm Coarsely phaneritic is 10mm – 30mm  Crystalline Texture Aphanitic Small crystals < 1mm diameter Crystals not generally discernible to eye  Crystalline Texture Aphanitic Microcrystalline has microlite crystals that can be identified with a petrographic microscope. Cryptocrystalline has finer-sized crystals that cannot be identified even with petrographic microscope.  Crystalline Texture Porphyritic Two distinctly different-sized crystals Large crystals are Phenocrysts Small crystals are Groundmass  Crystalline Texture Porphyritic Porphyritic-phaneritic has phenocrysts embedded in phaneritic groundmass. Porphyritic-aphanitic has phenocrysts embedded in aphanitic groundmass.  Crystalline Texture Pegmatitic Aphanitic  Crystalline Texture Porphyritic-phaneritic Porphyritic-aphanitic Non-crystalline Igneous Texture Non-crystalline Texture Glassy Glass-like texture resulting from near-instant solidification of melt, preventing the formation of mineral crystal structure. Has microlites and cryptocrystalline minerals. Non-crystalline Texture Glassy Quenching occur when melt is in contact with liquid water or air. Rapid Loss of Dissolved Gas is limited to silicic melts; why obsidian is more common than basaltic glass Non-crystalline Texture Glassy Vitrophyric Texture has recognizable phenocryst in glassy groundmass. Spherulitic Texture has crystals growing outward from existing crystal nuclei Non-crystalline Texture Glassy Devitrification occurs when glass crystallizes on pre-existing microlites or cryptocrystalline nuclei. Perlitic Texture has cloudy appearance and curved cooling cracks.  Non-crystalline Texture Vesicular Has spherical or ellipsoidal void spaces due to exsolution and entrapment of gas bubbles in lava. Vesicular rock name if > 30% vesicles by volume Vesicular + rock if 5% - 30% vesicles Vesicular-bearing + rock if < 5% vesicles  Non-crystalline Texture Pyroclastic Fragmented particles of various sizes from volcanic eruptions. Classified based on Composition, Size, & Shape  Non-crystalline Texture Pyroclastic Based on Composition: Lithic Pyroclasts are rock fragments Vitric Pyroclasts are glassy fragments Crystals Pyroclasts are minerals  Non-crystalline Texture Pyroclastic Based on Shape and Sizes: Blocks/Bombs if > 64mm pyroclasts Lapilli if 2mm – 64 mm pyroclasts Ash if < 2mm pyroclasts Dust if < 0.0625 mm pyroclasts; Fine Ash  Non-crystalline Texture Pyroclastic Welding Fragments become progressively fused together as porosity decreases during compaction, resulting to elongated, flat, parallel fragments.  Non-crystalline Texture Glassy Vitrophyric  Non-crystalline Texture Spherulitic Perlitic  Non-crystalline Texture Scoria Pumice  Non-crystalline Texture Pyroclastics Welded pyroclastics Questions?