Petrogenetic Significance of the Textures and Structures of Igneous Rocks | Geology Optional Notes for UPSC PDF Download

Textures of Igneous Rocks

• Definition: Igneous rocks form from molten magma or melt, resulting in various textures based on the proportions of glass and mineral grains, their sizes, shapes, and arrangements.
• Importance of Textures: Textures and structures in igneous rocks offer insights into the geological processes during crystallization, as well as information about the physical chemistry and cooling history of the rocks.
• Comparison between Granite and Basalt:
  • Colour: Granite is light-colored (leucocratic), while basalt is dark-colored (melanocratic).
  • Grain Size: Granite is coarse-grained, whereas basalt is fine-grained or sometimes glassy.
  • Mutual Relationship: In granite, crystals are large enough to be seen with the naked eye, unlike basalt.
• Microscopic Examination: Petrological microscopes reveal further differences between granite and basalt textures that are not visible to the naked eye.
• Identification of Rock Types: Textures aid in distinguishing different rock types based on their unique characteristics.

Textures and Structures of Igneous Rocks

• Crystallinity/ Degree of Crystallization
• Granularity/ Grain Size
  • Shape of the Mineral Grains
• Mutual Relationship Amongst Crystals and Glass

Fabric in igneous rocks refers to the arrangement, orientation, and mutual relationship of mineral grains, crystals, and/or glass. It is a non-compositional property of rocks, encompassing both textures and structures.

Watch this video for more about the fabric of igneous rocks: Igneous Textures, Processes, and Pathways: An Overview

2.2.1 Crystallinity

Crystallinity, or the degree of crystallization, indicates the amount of crystals formed during the solidification of magma. Igneous rocks can be entirely crystalline, partially crystalline, or completely glassy. The degree of crystallization is determined by the ratio of crystallized matter to glass in the rock, ranging from 0% to 100%.

Here are some related terms:

• Crystallites: Embryos of crystals that are organized towards full crystalline status, often with varied shapes.
• Longulites: Cylindrical rods with rounded ends.
• Globulites: Minute spherical drops or pellets, typically opaque and containing iron oxide. When aligned like a string of beads, they form margarites.
• Scopulites: Rods or needles with divergent plumes.
• Trichites: Filamentous or hair-like structures.
• Microlites: Somewhat larger bodies resembling minute crystals, usually rod or needle-shaped with crystal outlines specific to their mineralogy.

Glass in the context of igneous rocks is a highly viscous liquid with a disordered atomic structure, formed by rapid cooling of silicate melts during crystallization. It refers to a rock or portion of a rock devoid of a crystalline structure, resulting from the rapid supercooling of a highly viscous magma.

• Types of Textures in Igneous Rocks:
  • Holocrystalline Texture: This texture indicates that the rock is entirely composed of well-defined crystal faces of constituent minerals. For example, granite containing orthoclase or gabbro with augite. Holocrystalline texture is commonly found in plutonic rocks.
  • Hemicrystalline/Microcrystalline Texture: When a rock consists of both crystalline and glassy material, such as dolerite or basalt, it exhibits hemicrystalline texture. This texture is typically observed in rocks that have crystallized near the surface or at intermediate depths. Synonymous terms include merocrystalline or hypocrystalline.
  • Holohyaline Texture: Rocks with holohyaline texture are entirely made up of glassy or non-crystalline matter like crystallites and microlites. This texture results from rapid cooling and is commonly seen in volcanic rocks like obsidian, pitchstone, and nephelinite.

Crystallinity Factors

• Rate of Cooling:
  • Faster cooling results in finer crystals, seen in volcanic glass formation.
  • Slower cooling allows for the growth of larger crystals.
• Depth of Cooling and Magma Volume:
  • Greater depth leads to slower cooling rates.
  • Volume of magma influences crystal growth, with larger volumes leading to slower cooling and larger crystals.
• Composition and Viscosity of Magma:
  • Viscous lavas like rhyolite favor non-crystalline or glassy rocks.
  • Less viscous basaltic lavas result in crystalline rocks like phyric and aphyric basalts.
  • Higher viscosity magmas have more volatile content compared to less viscous ones.

Textures and Structures of Igneous Rocks

Explore the following videos for further insights:

• Igneous Textures, Processes and Pathways: An Overview
• Igneous Textures, Processes and Pathways: Volcaniclastics

Granularity

In igneous rocks, granularity varies widely, ranging from meter-sized pegmatites to microlites smaller than 0.01 mm, sometimes appearing glassy in volcanic rocks. The terms phaneritic and aphanitic describe coarse- and fine-grained rocks, respectively.

Phaneritic vs. Aphanitic Rocks

Phaneritic rocks have visible crystals, while aphanitic rocks require a petrological microscope for mineral identification.

Alternative Descriptions

Phyric and aphyric are alternative terms for phaneritic and aphanitic rocks, respectively.

Fig. 2.2: a) Phaneritic; and b) Aphaneritic texture

Unit 2: Aphanitic Rocks and Igneous Textures

Aphanitic Rocks:

• Characteristics:
  • Fine-grained with crystals too small to be seen with the naked eye or a hand lens.
• Classification:
  • Microcrystalline:
    • Grains visible only under a microscope.
  • Cryptocrystalline:
    • Only a felty mass is visible; mineral grains not seen under a microscope.

Textures and Structures of Igneous Rocks

• Photomicrographs:
  • Microcrystalline texture shown in Fig. 2.5a.
  • Cryptocrystalline texture shown in Fig. 2.5b.

Factors Controlling Grain Size in Igneous Rocks:

• Rate of Cooling:
  • Slow cooling leads to large crystal development due to delayed crystallization.
• Viscosity:
  • Viscous magma hinders crystallization by opposing ionic diffusion.
  • Viscosity is related to water vapor, gases, silica, and alumina contents.
  • Example: Rhyolite (obsidian) results from siliceous viscous magma.
• Volatile Content:
  • Presence of volatiles, especially H2O, reduces viscosity and promotes larger crystal growth.
  • Pegmatites from hydrous magmas have coarser crystals than granites.
  • Aplites are fine-grained as they form from dry magmas.

Summary of Igneous Petrology Concepts

Block 1

• CO2 and Volatile Constituents: CO2 and other volatile constituents in magma, such as common metal cations like Fe, Mg, Ca, Na, K, and dissolved water, affect magma viscosity. CO2 has a drying effect on magma, while water-rich magma is more fluid.
• Molecular Concentration and Chemical Activity: Minerals like zircon, apatite, sphene, rutile, and ilmenite form small-sized grains due to low concentrations of their constituents in magma.
• Complexity of Magma Composition: The complexity of magma increases with the number of chemical components present.
• Shape of the Crystals:
  • Euhedral: Refers to mineral grains with fully developed outlines and perfectly developed faces. Examples include pegmatite.
  • Subhedral: Describes crystal forms with less developed faces or grain boundaries. Examples include gabbro and granite.
  • Anhedral: Used for mineral grains lacking crystal outlines. Examples include aplite.

Textures and Structures of Igneous Rocks

Euhedral Magnetite

• Crystal grains develop in length, breadth, and height, showing three-dimensional geometry during crystallization.

Plagioclase

• Mineral grains are equidimensional, equally developed in all dimensions, like garnet, olivine, leucite.

Quartz

• Mineral grains are irregularly developed in all dimensions, e.g., quartz in granite.

Sutured Margin of Quartz Crystal

• Mineral grains are tabular, with greater length development compared to width, e.g., plagioclase, orthoclase.

Shapes of Crystals and Resulting Textures

• Equidimensional: Mineral grains equally developed in all dimensions, e.g., garnet, olivine, leucite.
• Prismatic: Mineral grains show distinct growth in one direction, e.g., augite, hornblende. If width and breadth are insignificant compared to length, it's called acicular or needle-shaped, e.g., sanidine.
• Tabular: Mineral grains have greater development in length compared to width, e.g., plagioclase, orthoclase.
• Platy or Sheet: Mineral grains developed in length and breadth in relation to height, e.g., mica.
• Irregular: Mineral grains irregularly developed in all dimensions, e.g., quartz in granite.

Igneous Petrology

Watch the following video to learn about homogenous textures:

• Igneous Textures, Processes and Pathways: Homogenous Textures
• Watch the video here

Factors Controlling Grain Shape in Igneous Rock:

• Crystal habit: Controlled by internal structure of the mineral.
• Conditions of growth: Different conditions of crystallization result in varied crystal shapes.
• Order of crystallization: Crystals' shapes are influenced by their position in the crystallization sequence.
• Slow cooling and diffusion process: Allows for the formation of euhedral crystals.

Exercise - Self-Assessment Questions (SAQ):

• a) Define texture.
• b) Define crystallite, microlite, and glass.
• c) List the textural elements.
• d) List three types of granular textures based on crystal shape, with examples of igneous rocks.
• e) List mineral grains based on their three-dimensional geometry and provide examples.

Mutual Relationship between Crystal and Non-Crystalline Material:

Equigranular Textures:

• Majority of grains are of equal size, seen in rocks like granite and gabbro.
• 
  
  • Microgranitic: Occurs in fine-grained rocks with mostly anhedral or subhedral grains.
  • Orthophyric: Euhedral grains in fine-grained rocks.

Unit 2: Inequigranular Textures in Igneous Rocks

Felsitic Texture

• If grains are very fine or microcrystalline or cryptocrystalline.

Inequigranular Textures

• In an igneous rock, if the grain size difference becomes pronounced with one set of grains being distinctly larger and associated with another set much finer in size, it is termed as inequigranular.

Types of Inequigranular Textures:

• Porphyritic Texture
• Poikilitic Texture
• Intersertal/Intergranular Texture

Usually, there is a significant difference in grain size in the rock. However, if the variation from larger to smaller grains is systematic and gradual, it is known as seriate texture.

Porphyritic Texture

In porphyritic texture, larger grains (phenocrysts) are surrounded by a groundmass consisting of smaller grains (microcrystals) or a glassy part.

The texture is visible both megascopically and microscopically. It results from changes in physicochemical conditions, molecular concentration, and insolubility.

Types of Porphyritic Texture:

• Vitrophyric: Phenocrysts are enclosed by a glassy groundmass.
• Felsophyric: Phenocrysts are enclosed by a cryptocrystalline groundmass.
• Glomeroporphyritic: Phenocrysts are early formed crystals of minerals clubbed together to form distinct clusters or crystal aggregates.

The term microphenocryst is used subjectively to distinguish finer phenocrysts from coarser ones, which are termed megaphenocrysts.

Porphyritic Texture Example:

Fig. 2.7 shows phenocrysts in a mottled pinkish groundmass.

Fig. 2.7b displays plagioclase phenocrysts in a groundmass of fine-grained plagioclase, augite, glass, and iron oxide.

Porphyritic texture is essential for understanding the formation processes of igneous rocks and the conditions under which they solidify.

Key Concepts in Igneous Petrology

Phenocryst

• An aggregate of chlorophaeite that contributes to the glomeroporphyritic texture.

Porphyritic Texture

• Types:
  • Vitrophyric texture: Phenocrysts in a glassy groundmass.
  • Glomeroporphyritic texture: Cluster of plagioclase crystal as phenocryst in groundmass with fine-grained plagioclase, augite, palagonite, and iron oxide.
• Examples:
  • Basalt: Euhedral grains of plagioclase as phenocrysts.
  • Alkali gabbro: Cluster of olivine grains as phenocryst in the groundmass of augite.

Groundmass

• The portion of rock where the phenocrysts are embedded.

Mineral Composition

• Includes olivine, augite, plagioclase, biotite, iron oxide, and orthoclase.

Additional Information

• Microscopic diagrams under crossed nicol magnification.
• Illustrations by Prof. J. P. Shrivastava.

Unit 2: Igneous Rocks Textures

Poikilitic Texture

• Poikilitic texture involves larger grains enclosing smaller mineral grains.
• The larger crystal is called oikocryst, while the enclosed crystals are known as chadocrysts.
• There are three types of poikilitic textures:
• 
  
  • Ophitic Texture:
    • Common in fine to medium-grained mafic rocks like dolerite and basalts.
    • Augite encloses smaller laths of plagioclase feldspar.
    • Example: In dolerite, augite surrounds plagioclase laths.
  • Subophitic Texture:
    • Augite grains are smaller and partly enclose plagioclase laths.
    • Result of simultaneous crystallization of plagioclase and pyroxene minerals.
    • Example: Subophitic texture in certain basaltic rocks.
  • Hyalophitic Texture:
    • Similar to ophitic texture but with diversely oriented plagioclase grains completely surrounded by glass.
    • Example: Hyalophitic texture in specific rock formations.

Intersertal/Intergranular Texture

• Mafic rocks like basalt exhibit various textures based on grain relationships.
• Groundmass comprises plagioclase, pyroxene, and glass, which may be altered.
• Intergranular Texture:
  • Corner touching plagioclase laths form a network filled with granular pyroxene.
  • Example: Intergranular texture found in certain basalt formations.
• Intersertal Texture:
  • Interlath spaces filled with glass or its devitrified product.
  • Example: Intersertal texture observed in specific volcanic rock compositions.

Igneous Petrology

Intergrowth Textures

• Intergrowth texture arises from the interlocking of grains of two different minerals due to simultaneous crystallization or eutectic crystallization of magma components at specific temperatures.
• Graphic Texture:
  • Common in granites, graphic texture is characterized by the intergrowth of quartz and orthoclase.
  • The alignment of quartz blebs parallel to a crystallographic orientation gives the appearance of cuneiform writing on a background of K-feldspar.
  • Quartz is arranged in prismatic wedge-shaped areas intersecting at an angle of about 60 degrees.
  • At a microscopic level, this texture is known as micrographic, observed under a microscope, and the rock is termed granophyre.
  • Examples include the elongated grey quartz blebs in a white orthoclase host in a granite, showcasing graphic texture.

Unit 2: Igneous Rock Textures and Structures

Myrmekitic Texture

• Myrmekitic texture arises from the intergrowth of quartz and plagioclase, typically oligoclase. The quartz intergrowth appears as worm-like rods within the plagioclase, also known as symplectite. This texture is commonly found in certain granite and metamorphic rocks.

Corona Texture

• Corona texture results from reactions in magmas and displays a concentric arrangement of characteristic minerals. The area of reaction products encircling a mineral is termed a reaction rim or kelyphitic borders. For instance, olivine may be surrounded by a pyroxene rim, with multiple successive rings possible around the central mineral.

Exsolution Textures

• Exsolution textures depict the chemical breakdown of an initially homogeneous solid solution as it cools. This process involves the separation of components during cooling, forming intergrowths.

Perthitic Texture

• Perthites are produced through the exsolution of alkali feldspar and albite, comprising thin strings, films, and patches of albite oriented within a K-feldspar host like orthoclase or microcline. Microperthitic texture, observable under a microscope, commonly develops in coarse plutonic rocks such as granite and gabbro.

Antiperthite Texture

• Antiperthite texture is the opposite of perthitic texture, where belbs and patches of orthoclase or microcline occur within plagioclase, creating antiperthic intergrowth.

Igneous Petrology Summary

Directive/Flow Texture

• When magma crystallizes with flow movement, minerals align in directional bands.
• Flow texture results in streamlines without distorting larger crystals.
• Flow structure in rocks like rhyolite showcases parallel arrangements of plagioclase crystals.

Trachytic Texture

• Observed in volcanic rocks like trachyte.
• Calcic plagioclase and glassy material align due to magma flowage.
• Referred to as trachytic texture.

Eutaxitic Texture

• Term used for the alignment of pumice fragments in ignimbrites.
• Ignimbrites form from widespread ash flow deposition.
• Aligned lithic, pumice, and crystal fragments are welded together in a groundmass.

Unit 2: Textures and Structures of Igneous Rocks

In this unit, we delve into the textures and structures of igneous rocks, exploring key concepts such as welding, devitrification, and more.

Welding

• Pumice fragment welding is a process where stretched pumice fragments fuse together, creating a cohesive structure.
• Fig. 2.15 illustrates a photomicrograph showcasing medium grained pyroclastic rock with an eutaxitic texture, highlighting the welding of pumice fragments.

Devitrification

• Devitrification occurs when highly viscous magma undergoes rapid supercooling, preventing proper crystalline formation.
• When a glassy material is subjected to increased pressure and temperature with chemically active fluids, devitrification is promoted.
• Natural glasses undergoing devitrification form minute crystals of cryptocrystalline character, leading to the creation of felsites.
• Perlitic cracks, observed in glassy or devitrified igneous rocks, are evidence of their original glassy state, resulting from contraction during rapid cooling.
• Fig. 2.16(a) depicts perlitic cracks in pitchstone, showcasing curved or spherical cracks, while Fig. 2.16(b) demonstrates a spherulitic texture.

Igneous Petrology Summary

Textures in Igneous Rocks

• Spherulitic Texture:
  • Occurs due to rapid cooling and crystallization of viscous magma.
  • Characterized by radiating arrays of fibrous or needle-like crystals.
  • Common in glassy felsic volcanic rocks.
• Spinifex Texture:
  • Found in komatiitic rock, an ultramafic volcanic rock.
  • Consists of randomly oriented, extremely fine-grained, hollow crystals.
  • Formed by rapid cooling or quenching of ultramafic lavas.

Exercise: Short Answer Questions

• a) Distinguish between equigranular and inequigranular textures.
• b) What is seriate texture?
• c) Define graphic texture and explain its formation.
• d) Explain exsolution intergrowth with an example.

Structures of Igneous Rocks

Structures in igneous rocks differ from textures and refer to larger features observed in the field.

Vesicular and Amygdaloidal Structures

Lavas rich in gases and volatiles form cavities upon eruption due to pressure changes.

• Vesicular Structure:
  • Formed by gas escape, creating cavities of various shapes.
• Amygdaloidal Structure:
  • Vesicles filled with secondary minerals like quartz or calcite.
  • Named for their almond-like shapes.

Textures and Structures of Igneous Rocks

Scoriaceous and Pumiceous Structures

• When lava is rich in volatiles and gases and cools, it forms structures known as scoriaceous and pumiceous.
• Scoriaceous Structure:
  • Scoria is a type of extrusive basalt with a clinkery appearance and high vesicles content.
  • If over 50% of the rock is vesicles and the density is greater than 1, it is classified as scoriaceous.
• Pumiceous Structure:
  • Pumice is a volcanic rock with numerous irregular cavities or vesicles left behind after volatile gases escape during cooling.
  • If over 50% of the rock is vesicles and the density is less than 1, it is termed pumiceous.
• Vuggy Structure:
  • Vugs are angular cavities in rocks formed by volatile fluids collecting between existing crystals.
  • This results in a structure known as vuggy structure.

Lava Tunnel

• When lava cools and solidifies, fluid lava inside can drain out through channels, creating lava tunnels.
• Lava Tubes:
  • Hardened basaltic flows often contain cave-like structures called lava tubes.
  • These tubes act as conduits for lava flow and develop within the flow where temperatures remain high.

Igneous Petrology

Lava Tubes

• Lava tubes are pathways that help lava move efficiently over long distances by providing insulation.

Blocky and Ropy Lava

• Blocky lava, known as 'aa', consists of rough, jagged blocks with sharp edges.
• Ropy lava, or 'pahoehoe', has a smooth, often glazed surface with wrinkled, rope-like forms.
• Pahoehoe lava forms at higher temperatures and is more fluid compared to aa lava.
• Pahoehoe can transition into aa lava, but not vice versa.

Platy and Sheet Structure

• Platy structure involves parallel partings or joints, forming plates of rock mass.
• Sheet structure consists of well-defined horizontal joints or surfaces.

Pillow Lava

• Pillow lava is ellipsoidal and pillow-shaped, commonly found in basic/mafic lavas.
• These lava formations resemble small pillows or cushions and are created by lava extrusion under various conditions.
• Pillows typically have a vesicular crust or glassy skin due to rapid cooling.

Unit 2: Textures and Structures of Igneous Rocks

Columnar/Prismatic Structure

• Uniform cooling and contraction in a uniform magma lead to the formation of columnar or prismatic structures with regular sides.
• This structure typically features four, five, or six sides and is a result of centers of nucleation-contractions forming at equally spaced intervals during cooling.
• Example: Columnar jointing in basalt and rhyolite as illustrated in Fig. 2.21a and b.

Lava Flow Structure

• During lava eruption, viscosity impedes movement, causing the formation of elongated lenticular patches arranged parallel to the lava flow, known as directional or flow structure.
• Example: Flow structures in rhyolite as depicted in Fig. 2.22.

Summary of Igneous Petrology Concepts

Rift and Grain Structure

• Developed from three sets of equally spaced joints, creating cubical blocks called mural jointing.
• Quarrymen utilize this structure to break down large granite blocks into smaller pieces.

Perlitic Structure

• Forms spherical balls with a pearl-grey shine due to shelly cracks from rapid cooling of lava or magma.

Rapakivi Structure

• Termed "rotten or crumbled stone" in Finnish, describes rounded potassic feldspar crystals surrounded by sodic feldspar rims.
• Common in granitic rocks, featuring large orthoclase crystals mantled by plagioclase.

Watch this video to learn more about Rapakivi Structure: Rapakivi Texture

Xenoliths

• Accidental rock fragments trapped in igneous rocks, may be of igneous, sedimentary, or metamorphic origin.
• Xenoliths can be foreign (xenolithic) or related (cognate) to their host rocks, with some crystallizing from the same magma.
• Unrelated xenoliths are older than host rocks, as they existed before surrounding magma solidified.

Xenocrysts vs. Xenoliths

• Xenoliths and Xenocrysts

  • Xenoliths: Xenoliths are rock fragments found within an igneous rock mass. These fragments are distinct from the surrounding rock.
  • Xenocrysts: In contrast, xenocrysts are individual mineral fragments that unintentionally became part of the magma and were preserved in a partially resorbed state.

• Origin of Xenocrysts

  • Xenocrysts may originate from a foreign source or come from the same country rock where the magma formed.

• Cognate Xenocrysts

  • Cognate xenocrysts: These are xenocrysts that are genetically related to the surrounding rock.