Plane/cross-polarized light, field width is 1.2 mm. Atlas of igneous rocks and their textures A companion volume to the Atlas of Rock-forming Minerals in Thin Section, this full-colour handbook is designed to be used as a laboratory manual both by elementary students of earth sciences undertaking a study of igneous rocks in thin section under the microscope, and by more advanced students and teachers as a reference work. This diabase from a thick lava flow contains abundant interstitial granitoid material, brown, isotropic altered glass, skeletal magnetite, and other interesting features. All such systems are founded on the assumption that the chemical composition of the ultimate igneous rocks is definitely related to the original chemical composition of the magma. Composition refers to both the types of minerals within a rock and the overall chemical makeup of the rock (the two are obviously related). They are all connected in the endless rock cycle, moving from one form to another and changing shape, texture, and even chemical composition along the way. Iceland. Environmental Geology (DEL-GEOL-002) Academic year. The chlorite is pale-green to pale-brown in plane light, with low first-order anomalous Berlin blue interference colors. Adcumulate texture, where essentially all trapped liquid was able to exchange material with the nearby magma, allowing overgrowths to fill the trapped liquid pockets. Granophyre in a highly fractionally crystallized gabbro. Many of the images have two views, most showing paired plane- and cross-polarized light images. The norm minerals are divided into two groups namely, salic and femic minerals. Properties of Igneous Rock Textures . The mineralogical composition of an igneous rock is actually an expression of the chemical composition of the parent magma and cooling history of the rock. In igneous rocks the mineral crystal are scattered randomly, but they are tightly interlocked. When the host mineral is identified as augite and the inclusions are of plagioclase felspars, the poiklitic texture is further distinguished as ophitic. Vesicle fillings can include a wide array of low-temperature minerals which, besides those shown here, may include zeolites, albite, K-feldspar, epidote, clay, and others. Igneous rocks can be easily identified with their texture, density, color, and mineral composition. Skeletal growth takes place during strong supercooling, when crystals grow projections into the surrounding liquid. This texture is common in aphanitic, or glassy, igneous rocks that have come to the surface of the earth, a process known as extrusion. They are Aphanitic, Phaneritic, Pegmatitic, Porphyritic, Vesicular, Pyroclastic, and Glassy. The texture is sometimes further distinguished into mega-porphyritic and microporphyritic depending upon the size of the phenocrysts. The glass colorless to light-brown, and hard to see with all the air bubbles entrained in the epoxy (bubbles in epoxy have thin, birefringent rims). There are also phenocrysts of magnetite and augite. Alteration without deformeation changed this assemblage to one largely made of actinolite and hornblende, chlorite, and sericite. Kinsman Granite, New Hampshire. Texture refers to the size and arrangement of the minerals or grains that make up a rock. Poiklitic Texture is characterised with the presence of fine-grained crystals within the body of large sized crystals. You cannot see the grains with the unaided eye. Shand and others developed a method of classifying igneous rocks into three divisions on the basis of predominant chemical composition as expressed by the relative abundance of the rock forming minerals. Disclaimer 8. Those textures that indicate the result of flow of magma during the formation of rocks are known as directive textures. Perthite exsolution texture, with irregular albite exsolution lamellae in microcline host crystals. Glomerocryst made of plagioclase and olivine in basalt. Glasses are sometimes referred as supercooled liquids. Porphyries are rocks with a large fraction of phenocrysts, typically around 50%. This example, from the marginal border series of the intrusion, has augite enclosing plagioclase and olivine. The left piece is more oxidized, and the right one relatively fresh. Cross-polarized light, field width is 1.2 mm. Uploader Agreement, Granites and Basalts | Igneous Rocks | Geography, Igneous Rocks: Formation, Textures and Uses | Petrology, Characteristics of Igneous Rocks | Types|Rocks | Geography, Weathering of Rocks: Types, Factors and Products | Geology, International Tourism in India: Introduction, History, Trends, Opportunities and Future, Forestry: Definition, Branches, Costs, Programme and Conclusion | Geography, Contribution of Russia to World Geography (In Hindi), French Scholars and their Contribution to Geography in Hindi. Note that the hornblende rims are brown on augite and magnetite, and green on parts of the olivine. Here, the granophyre formed in the last bit of interstitial liquid, among cumulate olivine, plagiocalse, magnetite, apatite, brownish magmatic augite, and, shown here, green augite that is inverted ferrobustamite. When the same magma (carrying with it these large crystals) moves upward, the pressure and temperature acting on it are greatly reduced. It is difficult to further classify the rocks on the basis of texture. Cross-polarized light, field width is 6 mm. Wyoming. In the granitic texture, the constituents are either all coarse grained or all medium grained and the crystals show euhedral to subhedral outlines. Report a Violation 10. In general, it's difficult to tell xenocrysts from phenocrysts if they are the same mineral. These are distinct from the brownish spherulites that occur in the same sample. The development of ophitic texture in rocks is yet incompletely understood. (e) The relative abundance of felsic and mafic minerals serves as a basis for broadly defining the saturation of the rock with silica. The more slowly-cooled interior first grew spherulites, which increase in abundance inward, coalescing into a solid mass of them on the right side of this image. The igneous rock texture helps in identifying the rock origin, mode of crystallization, and classification of the rock. Account Disable 11. Igneous textures in which the majority of constituent minerals show marked difference in their relative grain size are grouped as inequigranular texture. These form the most important part of the classification programme and involve frequent use of tables of molecular weights, percentage weight (of oxides) and molecular values etc. I. Igneous Textures A. In this case, the rock is a cumulate syenite, with large, slab-shaped cumulate feldspars. Subhedral crystals have some flat crystal faces, or shapes that approximate them. commonly show this texture. Xenolith in rhyolite. Hornblende replacing olivine and augite. At the same time, a few methods, as mentioned below, have been variously used successfully for classifying these rocks for different purposes. (iii) The molecular numbers and the various oxides are then arranged in an orderly manner. Phenocrysts of olivine and augite in an otherwise microcrystalline basalt. Red Hill, Center Harbor, New Hampshire. Filled vesicles, called amygdules (amygdales in the Old World). Iceland. The quartz grains reacted with the magma to form a garland of small augite crystals. Microcrystalline texture, showing relatively even grain size with a few larger mafic minerals. The texture of igneous rocks depends on the composition of the magma and the conditions surrounding the magma’s cooling. Intrusive rocks are characterized by a holocrystalline texture, in which all the rock material is crystallized. Stillwater Complex, Montana. Texture - refers to degree of crystallinity, grain size, and geometrical relationships between the cons tituents of a rock (fabric). For example, in a given rock, silica, SiO2 is 72.67 per cent, its molecular number will be 72.67/60 = 1.21. There are numerous tiny microlites with some flow alignment, and some small spherulites. Remember that orthopyroxene has much lower birefringence than augite. the rock was likely formed by a violent volcanic eruption. TEXTURES OF IGNEOUS ROCKS . Start studying Chapter 3: Textures of Igneous Rocks. This is a biotite granite, with visible quartz, plagioclase, and perthite. Several other samples in this category have microlites, but these are big enough to be easy to see. Northwestern New Hampshire. Farther in, sizable crystals grew in the matrix to give more typical basaltic textures. Granophyre, or 'graphic granite,' is an ordered intergrowth of quartz and alkali feldspar. Basalts are best examples. In this case, the olivine basalt has xenocrysts of quartz, which could not have crystallized from this magma. Holocrystalline texture is characterized by deep-seated or intrusive igneous rocks. There are three great categories of rocks: igneous, sedimentary, and metamorphic. Glassy Texture: Lava may cool so fast that its constituent atoms do not have enough time to become arranged in the ordered, three-dimensional frameworks of minerals. Sometimes the term Intermediate rocks are also used for silica percentage between 52-66 per cent. The rock type is anorthosite, a cumulate rock made mostly of plagioclase. Spherulites are spots where fibrous crystals radially grow from a nucleation point, usually during strong supercooling of the magmatic liquid. Glassy texture in rhyolite obsidian that has only about 2% crystals, most minute. In graphic texture, the intergrowth is most conspicuous and regular between quartz and felspar crystals. Textures of Igneous Rocks. Rocks like syenite, monzonites, peridotites etc. This is a composite term expressing the relative grain size of different mineral constituents in a rock as well as the degree of perfection in the form of the crystals of the individual minerals. These igneous rocks are composed chiefly of fine-grained (grain size less than 1 mm) constituents. Use of magnifying lens often becomes necessary for identifying all the constituent mineral components. Example: Granite, Gabbro, Syenite. There are basically seven types of igneous rock textures. Move the cursor over the visible image to see the other view. When the average grain size lies between 5 mm and 1 mm. The relative abundance of minerals of these groups is made the basis for defining the rock divisions, groups, and series. Subclasses, orders and sub orders are further distinguished on the basis of individual predominance of different normative minerals. Igneous textures include the rock textures occurring in igneous rocks.Igneous textures are used by geologists in determining the mode of origin igneous rocks and are used in rock classification. This sample of rhyolite also has strong flow alignment of the microlites. Chlorite replacing garnet. Shand) distinguish a special type of igneous rocks excessively rich in mafic minerals with a colour index above 90 as Hypermelanic. Plane/cross-polarized light, field width is 0.6 mm. Igneous rocks are classified based on texture and composition. In igneous rocks, these textures are shown by granites and felsites and hence are also often named as granitic and felsitic textures. They are generally fine grained in texture. In igneous rocks, these textures are shown by granites and felsites and hence are also often named as granitic and felsitic textures. It has also been used for classifying rocks into three divisions. Small grains of olivine, pyroxene, magnetite, and apatite hint indicate the original trapped liquid. The texture of igneous rocks can be analyzed to understand how the rock became solid or crystallized from liquid, melted rock. The hypabyssal rocks are of intermediate type, formed neither at great depths nor on the surface. Iceland. These are silica-deficient igneous rocks, the silica percentage being less than 45 per cent. The green mafic minerals are aegirine, some of which have nearly colorless augite cores. Ø The term texture is defined as the mutual relationship of different mineralogical constituents in a rock. Chapter 3: Igneous Textures • 3.1.1 Rates of nucleation, growth, & diffusion – The relative rates of initial nucleation, crystal growth and diffusion will have considerable influence on the ultimate texture of the resulting rock • However, whichever rate is the slowest will … The complex textures suggest magma mixing. The original gabbro probably was made of plagioclase, augite, magnetite, and ilmenite, among other things. In such rocks, identification of the constituent mineral grains is possible only with the help of microscope for which very thin rock sections have to be prepared for microscopic studies. The former is characteristic of certain felspathic lavas and is recognised by a parallel arrangement of felspar crystals; the latter is found in some syenites. Sometimes an aphinitic rock may show a few well-developed coarse crystals present as phenocrysts. At this phenocryst fraction, grain-on-grain contact would make them extremely viscous. Unknown source. Mesocumulate texture, in which liquid trapped between cumulate crystals was able to exchange material with the nearby magma, allowing more extensive overgrowths of cumulate minerals and a smaller volume of ultimately trapped liquid. Crystal size primarily reflects the rate of cooling, but is also often strongly affected by rock composition (especially water or gas content). Iceland. helpful 0 0. Basalt dike in the Skaergaard Intrusion, Greenland. Feathery texture of pyroxene in the matrix of basalt, in the matrix between glomerocrysts. It was fractured, and aqueous fluid caused it to react to form chlorite. Types of Igneous Rock Textures. There are six main types of textures; phaneritic, aphanitic, porphyritic, glassy, pyroclastic and pegmatitic. 2015/2016. Here the basalt originally had phenocrysts of plagioclase and numerous bubbles. Subhedral crystals of plagioclase, olivine, augite, and magnetite in a cumulate gabbro. As per Wikipedia, “Igneous rock (derived from the Latin word ignis meaning fire) is one of the three main rock types, the others being sedimentary and metamorphic.Igneous rock is formed through the cooling and solidification of magma or lava. Igneous rocks with porphyritic texture are designated as porphyry, as in granite porphyry or basalt porphyry. Moreover, the mineralogical composition of most of the igneous rocks can be determined fairly accurately megascopically or at best by making thin sections and studying them under microscope. Traditionally the phenocryst matrix in porphyries is relatively fine-grained, suggestive of extrusive rocks or shallow levels of intrusion. In this case, the molecular concentration of host minerals should be greater and these should crystallise under favourable conditions only towards the end of the process so that these are capable of enclosing many other crystals formed at earlier stages in the process. Most of the time, they're simple to tell apart. These are formed at shallower depths, generally less than 2 km below the surface where conditions of crystallisation favour porphyritic texture. These may be defined as rocks of zero grain size. Some mineral constituents may be relatively insoluble in the magma than the others. Volcanic ash, with oddly-shaped, colorless glass shards that are broken pumice vesicle walls. Which of the following igneous rocks contains no mineral crystals? Salem, Massachusetts. Vinalhaven Island, Maine. Myrmekite is a wormy intergrowth of quartz and feldspar, generally forming as a subsolidus reaction product between feldspar and migrating fluids. Plane/cross-polarized light, field width is 3 mm. The salient features of this classification are summarized as below: It is a set of standard minerals of calculated chemical composition. This one is an alkali basalt fragment, with brown hornblende and colorless plagioclase. Sieve texture, in microcline in rhyolite obsidian. It is similar to ophitic textures, but the enclosing mineral need not be pyroxene. Though it looks entirely crystalline, about 25% glass remains in this sample. Trachyandesite, Vulsini, Valentano, Italy. Chilled margin from a pillow lava rim. when some minerals are crystallized and others are of glassy character in the same rock. Cumulate rocks typically have the same minerals about the same size. The convention is to use the prefixes per and do to the normative group dominating in a given rock signifying domination of high or moderate order respectively. An igneous rock may contain crystals of any one type in a predominating proportion; hence its fabric will be defined by one of the following three terms related to fabric: When majority of the components are in fully developed shapes; The rock contains crystals of all the categories- euhedral, subhedral or anhedral; When most of the crystals are of anhedral or irregular shapes. Slower growth later closed off the space between skeletal projections, trapping liquid. The rock is somewhat altered and oxidized. Copyright 9. What are Igneous Rocks? Iceland. The dark blobs that make up the sieve appearance are melt inclusions that formed during rapid, probably skeletal crystal growth. Equigranular biotite granite. The results are expressed in terms of saturation with SILICA. University. TiO2 replacing ilmenite in a highly altered gabbro. Hornblende rimming quartz. Glomerocrysts are clumps of large crystals set in a fine-grained matrix, a variety of phenocrystic texture. Continental Divide near Estes Park, Colorado. lecture notes of the topic textures of igneous rocks. Rocks with holocrystalline texture are also termed as phaneric and the holohyaline rocks are referred as aphinitic. Stillwater Complex, Montana. In some cases these are found in pegmatites and other felsic rocks, but the textures are also found in mafic rocks as the last bit of evolved, felsic liquid crystallizes. After having known various factors that define the texture types, it will be easy to understand the important textures as exhibited by the igneous rocks. Medium-grained, 1-5 mm. Unknown source. Skaergaard Intrusion, Greenland. Black Hills, South Dakota. Igneous rock (derived from the Latin word ignis meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic.Igneous rock is formed through the cooling and solidification of magma or lava.. Antiperthite exsolution texture, in which K-feldspar has unmixed from a plagioclase host. Microlites are small crystals, typically elongate, typically more or less uniform in size, and typically scattered more or less evenly through a glassy sample. Hence these are coarse to medium grained in texture. Basalt with a few small olivine phenocrysts. Iceland. Other factors include the diffusion rate, which is how atoms and molecules move through the liquid. Igneous Rock Textures By: Ahmed Essam 2. Course. The fine-grained which is less than 1 mm is Aphanitic. Which of the following textures indicates two stages of cooling and crystallization? This is a somewhat altered, augite-bearing granitoid from a dike. Skaergaard intrusion, Greenland. What does an igneous rock with a pyroclastic texture tell a geologist? Similarly, the shape or form of the crystals, which is best seen only in thin sections under microscope, may be described as perfect, semi perfect or totally irregular. Holohyaline The rock which is entirely composed of glassy materials or glass is known as Holohyaline. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka ( 7. These are called rock-forming minerals and are divided into two main classes: These are generally light in colour and lower in density and include: quartz, felspar group and felspathoid group of minerals; These are dark in colour, comparatively heavier in density and contain ferro-magnesian minerals such as amphiboles, pyroxenes, micas, olivine, oxides of iron etc. Chemical composition of a given rock is ultimately expressed in terms of these normative minerals. Trachytic and Trachytoid textures are common examples. Probably very close to solidus temperatures olivine and augite reacted with the hydrous residual liquid to grow hornblende rims on both olivine and augite. Iceland. The primary magmatic minerals include plagioclase, olivine, augite, biotite, and late brown hornblende. These are coarse-grained (average grain size greater than 5mm) in which all the constituent minerals can be identified megascopically, that is, with unaided eyes. Atlas of igneous rocks and their textures A companion volume to the Atlas of Rock-forming Minerals in Thin Section, this full-colour handbook is designed to be used as a laboratory manual both by elementary students of earth sciences undertaking a study of igneous rocks in thin section under the microscope, and by more advanced students and teachers as a reference work. Imperfections in the augite, possibly trapped liquid inclusions or exsolution lamellae, also reacted to grow hornblende within augite crystals. Mixed with them are finer-grained ash, partly altered, and small crystals of biotite and feldspar. The ultimate result (of defining a class etc.) Three classes are recognized on this basis: The rocks in which silica percentage is greater than 66 and is indicated by abundance of minerals of primary/pure silica composition as Quartz, SiO2. You can actually get a uniaxial positive interference figure from this grain. Scoria. (c) Each class of the above rocks is then subdivided into rock subclasses according to the relative abundance of following rock forming mineral assemblages: (d) Further subdivision of some of these classes is made on the basis of predominance of particular type of a rock-forming mineral. The TiO2 polymorph is commonly anatase. Xenolith in the Skaergaard Intrusion, Greenland. make these rocks. This results in a rock almost completley made of the cumulate minerals, and so very simple mineralogy. 2. A field geologist and a civil engineer or mining engineer are concerned more with a practical rather than theoretical classification system. Flow-aligned plagioclase crystals in an olivine basalt. COARSE GRAINED TEXTURE (), mineral grains easily visible (grains several mm in size or larger)A hand specimen of granite with phaneritic (coarse grained) texture. Granites and syenites are common examples. Texture describes the physical characteristics of the minerals, such as grain size. The igneous complex is mostly granitic in composition, but this piece is a tonalite, possibly a cumulate of some sort. These spaces get filled subsequently during the process of rock formation by crystalline or glassy masses of other minerals. Please sign in or register to post comments. The magma can be derived from partial melts of existing rocks in either a planet's mantle or crust. Euhedral olivine phenocryst in basalt. Iceland. Crystallisation in the upper levels of magma becomes very rapid resulting in a fine-grained matrix that contains the big sized crystals formed earlier. There are six main types of textures; phaneritic, aphanitic, porphyritic, glassy, pyroclastic and pegmatitic. During the formation of the igneous rocks, sometimes two or more minerals may crystallize out simultaneously in a limited space so that the resulting crystals are mixed up or intergrown. Laconia, New Hampshire. In this case, grain textures look somewhat annealed, possibly because of deformation and annealing when it was still very hot. While calculating the molecular number, some negligible oxides are either neglected or added to some other related oxides. Hypabyssal Rocks – Porphyries [PO]; Microporphyries [M] and Glasses [G], iii. This relates to the cooling history of the molten magma from which it came. Orthocumulate texture, with cumulate plagioclase and olivine crystals enclosing quenched basaltic liquid. When the average grain size is less than 1 mm. It is then distinguished as porphyritic aphanite. In this article we will discuss about the texture and classification of igneous rocks. The three rock divisions recognised on the basis of colour index are given below: Some authors (e.g. This rock is a gabbro which originally had crystals of homogeneous pigeonite and augite. These can be broadly divided into five categories: Following is a brief account of these textures of igneous rocks: All those textures in which majority of constituent crystals of a rock are broadly equal in size are described as equigranular textures. The textures of igneous rocks differ mainly in the sizes and composition of the mineral crystals. Igneous textures include the rock textures occurring in igneous rocks.Igneous textures are used by geologists in determining the mode of origin igneous rocks and are used in rock classification. The quartz reacted with the magma to grow a rim of hornblende. The individual grains are fine in size but not microgranular. Iceland. Mode of Occurrence of Igneous Rocks Magmas erupted from volcanoes are either poured out as … Plagiarism Prevention 5. (ii) The percentage weight of each of the oxide is divided by its molecular weight to obtain what is called, molecular number of that oxide. is achieved in following steps: (i) The rock is chemically analyzed and the data obtained is converted and expressed in percentage terms of the respective oxides, (which is done by consulting tables). Factors Explaining Texture Composition refers to the rock’s specific mineralogy and chemical composition. Skaergaard Intrusion, Greenland. The resulting rock textures are then described as: When all the constituent minerals are distinctly crystallized; When all the constituents are very fine in size and glassy or non-crystalline in nature. Subophitic texture, in which randomly-oriented plagioclase crystals touch one another, generally enclosing pyroxenes and other minerals. Learn vocabulary, terms, and more with flashcards, games, and other study tools. The salic minerals are- Quartz, felspars orthoclase, albite and anorthite; leucite; nephelite; corundum; zircon and halite etc. The plutonic rocks are formed from magma at greater depths under conditions very favourable to formation of good crystals. In this case, grain textures look somewhat annealed, possibly because of deformation and annealing when it was still very hot. Iceland. Continental Divide, Montezuma, Colorado. At the time the xenolith became trapped, the magma was undersaturated with respect to quartz. Prohibited Content 3. These exhibit perfect or semi-perfect parallelism of crystals or crystallites in the direction of the flow of magma. University of Delhi. You must be able to identify the common textures of igneous rocks (highlighted in bold text below) and understand how they form (Figures 1 and 2). When the average grain size is above 5 mm; the constituent minerals are then easily identified with naked eye. Iceland. Igneous rock texture. As ilmenite broke down, the TiO2 in its structure precipitated in-place to partially pseudomorph the original ilmenite. The feathery texture is a variety of rapid growth between spherulitic and skeletal. In certain igneous rocks crystals formed at earlier stages may get so arranged that polygonal or trigonal spaces are left in between them. (ii) Change in Physico-Chemical Conditions: Every magma is surrounded by a set of physico-chemical conditions like temperature, pressure and chemical composition, which influence the trend of crystallisation greatly. These phenocrysts are medium-grained, and euhedral to subhedral. The following three factors will primarily define the type of texture in a given igneous rock: In an igneous rock, all the constituent minerals may be present in distinctly crystallized forms and easily recognized by unaided eye, or, they may be poorly crystallized or be even glassy or non- crystallized form. Geologists like igneous textures because they reveal so much about how a rock formed. The term microcrystalline is used for the textures in which the minerals are perceivably crystallized but in extremely fine grain. Microscopic examination of thin sections of these rocks becomes essential to determine their mineralogical composition. The original pigeonite, represented by the large gray grain to the left, exsolved augite lamellae as it inverted to orthopyroxene. Texture and structure of igneous rocks. Igneous rock textures 1. No prefix is used when the two groups are equally represented. Crystallinity , Granularity, Shape of grains, Mutual English as well as hindi.. Myrmekite. Graphic and granophyric textures are examples of the intergrowth textures. Before uploading and sharing your knowledge on this site, please read the following pages: 1. Content Filtration 6. -1-TEXTURES OF THE IGNEOUS ROCKS* Structure - large-scale features recognizable in the field, such as banding, lineation, jointing, and vesicularity. The fragments have not been compressed, and the rock is only loosly held together. An attempt has been made in the following table to present a tabular classification with above end in view: The essential features of the tabular or field classification are as follows: (a) The igneous rocks are first divided into three main subdivisions based on their mode of formation as indicated by their textural and structural features – plutonic, hypabyssal and volcanic. For instance if on the basis of chemical analysis and above process, it is observed that in a given rock, 70 % minerals are of salic group and 30% of femic group, the ratio obtained is 2.33 which places the rock into Dosalic group. Vesicular texture is a volcanic rock texture characterized by a rock being pitted with many cavities (known as vesicles) at its surface and inside. This represents subsolidus unmixing of an originally homogeous K-Na feldspar solid solution. Anhedral crystals in a thin hornblende-biotite granite dike. Much of the hornblende is nearly black because of oxidation in air around the time of eruption. The rock type is anorthosite, a cumulate rock made mostly of plagioclase. Welded tuff, with pumice fragments, glass shards, vesicles, and rock fragments variously flattened and folded. The unsaturated minerals like leucite, nepheline, sodalite, olivine etc. Xenocryst in basalt. Plane-polarized light, field width is 3 mm. Iceland. It may be due to operation of factors mentioned above (for porphyritic texture) but in a different manner. This is an equigranular biotite-hornblende granite, with abundant quartz, plagioclase, and microperthite. Fine-grained, 0.2-1 mm. The radial growth causes them to characteristally have an extinction cross, typically with the N-S and E-W directions being at or close to extinction. Iceland. Abrupt and discontinuous changes in these textures may result in the formation of the crystals of unequal dimensions. On the left is a magmatic garnet in a peraluminous granite. Union, Connecticut. In the felsitic texture, the rock is microgranular, the grains being mostly microscopic crystals but these invariably show perfect outlines. Many attempts have been made to classify igneous rocks on the basis of their chemical compositions. Its texture depends on the shape, size, time period to cool down and solidify, and the arrangement of crystals in the rock. Anhedral crystals have no flat crystal faces and generally don't have the approximate shapes of a euhedral crystal. There are several phenocrysts of brown to green pyroxene, magnetite, and plagioclase. classification, the igneous rocks are divided into five classes, which have been named according to salic/femic mineral ratio. Sometimes the texture is specifically termed intersertal if the material filling the spaces is glassy in nature. In the granitic texture, the constituents are either all coarse grained or all medium grained and the crystals show euhedral to subhedral outlines. The textural classification is in a general way, a megascopic or field classification. The subject of classification of igneous rocks is perhaps one of the most thoroughly discussed problems of petrology and yet without any universally accepted solution. Pumice, made of about 90% gas bubbles. In this sample, plagioclase grains are subhedral, but olivine, augite, and magnetite are anhedral. Notice the strong zoning in the alkali feldspars, with orthoclase cores, a layer of plagioclase lath inclusions, and a rim of highly twinned alkali feldspar, possibly albitic. The felsic minerals include quartz, tridymite, cristobalite, feldspars (plagioclase and alkali feldspar), feldspathoids (nepheline and leucite), muscovite, and corundum. It has been observed over wide areas that only a few minerals make bulk of igneous rocks. Dissolution is faster along crystal defects, which can give such crystals odd shapes. Such fine-grained TiO2 is sometimes called leucoxene. The original grains have recrystallized into approximately equant, anhedral grains of colorless plagiocase, colorless, high-index olivine, pale brown augite, and black magnetite. Igneous rocks are also divided into three divisions on the basis of their mode of formation as Plutonic, Volcanic and Hypabyssal rocks. In other words, it is just the reverse of the porphyritic texture. The material of the rock had no time to differentiate into individual grains or crystals. Porphyritic texture in a trachite. These are characterized by silica percentage between 45-66; free silica lies between 0-10 percent.