Textures in igneous rock

TEXTURES IN IGNEOUS ROCK

Introduction To Igneous Rocks Factors Common textures in igneous rocks :

Phaneritic Texture Aphanitic Texture

Glassy Texture Vesicular Texture

Porphyritic Texture CRITERIA FOR CLASSIFICATION

- FABRIC- SHAPES& INDIVIDUAL GRAINS- MICROSCOPIC EXAMINATION

CONCLUSION REFERENCES

CONTENTS

Introduction to Igneous Rocks

An igneous rock is any crystalline or glassy rock that forms from cooling of a magma.

A magma consists mostly of liquid rock matter, but may contain crystals of various minerals and volatiles.

Magma cools to form an igneous rock either on the surface of the Earth – which forms as a volcanic or extrusive

igneous rock , or beneath the surface of the Earth - which forms as a plutonic or intrusive igneous rock.

Extrusive rocks cool quickly at the Earth’s surface and are fine-grained (small <1 mm)

Intrusive rocks cool slowly deep beneath Earth’s surface and are coarse-grained (large >1 mm)

TextureTexture is used to describe the overall appearance of a is used to describe the overall appearance of a rock based on the size, shape and arrangement of rock based on the size, shape and arrangement of

interlocking minerals.interlocking minerals. Texture provides direct evidence about the origin of the Texture provides direct evidence about the origin of the

rock and always forms a part of the rock description.rock and always forms a part of the rock description.Factors affecting crystal sizeFactors affecting crystal size

Rate of coolingRate of coolingSlow rate = fewer but larger crystalsSlow rate = fewer but larger crystals

Fast rate = many small crystalsFast rate = many small crystalsVery fast rate forms glassVery fast rate forms glass

The main factor that determines the texture of an igneous rock is the cooling rate (dT/dt)

Also the physical conditions during emplacement, eruption, and crystallization of the magma are more important in

controlling the texture of the final rock. Other factors involved are:

The diffusion rate - the rate at which atoms or molecules can move (diffuse) through the liquid.

The rate of nucleation of new crystals - the rate at which enough of the chemical constituents of a crystal can come

together in one place without dissolving.The rate of growth of crystals - the rate at which new

constituents can arrive at the surface of the growing crystal. This depends largely on the diffusion rate of the molecules.

FACTORS

1. Coarse Grain (Phaneritic)2.Fine Grain (Aphanitic)

3.Glassy 4.Vesicular 5.Porphyritic

COMMON TEXTURES IN IGNEOUS ROCKS

COARSE GRAIN TEXTURE(PHANERITIC)Forms deep beneath the surface in a plutonic environment where the molten rock cools slowly.

also called Phaneritic texture.

minerals can be identified with the unaided eye.

Quartz

Orthoclase Feldspar Amphibole

Examples:

Granite, Gabbro, Diorite

larger crystals of uniform size.

SYENITE

• We use the grain size ranges to describe the texture (phaneritic)

Fine Grain Texture(Aphanitic )

Examples:

Basalt, Rhyolite

Forms at or near the Earth’s surface in volcanic environment where the molten magma cools quickly.

very small crystals (only seen under microscope)

may contain vesicles (gas bubbles)

Glassy TextureForms when the ions in the molten rock do not have time to organize into an orderly pattern to form crystals because it cools very quickly.

The result is volcanic glass.

Example:

Obsidian

Minerals cannot be identified with unaided eye

Vesicular TextureForms when gas bubbles escape from molten rock and are trapped as it cools and crystallizes.

This texture can form near the top of lava flows.

Example:

Pumice

Minerals cannot be identified with unaided eye

Porphyritic TextureThis texture results when magma with crystals already formed escape to the surface and cools quickly forming a fine grained igneous rock with large crystals inside.

This texture is a result of two stages of cooling;

1) Slow cooling forming the larger crystals.

2) Rapid cooling forming the finer crystals.

Examples:

Porphyry

labradorite

We use grain size of the groundmass or matrix to describe the phenocrysts (porphyritic)

Contd..Two stages of cooling, i.e. slow cooling to grow a few large crystals, followed by rapid cooling to grow many smaller crystals could result in a porphyritic texture, a texture with two or more distinct sizes of grains. Single stage cooling can also produce a porphyritic

texture. In a porphyritic texture, the larger grains are called phenocrysts and the material surrounding the

phenocrysts is called groundmass or matrix

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:

1. If most of the grains are euhedral - that is they are bounded by well-formed crystal faces. The fabric is said

to be idiomorphic granular.

2. 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.

3. 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.

TRACHYTE,XPL GRANITE,XPL

Other terms may apply to certain situations and should be noted if found in a rock.

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.

Glomeroporphyritic - If phenocrysts are found to occur as clusters of crystals, then the rock should be described as glomeroporphyritic instead of porphyritic

VESICULAR BASALT

Pumiceous - If vesicles are so abundant that they make up over 50% of the rock and the rock has a density less

than 1 (i.e. it would float in water), then the rock is

pumiceous.

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 refilled vesicle.

.

Scoriaceous- If vesicles are so abundant that they make up over 50% of the rock and

the rock has a density greater than 1, then the rock

is said to be scoriaceous.

Graphic - A texture consisting of intergrowths of quartz and alkali feldspar This texture is most commonly observed in pegmatites.

SCORIA

Spherulitic - A texture commonly found in

glassy rhyolites wherein spherical

intergrowths of radiating quartz and

feldspar replace glass as a result of devitrification.

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.

Other textures that may be evident on microscopic

examination of igneous rocks are as follows:

Myrmekitic texture - An intergrowth of quartz and

plagioclase that shows small wormlike bodies of quartz

enclosed in plagioclase. This texture is found in granites.

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 gabbros.

Subophitic texture - Similar to ophitic texture wherein the plagioclase

grains are not completely enclosed in a matrix of

pyroxene grains.

Poikilitic texture – It is one in which large crystal like Feldspar grain, contain small grains of other minerals scattered inside them.

Intergranular texture - A texture in which the angular interstices

between plagioclase grains are occupied by grains of

Ferro magnesium minerals such as olivine, pyroxene,

or Fe-Ti oxides.

Intersertal texture - A texture similar to intergranular texture except that the interstices between plagioclase grains are occupied by glass or cryptocrystalline material.

Coarse grained Basalt Under-XPL

Hyalopilitic texture – A texture wherein

plagioclase are more abundant than groundmass,

and the groundmass consists of glass which

occupies the tiny interstices between plagioclase grains.

Trachytic texture - A texture where in plagioclase grains show a preferred orientation due to flowage, and the interstices between plagioclase grains are occupied by glass or cryptocrystalline material.

Olivine has been replaced by clay minerals and Feo2

- orthoclase phenocrysts in a groundmass of orthoclase with minor plagioclase, biotite, hornblende, augite etc..

Moth eaten texture (also called sieve texture)- This sometimes occurs in plagioclase wherein individual

plagioclase grains show an abundance of glassy inclusions.

Perthitic texture : Exsolution lamellae of albite occurring in orthoclase or microcline.

Perthite

CONCLUSIONIgneous rocks formed from molten material. Igneous rocks are

characterized by an interlocking crystalline texture without preferred orientations. Igneous rocks are classified according to their texture and composition. The composition tells where the source magma came from, the texture tells how it cooled and

reveals information about the origin of the rock. A few compositions of igneous rock are rather common, and

most possible compositions are fairly rare.The textures pumaceous, scoriaceous, tuffaceous, and glassy

immediately determine rock name. For phaneritic rocks, you need to determine the mineralogy

and use that to find a rock name. For aphanitic rocks, you determine mineralogy of the phenocrysts and overall rock color,

and use those to determine a rock name. The classification scheme used in this class is a greatly-

simplified taxonomy.

Hyndman, (1985),Petrology Of Igneous & Metamorphic Rocks, Second edition, McGraw-Hill Book Company, pp

49-61. Loren A.Raymond,(1995),The Study Of

Igneous,Sedimentary,Metamorphic Rocks, Wm.C.Brown Publishers, pp 20-29.

Prof Stephen A.Nelson,(2004),Petrology(pdf)-Textures Of Igneous Rocks, Tulane University

http://geology.about.comhttp://ericfdiaz.files.wordpress.com

www.wikipedia.orgwww.pitt.edu

www.jewelinfo4u.com

REFERENCES