Igneous and Sedimentary Rocks

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LU 5 IGNEOUS AND SEDIMENTARY ROCKS

IGNEOUS ROCKS

Igneous rocks form by direct crystallization of mineralsfrom a magma melt;

We see a surface expression of magmatic activity duringvolcanic eruptions.

Whats magma?

Molten or partially molten rock beneath the earth'ssurface

Magma is generated deep underground due to the high

temperatures and pressures inside the Earth

Magma tends to rise

It encounters colder rock and begins to cool to form igneous rocks.

The igneous rocks may again be divided into two typesdepending on the place of cooling.

Intrusive (plutonic) rocks crystallize at depth, whereas

extrusive (volcanic and pyroclastic rocks) rockscrystallize after the magma reaches the earth's surface.

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Chemical Composition: The Table below lists the major chemical constituents ofigneous rocks, expressed as oxides, along with the

weight percent ranges in those oxides in normal igneousrocks.

It is clear from this Table that igneous rocks haveextremely variable and complex compositions

Oxide Formula Weight Percent Range

SiO2 35-78TiO2 0.05-4.0

Al2O3 5-22

MgO 0.01-30

CaO 0.5-17

FeO* 0.5-15

Na2O 0.3-9

K2O 0.05-10

Note that FeO* is a combination of FeO (ferrous iron) and Fe2O3 (ferric iron).

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Mineralogy:

The Table below lists the most common minerals foundin igneous rocks.

Note that not all minerals are found in all igneous rocks.

In fact, most igneous rocks only contain four or five different

minerals.

Major Minerals in Igneous Rocks

Mineral Chemical Formula

QUARTZ SiO2

ALKALI-FELDSPAR (K,Na)AlSi3O8

PLAGIOCLASE CaAl2Si2O8-NaAlSi3O8

HORNBLENDE (Na,K)Ca2(Mg,Fe)5(Si,Al)8O22(OH)2

MUSCOVITE (K,Na)2Al4Si6Al2O20(OH)4BIOTITE K2(Mg,Fe)6Si6Al2)O20(OH)4

PYROXENE Ca(Mg,Fe)Si2O6

OLIVINE (Mg,Fe)2SiO4

MAGNETITE-ULVOSPINEL Fe3O4-Fe2TiO4

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ILMENITE-HEMATITE FeTiO3-Fe2O3

The minerals that make up an igneous rock reflect thechemical composition of the rock.

For example an igneous rock rich in SiO2 will tend to be richin the mineral QUARTZ and an igneous rock rich in MgOwill contain a lot ofOLIVINE and/or PYROXENE.

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Classification:

Igneous Rocks are classified in several ways, andmethods of classification have evolved a lot over the past

100 years.

Each classification is useful for a certain purpose andreflects a particular way of looking at igneous rocks.

Igneous rocks are often classified according to thepercentage of SiO2.

The Figures below are general guide to igneous rock

classification, showing the rock names and the differencesin mineralogy.

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Key to interpret the classification table of igneous rocksabove.

Felsic Mafic

Fast

Cooling

SlowCooling

Rhyolite Andesite Basalt

Small Crystals

Large

Crystals

Micro GraniteMicro

Grandiorite

Andesite

Granite

Granodiorite Gabbro

Acidic Basic

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Major Minerals of Igneous Rocks

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IGNEOUS ROCKS

Crystallization and Textures:

Crystallization is the process of mineral formation by thecreation of orderly bonds between atoms.

Bond formation leads to a 3D crystal structure

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Crystallization of Minerals:

As magma cools, crystals of certain minerals form (see

below).

Generalized Cooling and Crystallization of Magma

(5x magnified view)

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If such crystals settle out of the magma the compositionof the magma may be altered, leaving a differentcombination of elements in the magma to form the nextcrystals in the cooling process.

For example, if much of the Ca, Fe and Mg are removedfrom a melt through the formation ofCa-rich plagioclase,olivine and pyroxenes, the remaining magma may bemore enriched in Al, K and Na (felsic) and formintermediate to felsic rocks when cooled.

Bowen's Reaction Series:

This can be thought of as an idealized order of crystallization in a

cooling magma.

N. L. Bowen theorized that the formation of minerals,making up igneous rocks, begins at high temperatureswith two different chemical sequences that eventuallymerge into a single series at cooler temperatures.

One sequence, the discontinuous series, involves theformation of chemically unique minerals at discrete

temperature intervals The other sequence, known as the continuous series,

temperature reduction causes a gradual change in thechemistry of the minerals

Both of these go on simultaneously in a cooling melt.

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The discontinuous series-

The discontinuous series starts with the formation of

rocks that are primarily composed of the mineralolivine. At a certain temperature magma might produce olivine,

but if that same magma was allowed to cool further, nextmineral on the series pyroxene is produced.

Continue cooling and the pyroxene would produceamphibole and then biotite.

Continued temperature decreases change the mineralsdominating the composition of the rock from pyroxene, toamphibole, and then biotite.

This series is discontinuous because the reactionoccurs at a fixed temperature at constant pressurewherein the early-formed mineral is converted to a morestable crystal.

Most silicate minerals are made from slightly differentproportions ofthe same 8 elements, all thats really happening here

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is adjusting the internal crystalline lattice to achieve stability atdifferent temperatures.

Each mineral in the series displays a different silicate structure

that exhibits increased polymerization as the temperature drops;olivine belongs to the island silicate structure type; pyroxene, the

chain; amphibole, the double chain; and biotite, the sheet.

The continuous series-

The continuous series produces light colored rocks rich inplagioclase feldsparminerals.

Plagioclase is continuously reacting with the liquid as thetemperature decreases.

At high temperatures, the plagioclase feldspar

minerals are dominated with the element calcium (Ca). With continued cooling, the calcium in these minerals is

gradually replaced with sodium (Na).

Convergence of both series The convergence of both series occurs with a continueddrop in magma temperature. In both cases, the liquid was consumed in the reaction.

In the merged series, the minerals within the

crystallizing rock become richer in potassium and silica We get the formation of first potassium feldspars andthen the mineral muscovite.

The last mineral to crystallize in the Bowen reaction seriesis quartz. Quartz is a silicate mineral composed of justsilicon and oxygen (SiO2).

However, not all of these minerals will be crystallized

together in the same rock. A mafic magma will begin crystallizing olivine andcontinue with pyroxenes and calcium rich plagioclasefeldspar. Some amphiboles may also crystallize before themelt is used up. Mafic melts dont have enough silica tocrystallize potassium feldspar, quartz, etc.

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Felsic melts don't have enough iron, magnesium, andcalcium to form olivine, pyroxene, or calcium plagioclase.The first-formed minerals in a felsic melt are amphiboles(hornblende) or biotite mica, along with some intermediate

or sodium plagioclase. Eventually, as the melt continues tocool and becomes richer in silica (as the metal cations areused up preferentially in the double chain and sheetsilicates) potassium feldspar and quartz crystallize.

http://gly1000-01.su00.fsu.edu/ig/Ig8.html#six

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The continuous reaction series for Na-Ca Feldspar (plagioclase)reacting with a melt.

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Texture

Texture refers to the way in which individual grains

relate to grains immediately surrounding them. Texture deals with small-scale features seen in hand specimen

or under the microscope, such as the degree of crystallinity, grainsize, grain shape, grain orientation, grain boundary relations

and crystal intergrowths.

Textures are useful indicators of cooling and

crystallization rates and of phase relations betweenminerals and magma at the time of crystallization.

Igneous rocks with interlocking crystals have crystalline

textures.

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The crystallinity and dominant grain size in crystalline

igneous rocks are each described by one of a series ofterms, as shown in the Figures below.

Phaneritic - individual crystals large enough to bedetected with the naked eye.

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Large crystals are clearly visible to the eye with or withouta hand lens or binocular microscope. The entire rock is made up of large crystals, which are generally

1/2 mm to several centimeters in size; no fine matrix material is

present.

They are further described as fine grained (< 1 mm), medium

grained (1-5 mm) or coarse grained (5-10 mm).

This texture forms by slow cooling of magma deep

underground in the plutonic environment.

Aphanitic- individual crystals too small to be detectedwith the naked eye.

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Aphanitic texture consists of small crystals that cannot beseen by the eye with or hand lens. The entire rock is made up of small crystals, which are generally

less than 1/2 mm in size.

This texture results from a rapid cooling in volcanicor shallow subsurface (hypabyssal) environments.

Porphorytic-characterized by two distinct mineral grainsizes.

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Smaller crystals constitute the matrix orgroundmasswhich surrounds larger crystals known asphenocrysts.

Porphyritic rocks are composed of at least two mineralshaving a conspicuous (large) difference in grain size.

The larger grains are termed phenocrysts and the finer grains

either matrix or groundmass (see the diagram above).

Porphyritic rocks are thought to have undergone twostages of cooling; one at depth where the largerphenocrysts formed and a second at or near the surfacewhere the matrix grains crystallized.

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Pegmatitic - an extreme case of porphorytic texture withcrystals > 1cm.

Pegmatites are igneous rocks that have porhoryticrocks with crystals > 1 cm found in a matrix or groundmass

of smaller crystals. Aplites - Aplite is defined as a light-colored plutonic(intrusive) igneous rock characterized by a very fine-grained, granular to sugary texture.

Vesicular - contains cavities (vesicles) formed by gasbubbles trapped as lava cooled.

Vesicular basalt

Glassy - lava or magma quenched abruptly, forming anamorphous mass. No crystals evident even under highmagnification.

Obsidian = volcanic glass

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Aplites & Pegmatites:

Both pegmatites and aplites have the same mineralogiccomposition as granite

Although aplites may range in composition from granitic togabbroic, most tend to be granitic with a mineralogical makeup

consisting of essentially quartz, microcline (potassium feldspar)

and plagioclase.

In some cases, aplites may consist of nearly all of the mineral

albite (sodium feldspar). These albitic aplites (sometimesreferred to as saccharoidal albites) are often associated with

granitic pegmatites, which are typically extremely coarse-grained] but they have different textures.

Pegmatites are very coarse-grained.

Aplites are sugary textured.

The figure below shows a medium-grained pink granite thatis cross-cut by related pegmatite and aplite dikes

Granite with pegmatite and aplite

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Formation of Aplites and Pegmatites fromHydrothermal Fluids

Sources of Hydrothermal Fluids

Magmatic rocksexsolve water (called juvenile or

magmatic water) during the final stages of cooling.

Meteoric water (precipitation) and connatewater (formation)

The hot water fluids are called hydrothermal fluids.

Hydrothermal Fluids change the mineralogy as a result

ofinteraction of the rock with the hot water fluids.

The fluids cause hydrothermal alteration of rocks by

changing their composition by adding or removing or

redistributing components.

Temperatures can range from weakly elevated to boiling.

Fluid composition is extremely variable.

They may contain various types of gases, salts (briney fluids),

water and metals.

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Further crystallization causes an aqueous fluid of lowviscosity to begin to separatefrom the melt, in aprocess called resurgent boiling.

The watery vapor phase, mentioned above, thataccumulates atop a, say, magma chamber is asupercritical fluid.It has the properties of a gas and so themolecules bounce freely.It also has the properties of a liquid and thuscontains many dissolved ions.

The separation of a fluid phase from the saturated melt

gives the system three major phases (crystals,melt and fluid) and initiates the second stage.

Stage 2:Petrogeny's residua system has a granitic melt

and a supercritical fluid i.e. it has two fluidsand magmatic temperature is reduced,

The vapor (supercritical fluid) could unmix and

separate into a liquid and a gas. That is to say thatthe supercritical fluid has boiled.

This boiling or separation of a gas phase from a liquidphase is called second boiling or resurgent boiling.Unlike normal boiling that comes from increasedtemperature, resurgent boiling arises fromreduction of temperature or pressure.

If the supercritical fluid boils, then there will be 3fluids: silicate melt + watery fluid + gaseousfluid.

Thus, petrogeney's residua system might contain twoor three fluids.

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Stage 3:Marked by the end of crystallization from melt,

Commonly, the vapor phase hydro-fractures theearly formed rocks.

Petrogeney's residual system moves into the

fracture.

In these fractures

If the vapor boils, there will be three fluids.

In such a situation, water dissolves nuclei, so that onlya few survive, and ions diffuse more readilythrough water to aid the few surviving ones togrow to giant crystals ofpegmatites.

If the supercritical fluid does not boil, the growingcrystals do not have the benefit of water.

Many nuclei will yield a sugary textured aplite.

Pegmatite forms from 3 fluids while aplite forms from 2fluids.

Notice that pegmatites and aplites are dikes or veinsbecause they fill fractures.

It is possible that, the residua system might changefrom supercritical fluid to a three phase fluid, so thatthe vein fill might start as an aplite and proceed tobecome a pegmatite.

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Trace Elements

Trace elements are elements that occur in lowconcentrations in rocks, usually less than 0.1 % (usually

reported in units of parts per million, ppm).

Trace elements will be found in small quantities in all of theminerals that crystallize from magma.

The ratio of the concentration of trace element i in a crystal(Ci

C) to the concentration of trace element i in the liquid(Ci

L) is called the distribution coefficient (Di) where

Di CiC

/ CiL

IfDi

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Igneous and Sedimentary Rocks

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