Prinsip2 Geokimia

7/26/2019 Prinsip2 Geokimia

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PRINCIPLES OFPRINCIPLES OF

GEOCHEMISTRYGEOCHEMISTRY

LECTURES FORLECTURES FOR

UNDERGRADUATE STUDENTSUNDERGRADUATE STUDENTS


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INTRODUCTIONINTRODUCTION

ANDAND

HISTORICAL REVIEWHISTORICAL REVIEW


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HISTORICAL REVIEWHISTORICAL REVIEW

The nameThe name geochemistry was first introduced bygeochemistry was first introduced by SchonbeinSchonbeinsince more thansince more than150 years.150 years.Clark,Clark,who was a chief chemist of the US. Geological Survey from 1! towho was a chief chemist of the US. Geological Survey from 1! to1"#5$ has contributed very much to the science %geochemistry%.1"#5$ has contributed very much to the science %geochemistry%.The modernThe modernscience of geochemistry can be dated bac& to 'lar& who (ublished a veryscience of geochemistry can be dated bac& to 'lar& who (ublished a verylarge number of chemical analyses of the various roc&s in the earth)s crustlarge number of chemical analyses of the various roc&s in the earth)s crust.. *e*ecom(uted averages for each common roc& ty(e and studied the mutualcom(uted averages for each common roc& ty(e and studied the mutualabundance and distribution of many ma+or and minor elements.abundance and distribution of many ma+or and minor elements.Goldschid!Goldschid!,1 - 1"!/ contributed significantly to the roles of ionic sie$,1 - 1"!/ contributed significantly to the roles of ionic sie$

coordination and atomic substitution in crystal lattices. *e gave a (racticalcoordination and atomic substitution in crystal lattices. *e gave a (racticaldefinition for the science geochemistry$ as it deals withdefinition for the science geochemistry$ as it deals with1/ the abundance of elements in roc&$ mineral or crystal$1/ the abundance of elements in roc&$ mineral or crystal$#/ the distribution of the elements$ and#/ the distribution of the elements$ and2/ lows governing the abundance and distribution of elements in roc&$ mineral2/ lows governing the abundance and distribution of elements in roc&$ mineral

or crystal.or crystal.


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The contributions of the USS3 geochemists are remar&able$The contributions of the USS3 geochemists are remar&able$es(ecially after the im(rovement of the analyticales(ecially after the im(rovement of the analyticaltechni4ues at the early decades of the (resent century.techni4ues at the early decades of the (resent century.

There are several trials to classify elements on geochemical basis.There are several trials to classify elements on geochemical basis.ames such asames such assiderophile, chalcophile, lithophile, hydrophile,siderophile, chalcophile, lithophile, hydrophile,thalassophile, atmophilethalassophile, atmophileare commonly used to denote (articularare commonly used to denote (articular

geochemical affinity of elements.geochemical affinity of elements.6odern advances in geochemistry are enormous in different6odern advances in geochemistry are enormous in different

academic and a((lied disci(lines. The revolution in the analyticalacademic and a((lied disci(lines. The revolution in the analyticaltechni4ues$ es(ecially the cou(led inductively (lasma-masstechni4ues$ es(ecially the cou(led inductively (lasma-masss(ectrometry facilitated data with very high (recision on alls(ectrometry facilitated data with very high (recision on all

com(onents of the earth$ such as water$ soil$ sediments$ air$ roc&$com(onents of the earth$ such as water$ soil$ sediments$ air$ roc&$crystals and minerals.crystals and minerals.

3ecently$ huge data are accumulating on the geochemistry of the3ecently$ huge data are accumulating on the geochemistry of the377$ 8G6 and the inert gases.377$ 8G6 and the inert gases.


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Geochemical 9ffinity

:n the classification scheme of Goldschmidt$ elements are divided according to how they(artition between coe;isting silicate li4uid$ sulfide li4uid$ metallic li4uid$ and gas(hasei$ Te

Ce$ 'o$ i$ 3u$ 3h$ 8d$ ?s$ :r$8t$ 6o$ 3e$ 9u$ '$ 8$ Ge$ Sn

To first order$ the distribution of elements between core and mantle resemblese4uilibrium (artitioning between metal li4uid and silicates


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Ehat ma&es an element sidero(hile or litho(hileFEhat ma&es an element sidero(hile or litho(hileFotably$ the Goldschmidt categories are well-grou(ed inotably$ the Goldschmidt categories are well-grou(ed in

thethe #eriodic !able o$ !he eleen!s#eriodic !able o$ !he eleen!s


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:oniation (otentialenergy re4uired to

remove the least tightly bound electron7lectron affinityenergy given u( as an

electron is added to an element

7lectronegativity

quantifies the tendencyof an element to attract a shared electronwhen bonded to another element.

8ro(erties derived from outerelectrons


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ORIGINORIGIN

O%O%

ELE&ENTSELE&ENTS


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A! !he be'innin',,,A! !he be'innin',,,

Matter + antimatterMatter + antimatter

Matter has the advantageMatter has the advantage

baryonsbaryons quarks, leptons,quarks, leptons,

electrons, photons (noelectrons, photons (no

protons or neutrons)protons or neutrons)

HadronsHadrons protons,protons,

neutronsneutrons

Hydrogen, helium (1:10 H:He)Hydrogen, helium (1:10 H:He)

ORIGIN O%ORIGIN O%

ELE&ENTSELE&ENTS


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N(clear reac!ions ) !he be'innin'N(clear reac!ions ) !he be'innin'

HeCHN

NO

OHN

NHC

CN

NHC

!#

1#

11

15

15

15

15

11

1!

1!

11

12

12

12

12

11

1#

++

++

++

++

++

++

+

+

MeVHHHeHeHe

MeVHeHH

MeV

MeVHHH

5".1#

!"2.5

0#.1

!##.0

11

11

!#

2#

2#

2#

11

#1

#1

11

11

++++

+++

+

+++++

+

++

+

CHeBe

BeHeHe

1#

!#

!

!

!#

!#

8articles8articles

-- !!##*e*eHH(roton$ (ositron ,I/(roton$ (ositron ,I/or negatively chargedor negatively chargedelectron ,-/electron ,-/ - high energy (hoton- high energy (hoton neutrinoneutrino

*ydrogen fusion8roton-(roton chain

Tri(le-al(ha fusion


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Neutron-capture

Reactions

NeHeO

OHeC

#010

!#

1

1

!#

1#

+

+

++ NinNiG2

#

1

0

G#

#

hat about the rest

o! the elements""

++ +CuNi 2#"2#

eutrino ca(ture

3adioactive decay


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&AG&ATIC&AG&ATICDI%%ERENTIATIONDI%%ERENTIATION


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6?*? - J!0&m

Eiechert-Gutenbergdiscontinuity

CRUSTCRUST

CORECORE

* &a+or ones, - Transi!ions* &a+or ones, - Transi!ions

Ear!hEar!h.s Cheical Di$$eren!ia!ion.s Cheical Di$$eren!ia!ion


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Mantle Composition

6antle ma&es u( K#L2 of earth6antle ma&es u( K#L2 of earthMsMsmassmass'om(osition a((ro;imated by'om(osition a((ro;imated by

(yrolite$ an invention of 3ingwood(yrolite$ an invention of 3ingwoodMsMsto e;(lain the way these wavesto e;(lain the way these wavesmove through the mantle.move through the mantle.

Si?Si?## !5N!5N

6g?6g? 20-!0N20-!0N

Ce?Ce? -12N-12N9l9l##??22 2N2N

'a?'a? 2N2N*igh 6n$ 'r$ Ti*igh 6n$ 'r$ Ti

?;ides 6antle 'rust

Si?# !5 0

6g? 2 !

Ce? !

9l#?2 2 15

'a? 2

a#? 0.5 2

'r#?2 0.! 0.01

6n? 0.# 0.1

Ti?# 0.1 1

Mantle versus crustMantle versus crust


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&AG&ATIC DI%%ERENTIATION&AG&ATIC DI%%ERENTIATION

Oifferentiation (rocesses affect all ma+or roc& ty(es. Eide variety of s(ecificOifferentiation (rocesses affect all ma+or roc& ty(es. Eide variety of s(ecificreactions ha((en as igneous$ metamor(hic$ and sedimentary roc&s form$reactions ha((en as igneous$ metamor(hic$ and sedimentary roc&s form$change$ trans(ort ions$ andchange$ trans(ort ions$ and Pdecom(oseM which result in geochemicalPdecom(oseM which result in geochemicaldifferentiation.differentiation.

*ow does 6agma com(osition changeF*ow does 6agma com(osition changeF*ot material in different (arts of the mantleF*ot material in different (arts of the mantleF

6elts some roc&s into it H interacts with surrounding material ,8artial6elts some roc&s into it H interacts with surrounding material ,8artial6elting/6elting/

Cractional crystalliationCractional crystalliation crystals form and get se(arated formcrystals form and get se(arated form

source.source.

Wh/ ear!hWh/ ear!h.s cr(s! has 0idel/ 1ar/in' cheis!r/2.s cr(s! has 0idel/ 1ar/in' cheis!r/2

LiquiLiqui

Ma!"aMa!"aCractional crystalliation

Resiual "eltResiual "elt

Cr#stalli$e r%c&Cr#stalli$e r%c&


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8late Tectonics - :gneous Genesis

1. 6id-ocean 3idges#. :ntracontinental 3ifts

2. :sland 9rcs

!. 9ctive 'ontinental

6argins5.. >ac&-arc >asins

. ?cean :sland >asalts. 6iscellaneous :ntra-

'ontinental 9ctivity

&imberlites$

carbonatites$anorthosites$ etc.


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GEOCHE&ISTR3GEOCHE&ISTR3

O% THEO% THEROC45%OR&INGROC45%OR&ING

&INERALS&INERALS


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GEOCHE&ISTR3 O% IGNEOUS ROC4SGEOCHE&ISTR3 O% IGNEOUS ROC4S

'haracteriation of different ty(es ,having different chemistries/'haracteriation of different ty(es ,having different chemistries/

Ul!raa$icUl!raa$ic &a$ic&a$ic In!eredia!eIn!eredia!e %elsic%elsic

'om(osition commonly (resented in weight N of the o;ides ,!0-N'om(osition commonly (resented in weight N of the o;ides ,!0-NSi?Si?##and 1#-1N 9land 1#-1N 9l##??22//

Melt'=i4uid com(osed (redominantly of silica and o;ygen. =i&e water$=i4uid com(osed (redominantly of silica and o;ygen. =i&e water$other ions im(art greater conductivity to the solution. Si and ? areother ions im(art greater conductivity to the solution. Si and ? are

(olymeried in the li4uid to differing degrees ,how(olymeried in the li4uid to differing degrees ,how PrigidM thisPrigidM thisnetwor&/.networ&/.

@iscosity of the li4uid increases with increasing silica content$ i.e. it@iscosity of the li4uid increases with increasing silica content$ i.e. ithas less resistance to flow with more Si?has less resistance to flow with more Si?##related to (olymeriationFFrelated to (olymeriationFF

There is *There is *##? in the magma ,ty(ically #-N/. *? in the magma ,ty(ically #-N/. *##? decreases the? decreases the

overall melting T of a magma$ what does that mean for mineraloverall melting T of a magma$ what does that mean for mineralcrystalliationFcrystalliationF


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8rocesses of chemical differentiation 8artial 6elting8artial 6elting6elting of a different solid material into a hotter li4uid6elting of a different solid material into a hotter li4uid

Cractional 'rystalliationCractional 'rystalliationSe(aration of initial (reci(itates which selectivelySe(aration of initial (reci(itates which selectivelydifferentiate certain elements. 74uilibrium is Q7AF *otter tem(eratures meandifferentiate certain elements. 74uilibrium is Q7AF *otter tem(eratures meanfaster &ineticsfaster &inetics

6elting6eltingCirst bit to melt from a solid roc& is generally more silica-rich. 9tCirst bit to melt from a solid roc& is generally more silica-rich. 9tde(th in the crust or mantle$ meltingL(reci(itation is a 8-T (rocessde(th in the crust or mantle$ meltingL(reci(itation is a 8-T (rocess

6inerals which form6inerals which formare thus a function ofare thus a function ofmelt com(osition andmelt com(osition andhow fast they cool ,re-how fast they cool ,re-e4uilibrationF/. This ise4uilibrationF/. This isgoverned by thegoverned by thestability of thosestability of thoseminerals and howminerals and how4uic&ly they may or4uic&ly they may ormay not react with themay not react with thehosting melt duringhosting melt duringcrystalliationcrystalliation


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RR

AA

BB

6agma at com(osition R6agma at com(osition R,20N 'a$ 0N a/,20N 'a$ 0N a/coolscools first crystalfirst crystal

bytownite ,2N 'a$bytownite ,2N 'a$#N a/#N a/This shifts theThis shifts the

com(osition of thecom(osition of theremaining melt suchremaining melt suchthat it is more a-richthat it is more a-rich

,A/,A/Ehat would be the ne;tEhat would be the ne;t

crystal to (reci(itateFcrystal to (reci(itateFCinally$ the last bit wouldCinally$ the last bit would

crystallie from Bcrystallie from B

6elt-crystal e4uilibrium 1


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&el!5cr/s!al e6(ilibri( 7b&el!5cr/s!al e6(ilibri( 7b8reci(itated crystals react with

cooling li4uid$ eventuallywill re-e4uilibrate bac&.U=7SS it cools so 4uic&ly

the crystal becomes oned orthe early (reci(itates aresegregated and removedfrom contact with the bul& ofthe melt

Ehy arenMt all felds(ars onedF:C there is sufficient time$the crystals will re-e4uilibrate with the magmathey are in. They reflect thetotal a-'a content of themagma. :C not$ thendifferent minerals ofdifferent com(osition will be

(resent in oned (lagioclaseor segregated from each

other (hysically.


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'ombining (hase and com(ositiondiagrams for mineral grou(s

&ica !ernar/&ica !ernar/

8io!i!e series8io!i!e series

Anni!eAnni!e

4%e4%e**9AlSi9AlSi**OO7:7:;9OH;;9OH;--

"hlo'o#i!e"hlo'o#i!e

4&'4&'**9AlSi9AlSi**OO7:7:;9OH;;9OH;--

&(sco1i!e&(sco1i!e

4Al4Al--9AlSi9AlSi**OO7:7:;9OH;;9OH;--

No icasNo icas

&iscibili!/&iscibili!/Ga#Ga#


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S?=:O S?=UT:?

?ccurs in a crystalline solid when one element substitutes for another. Cor?ccurs in a crystalline solid when one element substitutes for another. Core;am(le$ olivine is often e;(resses as ,6g$ Ce/e;am(le$ olivine is often e;(resses as ,6g$ Ce/##Si?Si?!!. This means that. This means thatCe andLor 6g occu(y the same cation site. :t is (ossible to haveCe andLor 6g occu(y the same cation site. :t is (ossible to have

fayalite Cefayalite Ce##Si?Si?!! or forestrite 6g or forestrite 6g##Si?Si?!! or mi;ture of any or mi;ture of any(ro(ortions between these two(ro(ortions between these two end members$end members$i.e.$ fayalite ori.e.$ fayalite orforestrite.forestrite.

#Ce#Ce##Si?Si?!!I #6g #,6g$ Ce/I #6g #,6g$ Ce/##Si?Si?!! #6g #6g##Si?Si?!!I #CeI #Ce

9nother e;am(le garnet may have the com(osition9nother e;am(le garnet may have the com(osition

,6g,6g1.1.CeCe0."0."6n6n0.#0.#'a'a0.#0.#/9l/9l##SiSi22??1#1#. The garnet is a solid solution of the. The garnet is a solid solution of thefollowing end member com(onentsfollowing end member com(onents

,1/ 8yro(e 6g,1/ 8yro(e 6g229l9l##SiSi22??1#1# ,#/ S(essartine 6n ,#/ S(essartine 6n229l9l##SiSi22??1#1#

,2/ 9lmandine Ce,2/ 9lmandine Ce229l9l##SiSi22??1#1# ,!/ Grossular 'a ,!/ Grossular 'a229l9l##SiSi22??1#1#

A(OA(O))* + , +(O* + , +(O))* A* A


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G?=OS'*6:OTMS 3U=7S9O 3:GE??OMS 6?O:C:'9T:?

1. The ions of one element can e;tensively re(lace those of another in ionic1. The ions of one element can e;tensively re(lace those of another in ioniccrystals if their radii differ by less than a((ro;imately 15N.crystals if their radii differ by less than a((ro;imately 15N.

#. :ons whose charges differ by one unit substitute readily for one another#. :ons whose charges differ by one unit substitute readily for one another

(rovided electrical neutrality of the crystal is maintained. :f the charges(rovided electrical neutrality of the crystal is maintained. :f the chargesdiffer by more than one unit$ substitution is generally slight.differ by more than one unit$ substitution is generally slight.2. Ehen two different ions can occu(y a (articular (osition in a crystal2. Ehen two different ions can occu(y a (articular (osition in a crystal

lattice$ the ion with the higher ionic (otential forms a stronger bondlattice$ the ion with the higher ionic (otential forms a stronger bondwith the anions surrounding the site.with the anions surrounding the site.

3ingwood (ro(osed the modifications3ingwood (ro(osed the modifications

!. Substitutions may be limited$ even when the sie and charge criteria are!. Substitutions may be limited$ even when the sie and charge criteria aresatisfied$ when the com(eting ions have different electronegativitiessatisfied$ when the com(eting ions have different electronegativitiesand form bonds of different ionic character.and form bonds of different ionic character.

This rule was (ro(osed in 1"55 to e;(lain discre(ancies with res(ect to theThis rule was (ro(osed in 1"55 to e;(lain discre(ancies with res(ect to thefirst three Goldschmidt rules. Cor e;am(le$ afirst three Goldschmidt rules. Cor e;am(le$ a II and 'uand 'uIIhave the samehave the sameradius and charge$ but do not substitute for one another.radius and charge$ but do not substitute for one another.


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Co(#led a!oic s(bs!i!(!ions

3ule 1 Ehen ' V "$ rTh3ule 1 Ehen ' V "$ rTh!I!IV 1.1 W$ r'eV 1.1 W$ r'e2I2IV 1.#2W.V 1.#2W. O4O4

3ule # ?nly 1 charge unit difference.3ule # ?nly 1 charge unit difference. O4O4

3ule 2 :onic (otential ,Th3ule 2 :onic (otential ,Th!I!I/ V !L1.1 V 2.!# ionic (otential ,'e/ V !L1.1 V 2.!# ionic (otential ,'e2I2I/ V 2L1.#2/ V 2L1.#2V #.!!$ so ThV #.!!$ so Th!I!Iis (referredXis (referredX

3ule !3ule ! Th V 1.2Th V 1.2 'e V 1.1.'e V 1.1. O4O4

>ut we must have a cou(led substitution to maintain neutrality>ut we must have a cou(led substitution to maintain neutrality

ThTh!I!II SiI Si!I!I'e'e2I2II 8I 85I5I

AA-*-* * (* (.., +, +)*)** /* /)*)*

'an Th!I substitute for 'e2I in monaite ,'e8?!/F'an Th!I substitute for 'e2I in monaite ,'e8?!/F

8lagioclase a9lSi8lagioclase a9lSi22??- 'a9l- 'a9l##SiSi##??

aaII I SiI Si!I!I'a'a#I#II 9lI 9l2I2I

Gold and arsenic in (yrite ,CeSGold and arsenic in (yrite ,CeS##//

9u9uIII 9sI 9s2I2I#Ce#Ce#I#I

377 and a in a(atite ,'a377 and a in a(atite ,'a55,8?,8?!!//22C/C/

3773772I2II aI aII #'a#'a#I#I

Ot0er e1a"2lesOt0er e1a"2les


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:ncom(atible vs. 'om(atible:ncom(atible vs. 'om(atibletrace elementstrace elements

INCO&"ATI8LE ELE&ENTS


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Three ty(es ofThree ty(es oftrace-element substitutiontrace-element substitution

1/ '96?UC=9G7$ #/ '98TU37 and 2/ 9O6:SS:?1/ '96?UC=9G7$ #/ '98TU37 and 2/ 9O6:SS:?

'96?UC=9G7'96?UC=9G7?ccurs when the minor element has the same charge?ccurs when the minor element has the same chargeand similar ionic radius as the ma+or element ,same ionic (otential noand similar ionic radius as the ma+or element ,same ionic (otential no

(reference.(reference.

BrBr!I!I

,0.0 W/ *f,0.0 W/ *f!I!I

,0." W/. *f usually does not form its own mineral it,0." W/. *f usually does not form its own mineral itis camouflaged in ircon ,BrSi?is camouflaged in ircon ,BrSi?!!/./.'98TU37'98TU37?ccurs when a minor element enters a crystal (referentially to?ccurs when a minor element enters a crystal (referentially to

the ma+or element because it has a higher ionic (otential than the ma+orthe ma+or element because it has a higher ionic (otential than the ma+orelement.element.

Cor e;am(le$ Q-felds(ar ca(tures >aCor e;am(le$ Q-felds(ar ca(tures >a#I#I,1.!! W BLr V 1.2"/ or Sr,1.!! W BLr V 1.2"/ or Sr#I#I,1.#1,1.#1W BLr V 1.5/ in (lace of QW BLr V 1.5/ in (lace of QII,1.! W$ BLr V 0./. This re4uires,1.! W$ BLr V 0./. This re4uirescou(led substitution to balance charge Qcou(led substitution to balance charge QII I SiI Si!I!ISrSr#I#I,>a,>a#I#I/ I 9l/ I 9l2I2I

9O6:SS:?9O6:SS:?:nvolves entry of a foreign ion with an ionic (otential less:nvolves entry of a foreign ion with an ionic (otential lessthan that of the ma+or ion.than that of the ma+or ion.

7;am(le 3b7;am(le 3bII,1.5 W BLr V 0.2/ for Q,1.5 W BLr V 0.2/ for QII,1.! W$ BLr V 0./ in Q-,1.! W$ BLr V 0./ in Q-felds(ar. The ma+or ion is (referred.felds(ar. The ma+or ion is (referred.


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WeatheringWeathering

Lec!(re *Lec!(re *


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WEATHERINGWEATHERING

There is a distinctionThere is a distinctionbetween weathering andbetween weathering anderosionerosion

6ost of the earth6ost of the earthMs surface isMs surface iscovered by e;(osure ofcovered by e;(osure ofsediment or sedimentary roc&$sediment or sedimentary roc&$by area. >ut the sediment layerby area. >ut the sediment layeris thin in most (laces$ withis thin in most (laces$ withres(ect to overall crustalres(ect to overall crustal

thic&ness$ so sedimentary roc&thic&ness$ so sedimentary roc&is a minor volume fraction ofis a minor volume fraction ofthe crust ,in (art by definitionthe crust ,in (art by definitiononce buried to the mid-crust$once buried to the mid-crust$sediments get coo&ed tosediments get coo&ed tometasediments/.metasediments/.


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STA8ILIT3 O% &INERALS

AGAINST

WEATHERING

'hemical weathering is driven by'hemical weathering is driven bythermodynamic energy minimiation$ +ust li&ethermodynamic energy minimiation$ +ust li&echemical reactions at high tem(erature.chemical reactions at high tem(erature. The system see&s the most stableThe system see&s the most stable

assemblage of (hases.assemblage of (hases.

The differences are thatThe differences are that ,1/ &inetics are slow and metastability is,1/ &inetics are slow and metastability is

commoncommon ,#/ the stable minerals under wet$,#/ the stable minerals under wet$

ambient conditions are different fromambient conditions are different fromthose at highthose at high TTandand

,2/ solubility in water and its de(endence,2/ solubility in water and its de(endenceon water chemistry ,notably (*/ areon water chemistry ,notably (*/ arema+or determinants in the stability ofma+or determinants in the stability ofminerals in weathering.minerals in weathering.

9 fresh roc& made of olivine and (yro;enes9 fresh roc& made of olivine and (yro;eneswill end u( as clays and iron o;ides$ withwill end u( as clays and iron o;ides$ withother elements in solutionother elements in solution

9 fresh roc& made of felds(ars and 4uart will9 fresh roc& made of felds(ars and 4uart willend u( as clays$ hydro;ides$ and 4uart inend u( as clays$ hydro;ides$ and 4uart in

most waters.most waters.


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T3"ES O% WEATHERING REACTIONST3"ES O% WEATHERING REACTIONS


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Some minerals are congruently soluble in acidic water$ leaving no residue

The most abundant is calcite 'a'?2 I *#'?2 V 'a#I I #*'?2H ,the Tums reaction/

7ffects of dissolution ,and (reci(itation/ of calcite can be dramatic$ to say the least.

Sin&holeSin&hole S(eleothemsS(eleothems

Qarst terrainQarst terrain


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Formation of KaoliniteFormation of KaoliniteThe most common alteration (roduct of felds(ars isThe most common alteration (roduct of felds(ars is kaolini!ekaolini!e$$

9l9l##SiSi##??55,?*/,?*/!!$ which serves as a model for the formation of clays by$ which serves as a model for the formation of clays by

weathering generally.weathering generally.

The reactions of felds(ars to &aolinite illustrate some of the basicThe reactions of felds(ars to &aolinite illustrate some of the basictrendstrends Q$ a$ 'a are highly soluble and readilyQ$ a$ 'a are highly soluble and readily leachedleachedbyby

chemical weathering.chemical weathering. 7;cess Si can be removed as silicic acid although 4uart is7;cess Si can be removed as silicic acid although 4uart is

relatively insoluble.relatively insoluble. 9l is e;tremely insoluble$ and is essentially conserved as9l is e;tremely insoluble$ and is essentially conserved as

clays.clays. Eeathering is a hydration (rocess$ leaving *Eeathering is a hydration (rocess$ leaving *##? bound in? bound in

the altered minerals.the altered minerals.# Q9lSi# Q9lSi22??I " *I " *##? I # *? I # *II9l9l##SiSi##??55,?*/,?*/!!I # QI # QIII ! *I ! *!!Si?Si?!!

ote the *ote the *IIon the left-hand side


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8roduction of strong weathering agents

#CeS#CeS##I ?I ?##I #*I #*##?? #Ce#Ce#I#II ! S?I ! S?!!#-#-I ! *I ! *II

Ce o;idiers$ S o;idiersCe o;idiers$ S o;idiers

CeCe#I#II ?I ?##I *I *IICeCe2I2I??* I # *??* I # *II

**III S?I S?!!#-#-==*S?*S?!!--

3eduction by bacteria in swam(s and marches3eduction by bacteria in swam(s and marches'*'*##? I Ce??*? I Ce??* CeCe#I#II '?I '?##

'*'*##? I S?? I S?!!#-#-*S*S--I '?I '?##

6 f ff h


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6any factors affect therate at which a roc& willweather.

Some of these variables arelocal ,e.g.$ source roc&/$some are global. Theseinclude tem(erature and

('?#$ leading to the '?#-weathering feedbac& cycle.


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Sor(tion 9ttraction between a (articular mineral surface and an ionor molecule due to ,1/ 7lectrostatic interaction ,unli&echarges attract/$ ,#/ *ydro(hobicLhydro(hilic interactions$and ,2/ S(ecific bonding reactions at the surface

De$ini!ions


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:n addition$ can also have bi-dendate sor(tion reactions

Sor(tion to YS-?* sites

YYS-?* I 6#IS-?* I 6#I YS-?6I I *IYS-?6I I *I

YYS-?* I =#-S-?* I =#-

YS-=- I ?*-YS-=- I ?*-

S-OH

S-OH+ M2+

S-O

S-OM + 2 H+

7ffect of (* on surface charge ,' m-#/ of selected minerals. The7ffect of (* on surface charge ,' m-#/ of selected minerals. The

curve for calcite is valid for a sus(ension in e4uilibrium with air.curve for calcite is valid for a sus(ension in e4uilibrium with air.


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9dsor(tion of metal9dsor(tion of metal

Ehat factors determine this selectivityFEhat factors determine this selectivityF

,1/ :onic (otential defined as the charge over the radius ,BLr/.,1/ :onic (otential defined as the charge over the radius ,BLr/.

,#/ 'ations with low BLr release their waters of hydration more easily and can,#/ 'ations with low BLr release their waters of hydration more easily and canform inner-s(here surface com(le;es.form inner-s(here surface com(le;es.

,2/ :n a natural solution$ many metal cations com(ete for the available,2/ :n a natural solution$ many metal cations com(ete for the availablesor(tion sites.sor(tion sites.

6any isovalent series cations e;hibit decreasing sor(tion affinity with6any isovalent series cations e;hibit decreasing sor(tion affinity with

decreasing ionic radiusdecreasing ionic radius's'sIIK 3bK 3bIIK QK QIIK aK aIIK =iK =iIIK >aK >a#I#IK SrK Sr#I#IK 'aK 'a#I#IK 6gK 6g#I#IK *gK *g#I#IK 'dK 'd#I#IK BnK Bn#I#I

Cor transition metals$ electron configuration becomes more im(ortant thanCor transition metals$ electron configuration becomes more im(ortant thanionic radiusionic radius

'u'u#I#IK iK i#I#IK 'oK 'o#I#IK CeK Ce#I#IK 6nK 6n#I#I

:n a natural solution$ many metal cations com(ete for the available:n a natural solution$ many metal cations com(ete for the availablesor(tion sites. 7;(eriments show some metals have greater adsor(tionsor(tion sites. 7;(eriments show some metals have greater adsor(tion

affinities than others.affinities than others.


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:on e;change reactions:on e;change reactions :ons adsorbed by outer-s(here com(le;ation and diffuse-ion

adsor(tion are readily e;changeable with similar ions in solution.

Cation e&chan'e capacityCation e&chan'e capacity The concentration of ions$ in The concentration of ions$ inme4L100 g soil$ that can be dis(laced from the soil byme4L100 g soil$ that can be dis(laced from the soil byions in solution.ions in solution.

7;change reactions involving common$ ma+or cations are7;change reactions involving common$ ma+or cations aretreated as e4uilibrium (rocesses. The general form of atreated as e4uilibrium (rocesses. The general form of acation e;change reaction iscation e;change reaction is

n9n9mImII m>RI m>R m>m>nInII n9RI n9R

The e4uilibrium constant for this reaction is given byThe e4uilibrium constant for this reaction is given by

m

B

n

"

n

"

m

B

N

N

a

a(=


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'3AST9= '*76:ST3A'3AST9= '*76:ST3A?C '=9A 6:739=S?C '=9A 6:739=S

G h i f l i lG h i f l i l


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Geochemistry of clay mineralsGeochemistry of clay minerals

'lay minerals can be described as hydrated'lay minerals can be described as hydratedaluminosilicates having grain sie less thanaluminosilicates having grain sie less than

! microns. They are com(osed of two main! microns. They are com(osed of two mainstructural units namely tetrahedral ,silica/structural units namely tetrahedral ,silica/and octahedral ,alumina/and octahedral ,alumina/


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The crystal chemistry of clays controls their (hysical (ro(erties such asThe crystal chemistry of clays controls their (hysical (ro(erties such asswelling$ cationic e;change ca(acity and stability against burial diagenesisswelling$ cationic e;change ca(acity and stability against burial diagenesis

Schematic illustration of the two-layerSchematic illustration of the two-layer,T?/ and three-layer clays ,T?T/,T?/ and three-layer clays ,T?T/

SwellingSwelling

Schematic illustration of some three-Schematic illustration of some three-

layer clays ,note the interlayer cations/layer clays ,note the interlayer cations/

T? clays such as &aolinite and T?Tclays differ with interlayer cations


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'lays can have significant chemicalsubstitution$ they undergo (hase transitions

as diagenesis (roceeds

:lliteSmectite I 6g#II Ce#IISi?#I*#?

9l#Si!?10,?*/#Zn*#? I Q9lSi2?Q9l#,9lSi2/?10,?*#/ I ! Si?#,a4/ I n *#?


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CAR8ONATESCAR8ONATESGEOCHE&ISTR3GEOCHE&ISTR3


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The commonest form of carbonate in nature is 'a'?2which$ li&e

other carbonates$ dissolves in acid medium'a'?'a'?22 'a 'aI#I#I '?I '?22H#H#

Saturation state can be e;(ressed as follows

[ V :98 L Q[ V :98 L QEhere [ is the saturation inde; and :98 is the ion activity (roduct,V \'aI#]\'?2H#]/ for the solution$ and with the a((arent solubility

(roduct for a solution in e4uilibrium with solid 'a'?2$ and Q isconstant. Q de(ends on mineralogy where calcite ^ aragonite ^ 6g-calcite$ as well as on the (revailing T and 8 wheresolubility is higherin dee(er and colder water..

9t e4uilibrium [ V :98 L Q V \'a[ V :98 L Q V \'aI#I#]\'?]\'?22H#H#] V 1] V 1

)hen * + !, solution is supersaturated

)hen * - !, solution is undersaturated

Carbona!e Sol(bili!/Carbona!e Sol(bili!/


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Temperature Pressure

(C) (atm.) Calcite Aragonite

2 ! " ""

2 ! # "$

2 2%& '! !&!

2 2%& !&" !%2

Saturation o caronate ion !&-" mole*+g

I# -# I# -#2 in situ 2 in situ

) I# -#s( 2 saturation

\'a ]\'? ] \'a ]\'? ]

Q \'a ]\'? ] = =

?ften$ \'aI#] is considered as a constant function of salinity of ocean.Then$ [ can be e;(ressed in terms of carbonate ion saturation asfollows

:f \'aI#

] constant

-#2 in situ

-#2 saturation

\'? ]

\'? ]

Therefore$ it is (ossible to define _'?2H# as ,\'?2

H#] in situ - \'?2H#]sat/

8ressure$ tem(erature$ and res(iration will affect the \'?2H#] in situ. The \'?2H#]

will be lower in dee(er$ colder$ and older water

Carbonate ion saturation of seawater as function of T and for the two polymorphs

calcite and ara'onite


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3edo; PCrontsM

>oundary betweeno;ygen-rich ,o;ic/and more reduced,ano;ic/ waters

?;ygen in(ut throughentrainment ,wind$wave action$(hotosynthesis/

?;ygen consum(tion

from heterotro(hicconsum(tion$ reactionwith reduced forms ofCe$ 6n$ S

9no;ic

?;ic

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Prinsip2 Geokimia