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LU 5 SEDIMENTARY AND IGNEOUS ROCKS

SEDIMENTARY ROCKS

Sedimentary rocks form from pre-existing rock particles - igneous,metamorphic or sedimentary.

The parent rock undergoes weathering by chemical and/or physical

mechanisms into smaller particles.

These particles are transported by ice, air or water to a region of lowerenergy called a sedimentary basin.

Deposition takes place as a result of a lowering of hydraulic energy,

organic biochemical activity or chemical changes (e.g., solubility).

Once deposited, the sediments are lithified (turned into rock) throughcompaction (decrease in rock volume due to weight of overlying

sediment) and cementation (chemical precipitation in pore spacesbetween grains which "glues" the rock together.

The primary mineralogical and textural characteristics of the rock can bemodified as the sediments are buried deeper in the earth's crust andundergo an increase in both temperature and pressure. This low pressure,low temperature change process is termed as diagenesis.

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Compaction of sediments

Sediments

Sediments are a collection of loosened particles of solid rock originating from

weathering and erosion of preexisting rocks

chemical precipitation

organic secretion or organic debris

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Formation of Sedimentary Rocks

Five Processes Common to the Formation of Many Sedimentary Rocks

Rx=Rocks

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1. Weathering

The picture on the left below is of a coarse grained granite with pinkorthoclase feldspar that is almost pristine, with little weathering.

Notice the pink orthoclase, black amphibole, and clear glassyquartz. All these are hard, durable minerals, bright and shiny. Note

especially the pinkness of the orthoclase.

The picture in the middle is the same granite, but one that has begun

to weather; notice how the luster of the orthoclase has become dull,and the color has begun to fade.

The picture on the right has that the granite has crumbled into a pileof decomposing igneous minerals, and their weathering products -clay, and other new sedimentary minerals.

Such weathered material, commonly found at the base of granite

slopes, will be picked up by rain, streams, and rivers and begin itjourney to be deposited to become a sedimentary rock.

2. Erosion (means to pick up) removal of material by water, wind or ice

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4

Weather ing: decomposit ion of rocks

Th e re is a d i st in c t io n b e tw e e nweathering and e rosion:

Weather ing conver ts exposedrock to soil in place

Eros ion t ransport s d is so lvedor fragmented material fromthe source area whereweathering is occurring to adepositional environmentwhere it can form sediment.

Mo s t o f th e e a rt h s su rf a c e i scovered by exposure of sediment or sedimentary rock,by a rea .

But the sed iment layer i s th inin most places, with respect tooverall crustal thickness, sosedimentary rock is a minorvolume fraction of the crust(in part by definition: onceburied to the mid-crust,sediments get cooked tometasediments).

3. Transportation - moving material from one location to another

There are a number of different ways in which sediments are moved fromone place to another. His is done by the different transportation agents''.

Here's a list of some:

Rivers

Glaciers Wind

Ocean Waves/Tides/Currents

Rivers: Once an element enters into streams and rivers it becomes part ofthe hydrosphere.

Load

The rock particles and dissolved ions carried by the stream are the calledthe stream's load.Stream load is divided into three parts.

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o Bed Load

Coarser and denser particles that remain on the bed of the stream most ofthe time but move by a process of saltation (jumping) as a result of

collisions between particles, and turbulent eddies.Sediment can move between bed load and suspended load as the velocityof the stream changes.

o

o

o Suspended Load

Particles that are carried along with the water in the main part of thestreamsThe size of these particles depends on their density and the velocity of thestream.

Higher velocity currents in the stream can carry larger and denserparticles.

o Dissolved Load

Ions that have been introduced into the water by chemical weathering of

rocksThis load is invisible because the ions are dissolved in the water.The dissolved load consists mainly of HCO3

- (bicarbonate ions), Ca+2, SO4-2,

Cl-, Na+2, Mg+2, and K+.

These ions are eventually carried to the oceans and give the oceans theirsalty character.

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Streams that have a deep underground source generally have higher

dissolved load than those whose source is on the Earth's surface

All these modes can transport clastic particles of various sizes, and the sizeeach can transport depends on the force with which it flows.

Figure:This graph describes the relationship between stream flow velocity

and particle erosion, transport, and deposition.The curved line labeled "erosion velocity" describes the velocity required toentrain particles from the stream's bed and banks.

The erosion velocity curve is drawn as a thick line because the erosionparticles tend to be influenced by a variety of factors that changes fromstream to stream.

Also, note that the entrainment of silt and clay needs greater velocitiesthen larger sand particles.

This situation occurs because silt and clay have the ability to form cohesivebounds between particles. Because of the bonding, greater flow velocities

are required to break the bonds and move these particles.

The graph also indicates that the transport of particles requires lower flow

velocities then erosion.

This is especially true of silt and clay particles.

Finally, the line labeled "settling velocity" shows at what velocity certainsized particles fall out of transport and are deposited.

http://www.classzone.com/books/earth_science/terc/content/visualizations/es1303/es1303page01.cfm?chapter_no=visualization

http://www.classzone.com/books/earth_science/terc/content/visu

alizations/es0604/es0604page01.cfm?chapter_no=visualization

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4. Deposition - cessation of transport and accumulation of sedimentsSediments Settling Down!

These pebbles were deposited in a stream that once flowed over this area inIndiana, USA thousands of years ago.

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5. Lithification - process whereby loose material is converted into rock -processes involved in lithification:

Stratification

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Main components of sedimentary rocks

It is essentialto understand the four main components of sedimentary

rocks:

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Grains or particles are deposited after transportation, or may be a

residuum after weathering Matrix is the finer (deposited) material between most of the grains

Cements are minerals precipitated from waters in and passing throughthe pores

Porosity represents the holes in the rock: these may be primary,having existed immediately after sedimentation, or secondary, having

been produced by mineral dissolution or fracturing during diagenesis.

a. Compaction - weight of overlying sediment compresses and reducespore space (opening between grains) in the sediments below

b. Desiccation - loss of water mainly from compression as pore space isreduced - for clay-rich material compaction and desiccation are enoughto make it into a rock (claystone or shale)

c. Cementation - growth of minerals on grain surfaces, such that grainsbecome bound together as minerals fill the pore spaces between grains

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Compactionthe loss of porosity due to overburden pressure. With

compaction, sediments dewater and grains become more tightly packed.

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Mineralogy:

Because of their detrital nature, any mineral can occur in a sedimentaryrock.

Clay minerals, the dominant mineral produced by chemical weatheringof rocks, is the most abundant mineral in mudrocks.

Quartz, because it is stable under conditions present at the surface of the

Earth, and because it is also a product of chemical weathering, is themost abundant mineral in sandstones and the second most abundantmineral in mudrocks.

Feldspar is the most common mineral in igneous and metamorphicrocks. Although feldspar eventually breaks down to clay minerals andquartz, it is still the third most abundant mineral in sedimentary rocks.

Carbonate minerals, either precipitated directly or by organisms, makeup most biochemical and chemical sedimentary rocks, but carbonates are

also common in mudrocks and sandstones.

Principal Minerals in Sedimentary Rocks

Mineral Composition

Quartz SiO2

Plagioclase feldspar Composition varies between albite, NaAlSi3O8, and

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anorthite, CaAl2Si2O8

K-feldspar (orthoclase or

microcline)KAlSi3O8

Kaolinite (clay) Al2Si2O5(OH)4

Muscovite K,Al Mica KAl2(Al,Si3)O10 (OH)2

Biotite K (Mg,Fe) Mica K(Mg,Fe)3(Al,Fe,Si3)O10(OH)2

Goethite FeO(OH)

Hematite Fe2O3

Gypsum CaSO4.2H2O

Anhydrite CaSO4

Calcite CaCO3

Dolomite CaMg(CO3)2

Mudrocks Sandstones

Mineral Composition % %

Clay minerals 60 5

Quartz 30 65

Feldspar 4 10 - 15

Carbonate minerals 3

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Crystallisation of dissolved minerals in a shallow part of the

Clastic Rocks:

We classify clastic sedimentary rocks on the basis of texture andcomposition.

Texture is controlled by grain size, which is the basis of clastic sedimentaryrock classification.

The following table details the classification.

Clast Size Clast Name Rock Name

Greater than 2 mm Gravel Conglomerate or Breccia

1/16 to 2 mm Sand Sandstone

1/256 to 1/16 mm Silt Siltstone

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Less than 1/256 mm Clay Mudstone or Shale

Brecciaapplies to angular clasts; the clasts in a conglomerate arerounded.

Shale is used to describe rocks that cleave into sheet-like fragments;

mudstones break into irregular shapes.

Conglomerate. Sandstone

Siltstone Shale

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Chemical and Biochemical Rocks:

Chemical and biochemical rocks are classified by composition, rather thangrain size.

There are a number of chemical and biochemical sedimentary rocks asdescribed below.

Carbonates:

The most abundant carbonate rock is limestone, composed almost entirelyof CaCO3, usually as calcite.

This can be formed directly from reef carbonates or as accumulations ofcarbonate sands or muds produced by fossils (foraminifera) or direction

precipitation in warm shallow waters.

Limestone

Evaporites:

Evaporates are formed when saline water evaporates.

This requires an arid climate, little freshwater influx and limited connectionto open waters.

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As water evaporates, the dissolved components increase in concentration.When they become over saturated, they precipitate. The order of

precipitation moves from Calcite, to gypsum, to halite, to magnesium andpotassium chlorides and ends with sulfates.

Freshwater evaporation can also produce evaporites if evaporationproceeds for a long time with constant fresh influx, allowing salinity within

the lake to increase.

Gypsum Halite

Siliceous sediments:

Diatoms and radiolaria produce shells or casts of silica (SiO2).

These rain onto the ocean floor (or lake floor), producing siliceous oozes

that recrystallize into chert or flint, a rock consisting almost entirely of

microcrystalline silica, with further deposition and burial. Less recrystallized cherts contain opal (SiO2 * nH2O), a semi- precious

gemstone.

Diatom accumulations can also produce diatomite, a sedimentary rockused to make abrasives.

Iron oxides:

Iron formations, sedimentary rocks containing more 15% iron in the formof iron oxides and minor silicates, formed early in Earth's history.

Important sources of iron ore, these are indicative of a reducing (lowoxygen) environment that allowed Fe2+ to occur in solution and be

transported great distances.

What exactly caused them to precipitate is still not certain.

Peat, coal, oil and gas:

Coal is the only rock here, but all forms of organic matter occurring insedimentary rock are important as fossil fuels.

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The deposition of significant organic matter requires a reducingenvironment since oxygen, if present, will oxidize the organic material,

producing water and carbon dioxide.

Coal begins as peat. With burial, heat and time, it undergoes a series ofreactions, such as the release of methane (CH4), that increase the carbon

content of the residuum.

Oil and gas are fluids formed by the diagenesis of organic material in thepores of sedimentary rock.

Deep burial alters organic matter found in shales to fluids that escape,ascend and accumulate in porous reservoir rock (typically sandstone orlimestone).

Oil-bearing deposits are marine in origin. The organic matter they derive

from is thought to consist largely of phytoplankton (small plants, includingdiatoms) and bacteria.

CoalGlobal abundance of sedimentary rocks:

Sedimentary rocks account for about 75% of the rocks at the earth's surface:

Average thickness of sedimentary rocks: continents: 1.8 km; oceans: 300 m.

The thickest accumulations are found in shallow marine settings, and inmountain belts.

In mountain chains sedimentary rocks can gradually pass into metamorphic

rocks without an obvious interface.

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Abundance of Sedimentary Rocks

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Chemical Composition

Igneous

Rocks

Shale Sandstone Limestone Sedimentarya Sedimentaryb Sedimenta

SiO2 59.14 58.10 78.33 5.19 58.49 59.7 46.20

TiO2 1.05 0.65 0.25 0.06 0.56 - 0.58

Al2O3 15.34 15.40 4.77 0.81 13.08 14.6 10.50

Fe2O3 3.08 4.02 1.07 0.54 3.41 3.5 3.32

FeO 3.80 2.45 0.30 - 2.01 2.6 1.95MgO 3.49 2.44 1.16 7.89 2.51 2.6 2.87

CaO 5.08 3.11 5.50 42.57 5.45 4.8 14.00

Na2O 3.84 1.30 0.45 0.05 1.11 0.9 1.17

K2O 3.13 3.24 1.31 0.33 2.81 3.2 2.07

H2O 1.15 5.00 1.63 0.77 4.28 3.4 3.85

P2O3 0.30 0.17 0.08 0.04 0.15 - 0.13

CO2 0.10 2.63 5.03 41.54 4.93 4.7 12.10

SO3 - 0.64 0.07 0.05 0.52 - 0.50

BaO 0.06 0.05 0.05 - 0.05 - -

C - 0.80 - - 0.64 - 0.4999.56 100.0

0

100.00 99.84 100.00 100.0 100.13

a = Clarke b = Garrel and Mackenzie (1971) c = Ronov and Yaroshevsky (1969)

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SEDIMENTATION AND GEOCHEMICAL PROCESES

1. WEATHERING

Mineral Stability:

Many minerals in igneous and metamorphic rocks formed at hightemperature (T) and pressure (P), in dry environments.

When exposed to the atmosphere, and lower T and P, many minerals arechemically unstable.

They weather, producing sediments.

In one of the first attempts to study how minerals respond to chemicalweathering, Goldich studied the weathering of gneissic rocks (metamorphic

rocks) in the United States in the 1930's, and established the GoldichStability Series (see below)

This series is the same as the Bowenss Reaction Series in igneous

petrology.

The least stable minerals are those which form at the highest temperature

in a magmatic melt (e.g., olivine), whereas those minerals that crystallize

at lower temperatures (e.g. quartz) are generally more resistant toweathering.

Because these conditions differ from those under which most rocks form

minerals can be classified based on their stability under near surfaceconditions.

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Such a classification, with minerals listed in order of increasing stability isas follows:

Stability Under Present Surface Conditions Mineral

Unstable

Olivines*Pyroxenes*

Ca-rich Plagiocalse*

Hornblende*

Andesine - Oligoclase*

Less Unstable

Sphene

Epidote

Andalusite

Staurolite

Kyanite

Sillimanite

Magnetite

Garnet

Very Stable

Muscovite*

Albite*

Orthoclase/Microcline*

Clay Minerals

Quartz*

Tourmaline

Zircon

In this list, the igneous minerals have an asterisk (*).Note that the order in which they occur is in the same order that occur inBowen's reaction series.

Igneous minerals that crystallize at the highest temperatures are most outof equilibrium at the Earth's surface, and are therefore the most unstable.

Minerals that are very stable at the Earth's surface are minerals that either

form as a result of chemical weathering, or crystallize at the lowesttemperatures.

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Part of the explanation of mineral resistance to weathering relates to the

cation bond strengths of the different elements with oxygen (e.g. insilicate minerals).

The sequence from weakest to strongest bonds is: K, Na, Ca, Mn, Fe

2+

,Mg, Fe3+, Al, Ti, Si.

The K-O bond is weaker than the Mg-O bond, yet orthoclase (K-feldspar) ismuch more resistant to weathering than forsterite (Mg-rich olivine)...why?

The explanation lies in the structure of the minerals:

In olivines, the Si and O are arranged in isolated silica tetrahedra;

their -ve charges are balanced by Mg2+.

When Mg2+ is removed in weathering, the mineral structure collapses.

In contrast, the strong Si-O bonds in orthoclase (chains) prevent theframework structure falling apart despite removal of the K+ ions during

weathering.

The structure survives, and K-feldspar can become a detrital mineral insediments, especially in the feldspar-rich sandstones termed arkose.

Relative Mobilities

High mobility- ions easily removedHigh immobility relatively difficult to removeMobility is related to ionic potential

Ionic potential: Ratio of the valence or ionic charge (Z) to ionic radius (r)Useful first approximation

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Very mobile ions: Z/r 9.5

IONIC RADIUS AND IONIC CHA RGE(3.5/56)

However, controlled by major external factors: Leaching, pH, Eh etc.

Extent of weathering largely depends on the mobility of ions in the rock

Relative mobility of common cations:

(Ca2+, Mg2+, Na+)>K+>Fe2+>Si4+>Ti4+ >Fe3+>Al3+)

Mineral Solubility:

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Solubility, as measured in the laboratory, is the amount of mineral that

will dissolve in a fixed amount of water, at a certain temperature andis a property unique to each mineral composition.

Minerals differ in their tendency to dissolve in water.

In a natural system, the solubility of any mineral is a function of the

composition of the liquid surrounding it. Only rarely will that liquid be purewater.

Solubility of Calcite:

The main minerals in carbonate rocks are:

Calcite: CaCO3 Mg-calcite: CaCO3, but with several mol% Mg

(Typically > 4 mol %)

Aragonite: CaCO3

Dolomite: Ca Mg (CO3)2

Calcite and aragonite arepolymorphs they have the same chemicalcomposition but a different crystal structure

(Calcite is trigonal whereas aragonite is orthorhombic).

The common carbonate minerals differ in their solubility.

Calcite is less soluble (i.e. more stable) than aragonite in most fluids atthe earth's surface.

The solubility of Mg-calcite (high magnesium calcite) varies with the

amount of Mg substitution for Ca in the calcite crystal lattice, butMg-calcite is commonly more soluble than aragonite.

(Mg-calcite commonly recrystallizes to calcite during diagenesis; aragonite,

being more soluble the calcite (especially in fresh water), may dissolve toproduce secondary porosity or may alter to calcite during diagenesis.)

Several factors influence the solubility of CaCO3:

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At higher latitudes and with increased water depth, the solubility

increases due to decreasing temperature, and in the case of waterdepth, increasing pressure.

It is much easier for CaCO3 to precipitate in warm surface water,

which is why most (but not all) marine carbonate sedimentationtakes place in subtropical and tropical waters.

(Surface seawater is supersaturated with respect to both calcite

(~2.8X) and aragonite (~1.9X) in the tropics.)

Cool water carbonates that forming in temperate and high latitudes doexist, but are mainly accumulations of skeletal carbonate secreted byseveral groups of organisms.

Although supersaturated, surface seawaters in the tropics are notperpetually crowded with growing crystals of CaCO3. The main reasons

are:

1. Kinetics: it is difficult for crystals of calcite or aragonite to nucleate in

seawater: the level of supersaturation may be too low.

2. The high amount ofMg in seawater, which decreases the stability and

inhibits growth of nucleating crystals.o CO2The main reaction for the precipitation of calcite or aragonite is:

Ca2++ 2HCO3- = CaCO3+ CO2+ H2O

For saturated solutions, removing CO2 may induce precipitation.

Reversing the reaction (adding CO2) may induce dissolution of existing

calcite or aragonite.

Methods to decrease CO2 and promote mineral precipitation:

1. Increase the temperature

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Increasing the temperature decreases CaCO3 solubility. Degassing of CO2may result because gases are less soluble in warm water than cold:

2HCO3- = CO3

2- + H2O + CO22. AgitationAgitating the fluid may release CO2 that is in excess of that which would be inequilibrium with atmospheric CO2.

Wave action is important in this respect. This is similar to shaking apop bottle.

Gas is injected under high pressure. Removing the cap allows CO2 toescape as the fluid equilibrates with atmospheric CO2.

Shaking (agitating) the bottle accelerates the loss of carbon dioxide.

3. Increasing the salinityCarbon dioxide is less soluble in saline waters than fresh waters.

Salinity normally increases by evaporation which not only leads to lossof CO2, but also increases the amount of Ca

2+ and CO32- left in the

fluid.

This also increases the potential for mineral precipitation.

4. Biochemical activity

Carbonates are commonly produced directly or indirectly by the activities oforganism, especially microbes.

For example, during photosynthesis CO2 is removed from the

environment to provide organic carbon (e.g. in algae), and the release

of oxygen:106CO2 + 16NO3

- + HPO42- + 122H2O + 18H

+ + trace elements +

energy = C106H263O110N16P + 138O2

The reverse of this reaction the decay or organic matter leads todissolution of the carbonate and production of porosity:

CH2O (organic matter) + O2 = CO2 + H2O, thenCO2 + H2O + CaCO3 = Ca

2+ + 2HCO3-

Most marine carbonate precipitation is probably due either directly or

indirectly to the activities of organisms.

Silica Solubility:

Silica solubility in fluids increases with increasing pH, temperature andpressure:

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Reactions in Aqueous Solutions

Physio-Chemical Conditions

There are certain primary parameters that are needed to specify the

physio-chemical conditions present in the water system.

We refer to these as Master Variables.

Some Master Variables that influence the physio-chemical conditions are

Temperature

Pressure pH: A measure of the concentration of hydrogen and hydroxide ions.

The pH of a solution measures how acidic or alkaline it is.pH values range from 0 to 14.

A neutral solution has a pH of 7.A pH less than 7 indicates an acidic solution while a pH greater than7 indicates an alkaline solution.

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Redox Potential

- measures ability of environment to supply/take up electrons

- potential is sum of all possible reactions

- overall Eh more important than individual reaction

Natural Parameters or Variables and Oxidation-Reduction Reactions

The oxidising power or the redox potential and acidity are the two most

important parameters in an aqueous system.

Waters in different near-surface environments have their respective

Eh-pH ranges. These two parameters or variablesand their ranges normallyencountered in various near-surface natural aquatic environmentsmay beconveniently represented using an Eh-pH diagram.

A number of Eh-pH domains are seen for aqueous systems innatural aquatic environments.

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The top part of the diagram represents oxidizing environments and the

bottom reducing environments.The Eh-pH diagram also graphically shows the ranges of these two master

variables over which an aqueous species is stable. These are referred to astability fields.The Eh-pH diagram below shows the various stability fields in the

manganese-water system.

Eh-pH DIAGRAMS

Show both redox and acid-base reactions. Depict mineral stabilities and solubilities. Depict the predominant aqueous species. Important for understanding processes in environmental geochemistry,

exploration geochemistry, corrosion science, hydrometallurgy, and avariety of other fields.

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The H2O - CO2 - CaCO3 System

Chemical Reaction:

Carbonic acid is formed when atmospheric CO2 is dissolved in water.

1) Gas dissolution

2) Carbonic acid formation

This equation indicates that one molecule of water reacts with one moleculeof carbon dioxide to make carbonic acid H2CO3.

3) Carbonic acid equilibrium

Carbonic acid is involved in another reaction, breaking down into hydrogen

ions and bicarbonate ions.

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Note: Carbonic acid H2CO3, Bicarbonate ions, HCO3-

Dissolution of calcium carbonate

Precipitation and Formation of Chemical Sedimentary Rock

Unlike most other sedimentary rocks, chemical sedimentary rocks are notmade of pieces of sediment.

Instead, they have mineral crystals made from elements that are

dissolved in water.

All sorts of things can dissolve into water. If you put a spoonful of salt intowater, the salt will eventually dissolve.

The water in the oceans, lakes, and ground is often full of dissolvedelements.

Seawater tastes salty mainly because there are salty minerals such ashalite dissolved in it.

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Sometimes water becomes so full of dissolved elements that they will notall fit. Some are not able to remain dissolved and form solid mineral

crystals. This usually happens when some of the water has evaporatedaway, leaving less room for the dissolved elements.

If enough water evaporates, they do not all fit and some formcrystals of minerals such as halite, gypsum, and calcite.

In arid areas where it is hot and dry that water evaporates very quickly,leaving behind the minerals that were once dissolved in it.

5. LITHIFICATION

Burial

(>P and >T) + Compaction (reduction of pore space)

Diagenesis

Definition:All physical and chemical processes affecting a sedimentafter deposition before metamorphism

As sediments are deposited, they build layers of material.

These layers are buried by later sediments, which are buried by later

sediments, and so on and so on.

As more and more sediments are deposited, the sediments near the

bottom get heated up and squeezed more and more.

The sediments begin to undergo diagenesis, which is an umbrella term

for physical and chemical changes which turn sediment intosedimentary rocks.

Physical Processes

Physically, the primary change is compacting sediments and squeezingout fluid in the pores between sediment grains; imagine squeezing waterout of a sponge.

Chemical Processes:

Cementation

Development of new precipitates in pore spaces Carbonates (calcite) and silicates (quartz) most common

May be in response to groundwater flow, increasing ionic concentrationin pore waters, and increased burial temperatures

Overgrowths or microcrystalline cement when high pore-waterconcentrations of hydrous silica

Iron oxide (hematite, limonite) determined by oxidation state

Mineral Replacement - Authigenesis

Dissolution of one mineral is replaced by another, simultaneously

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No volume change

Carbonate replacement by quartz; chert by carbonates; feldspars and

quartz by carbonates; feldspars by clay mineralsMineral Recrystallization

Existing mineral retains original chemistry but increases in size

Volume change

Amorphous silica to coarse crystalline quartz; fine lime mud intocoarse sparry calcite

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Diagenesis The process of modification of newly

deposited sediments into sedimentaryrocks is diagenesis or lithification.

Processes include:

physical compaction by the pressure ofoverburden, accompanied by expulsion ofpore waters

Growth of new diagenetic minerals andcontinued growth of chemical sedimentsfrom pore waters.

Dissolution of soluble elements of clasticrocks.

Recrystallization and remineralization as

water chemistry, pressure, and temperatureevolve.

At the high-Tand P end, diagenesis mergessmoothly into the low-Tand P end ofmetamorphism. The distinction is arbitrary.