Carbonates 01, Mineralogy

7/29/2019 Carbonates 01, Mineralogy

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MINERALOGYOF

CARBONATE

ROCKS


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Michel-Levy Color Chart


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standard thin-section thickness

first order second order third order


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Carbonate minerals have very high order birefringence colors


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calcite (in a marble), xp light, note pressure twinning.


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Calcite, in a rather too thin slice of a fossil (Inoceramus): Note birefringence

color bands (at least third order colors)


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first order red

2nd

3rd


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Main carbonate rock minerals

Low Magnesian Calcite (LMC)

High Magnesian Calcite (HMC)

Ferroan Calcite

Aragonite

Dolomite

Ankerite and Ferroan Dolomite

Siderite


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Calcium carbonate minerals

LMC - low magnesian calcite - contains

less than 5% magnesium

HMC - high magnesian calcite - usuallycontains between 12 and 30%

magnesium

Ferroan calcite Aragonite - contains traces of strontium


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Calcium magnesium

carbonate minerals

Dolomite - ordered, stoichiometric (50-50 Ca and Mg in alternating layers

Protodolomite - disordered and non-stoichiometric, usually young and notdeeply buried

Ankerite - CaCO3.(Mg,Fe)CO3


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Carbonate Mineral

Identification

X-Ray Diffraction


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13/53LockX-Ray Diffraction permits determination of Ca++:Mg++ ratio in dolomite


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Carbonate Mineral

Identification

X-Ray Diffraction

staining


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Stains Used with Carbonate

Rocks Alizarin Red-S; stains calcite red

applied by dipping rock or thin section briefly in

slightly acid stain

Titan Yellow; stains dolomite yellow applied by boiling sample in strong alkaline

solution

Potassium Ferricyanide

stains iron-bearing mineral blue

may be applied together with alizarin


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calcite (stained red with Alizarin Red S) and unstained dolomite rhombs

l i h ll ( i d d i h Ali i R d S)


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calcite shell (stained red with Alizarin Red S)

ferroan calcite cements in mold from dissolved aragoniteshells (stained purple (Alizarin and potassium ferricyanide)

pore space

(blue dye)

glauconite

(natural green)


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Carbonate Mineral

Identification

X-Ray Diffraction

staining

S.E.M. - Scanning Electron Microscopy


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it dl (SEM)


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aragonite needles (SEM)

Eff t f t ti t i l ith d l it t l


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Effect of rotating stage in ppl, with dolomite crystals

A

B

B

A

d l it t i d ith Tit Y ll l i lith i li t


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dolomite, stained with TitanYellow, replacing ooliths in a limestone


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Non-carbonate minerals

Gypsum:

grey-silver birefringence colors, like quartz,

but good cleavages

Anhydrite

bright first order birefringence colors, right-

angled cleavages

Lenticular gypsum Note gray birefringence cleavage


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Lenticular gypsum. Note gray birefringence, cleavage

Anhydrite and dolomite Smackover Formation (Jurassic) Alabama subsurface


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Anhydrite and dolomite, Smackover Formation (Jurassic), Alabama subsurface.

anhydrite replacement of foraminiferal carbonate mud


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anhydrite replacement of foraminiferal carbonate mud

miliolid foram "ghost"

mud remnantdolomite rhombs

anhydrite and dolomite


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anhydrite and dolomite

Anhydrite note cleavages at right angles bright first order interference colors


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Anhydrite note cleavages at right angles, bright first order interference colors

anhydrite note the replacement of a shell fragment


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anhydrite note the replacement of a shell fragment


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anhydrite deep subsurface diagenetic replacement


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anhydrite, deep subsurface diagenetic replacement


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Halite:

isotropic, cubic

halite (SEM)


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( )

Halite cubes (isotropic) in porous limestone Sunniland Formation Florida


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Halite cubes (isotropic), in porous limestone, Sunniland Formation, Florida,

Note inclusion


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Quartz and chert

Chert replacing limestone.


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Chert replacing limestone.

Chert, replacing limestone.


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Chert, replacing limestone.

Quartz crystals, replacing limestone.


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Qua c ys a s, ep ac g es o e


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Glauconite


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LockAdams, MacKenzie and Guilford, 1984, "Atlas of Sedimentary RocksUnder the Microscope" Longman


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LockAdams, MacKenzie and Guilford, 1984, "Atlas of Sedimentary RocksUnder the Microscope" Longman


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Looking at Carbonate Rocks

Compare freshly broken surfaces withweathered surfaces the latter commonlyshow textures much better.

In the lab, slabbing, polishing, acid etching,varnishing all improve visibility.

In the field, etch with acid before using thehand lens.


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The Carbonate Equation

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

dynamic equilibrium, so that removingCO2, for example, will cause movementto the right, precipitating more CaCO3.

Note that CO2 + H2O = H2CO3 (carbonicacid)


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Carbon dioxide controls

CaCO3

precipitation

Adding CO2 will dissolve some

carbonate Removing CO2 will precipitate more

carbonate,


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The Carbonate Equation (2)

Controls on carbonate precipitation anddissolution:

temperature CO2 solubility increases with colder

temperature (compare warm and cold Coke) carbonates are readily precipitated in warm water

(where CO2 easily escapes to the atmosphere as

gas) carbonates dissolve in cold water, as in the deep

ocean, hence carbonate compensation depth (CCD)below which fine-grained carbonate sediments donot accumulate.


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Controls on carbonate precipitation and

dissolution:

pressure CO2 solubility increases with higher pressure

(compare Coke before and after being

uncapped)

another factor in dissolution of carbonates in thedeep ocean


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photosynthesis 6CO2 + 6H2O + sunlight energy = C6H12O6

(sugars)

this removes CO2, encourages CaCO3precipitation

respiration the reverse of this reaction, releases CO2 and

dissolves carbonates


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Controls on carbonate precipitation and

dissolution:

water agitation (compare to stirring Coke) allows CO2 to escape from the water,

encouraging carbonate precipitation


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oil generation is preceded by CO2generation during organic diagenesis

carbonate dissolution results in the deepsubsurface

Carbonates 01, Mineralogy

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