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Chapter 22. Continued. Aragonite Structural Type Aragonite CaCO 3. Orthorhombic Tabular prysmatic or commonly pseudohexonal twinned intergrowths Forms: As precipitates from mineral springs In oxidation zone of sulfide ores and weathering crusts Marine chemical sediments Karst caves

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chapter 22
Chapter 22

Continued

aragonite structural type aragonite caco 3
Aragonite Structural TypeAragoniteCaCO3

Orthorhombic

Tabular prysmatic or commonly

pseudohexonal twinned intergrowths

Forms:

As precipitates from mineral springs

In oxidation zone of sulfide ores and weathering crusts

Marine chemical sediments

Karst caves

Part of mother-of-pearl in mollusk shells

aragonite structural type witherite baco 3
Aragonite Structural TypeWitheriteBaCO3

Isomorphic series with aragonite and strontianite

Low T hydrothermal mineral associated with strontianite and barite

aragonite structural type strontianite srco 3
Aragonite Structural TypeStrontianite SrCO3

End-member of isomorphic series with aragonite, and with witherite (BaCO3)

Low T hydrothermal mineral

Concretions in sedimentary rocks

aragonite structural type cerussite pbco 3
Aragonite Structural Type CerussitePbCO3

Oxidation product from lead sulphide ore

Adamantine luster on faces, greasy luster on uneven or shell-like fracture surfaces

High S.G.

Lead ore, but not very NB

the malachite group malachite cu 2 co 3 oh 2 azurite cu 3 co 3 2 oh 2
Occur in oxidation zone of chalcopyrite and other Cu-sulphides

Both pseudomorphs of copper and cuprite

Malachite

Green

Green paint

Rare semi-precious stone

Azurite:

Blue

Blue paint

The Malachite GroupMalachite Cu2(CO3)(OH)2Azurite Cu3(CO3)2(OH)2
bastnaesite group bastnaesite ceco 3 f
Bastnaesite groupBastnaesite – CeCO3F

Rare earth element carbonates

Ce, La, Sr, Ba

Form when REE silicates decomposes in pegmatite veins

Other REE carbonate minerals:

Parisite, rontgenite, synchisite, carbocernaite, ancylite, lanthanite

formation conditions
Formation conditions

Sedimentary rocks – major occurrence

Carbonatites – rare rock type

A carbonate rock of igneous origin

Usually forms dikes or volcano plugs

Hydrothermal deposits – carbonates common

Medium and low T conditions

Surface conditions

Caves: stalactites and stalagmites

Weathering crusts of ore deposits

Metamorphic rocks

From sedimentary rocks: limestones

If limestone pure calcite – forms marble

If calcite, dolomite and quartz – new silicate minerals

CaCO3 + SiO2  CaSiO3 + CO2

Calcite Quartz Wollastonite

carbonates in sedimentary rocks
Carbonates in sedimentary rocks

Carbonate rocks = ¼ of all sedimentary rocks

Limestone, dolostone (less common) and rarely carbonate evaporites

In solution in seawater deeper than: 4.2 – 4.7km

Therefore only precipitates in shallow water environments

In sediments of continental shelf and coral reefs

Shallow oceans of intermediate, tropical zones

Dolomite more rarely precipitated than calcite

carbonates in sedimentary rocks2
Carbonates in sedimentary rocks

Continental carbonate sediments

Clastics, surface crusts, soil, lakes, evaporites

Carbonates in oceans

Clastic material transported from coast

Remnants of skeletons from living organisms

Authigenic minerals

Forms in place: reactions of primary minerals with porous solutions during diagenesis