what happens when granite is weathered?? first, unweathered granite contains these minerals: –na...

What happens when granite is weathered??

• First, unweathered granite contains these minerals: – Na Plagioclase feldspar – K feldspar – Quartz – Lesser amounts of biotite, amphibole, or muscovite

• What happens when granite is weathered?• The feldspars will undergo hydrolysis to form kaolinite

(clay) and Na and K ions • The Na+ and K+ ions will be removed through leaching • The biotite and/or amphibole will undergo hydrolysis

to form clay, and oxidation to form iron oxides.

Granite weathering, continued• The quartz (and muscovite, if present) will remain as

residual minerals because they are very resistant to weathering.

• Weathered rock is called saprolite. • What happens after this?

– Quartz grains may be eroded, becoming sediment. The quartz in granite is sand- sized; it becomes quartz sand. The quartz sand will ultimately be transported to the sea (bed load), where it accumulates to form beaches.

– Clays will ultimately be eroded and washed out to sea. Clay is fine-grained and remains suspended in the water column (suspended load); it may be deposited in quiet water.

– Dissolved ions will be transported by rivers to the sea (dissolved load), and will become part of the salts in the sea.

Sedimentary Minerals

• We will focus on some minerals which form from precipitation of dissolved ions other minerals in sedimentary rocks are derived from the source rocks!

• Clay, carbonate, and sulfate groups are key in sedimentary rocks – can ‘be’ the rock or cement fragments together!– SiO4

4-, CO32-, SO4

2- anionic groups, respectively

• Also consider halides (anion is Cl- or F-) and mineralization of silica

ClaysSheet Silicates – aka Phyllosilicates

[Si2O5]2- Sheets of tetrahedra Phyllosilicates

micas talc clay minerals serpentine

Sheet Silicates – aka Phyllosilicates

[Si2O5]2- Sheets of tetrahedra Phyllosilicates

micas talc clay minerals serpentine

•Clays talc pyrophyllite micas•Display increasing order and lower variability of chemistry as T of formation increases

Clays

• Term clay ALSO refers to a size (< 1mm = <10-6 m)

• Sheet silicates, hydrous – some contain up to 20% H2O together with a layered structure and weak bonding between layers make them SLIPPERY WHEN WET

• Very complex (even argued) chemistry reflective of specific solution compositions

Major Clay Minerals

• Kaolinite – Al2Si2O5(OH)4

• Illite – K1-1.5Al4(Si,Al)8O20(OH)4

• Smectites:– Montmorillonite – (Ca, Na)0.2-

0.4(Al,Mg,Fe)2(Si,Al)4O10(OH)2*nH2O

– Vermicullite - (Ca, Mg)0.3-

0.4(Al,Mg,Fe)3(Si,Al)4O10(OH)2*nH2O

– Swelling clays – can take up extra water in their interlayers and are the major components of bentonite (NOT a mineral, but a mix of different clay minerals)

SiO4 tetrahedra polymerized into 2-D sheets: [Si2O5]

Apical O’s are unpolymerized and are bonded to other constituents

Phyllosilicates

Tetrahedral layers are bonded to octahedral layers

(OH) pairs are located in center of T rings where no apical O

Phyllosilicates

Octahedral layers can be understood by analogy with hydroxides

Phyllosilicates

Brucite: Mg(OH)Brucite: Mg(OH)22

Layers of octahedral Mg in Layers of octahedral Mg in coordination with (OH)coordination with (OH)

Large spacing along Large spacing along cc due due to weak van der waals to weak van der waals bondsbonds

cc

Phyllosilicates

Gibbsite: Al(OH)Gibbsite: Al(OH)33

Layers of octahedral Al in coordination with (OH)Layers of octahedral Al in coordination with (OH)

AlAl3+3+ means that means that only 2/3 of the VI sites may be occupiedonly 2/3 of the VI sites may be occupied for charge-balance reasons for charge-balance reasons

Brucite-type layers may be called Brucite-type layers may be called trioctahedraltrioctahedral and gibbsite-type and gibbsite-type dioctahedraldioctahedral

aa11

aa22

Phyllosilicates

Kaolinite:Kaolinite: Al Al22 [Si [Si22OO55] (OH)] (OH)44

T-layers and T-layers and didiocathedral (Alocathedral (Al3+3+) layers ) layers

(OH) at center of T-rings and fill base of VI layer (OH) at center of T-rings and fill base of VI layer

Yellow = (OH)Yellow = (OH)

T T O O -- T T O O -- T T OO

vdwvdw

vdwvdw

weak van der Waals bonds between T-O groups weak van der Waals bonds between T-O groups

Clay building blocks• Kaolinite micelles attached with H

bonds – many H bonds aggregately strong, do not expend or swell

1:1 Clay

Phyllosilicates

Serpentine:Serpentine: Mg Mg33 [Si [Si22OO55] (OH)] (OH)44

T-layers and T-layers and tritriocathedral (Mgocathedral (Mg2+2+) layers ) layers

(OH) at center of T-rings and fill base of VI layer (OH) at center of T-rings and fill base of VI layer

Yellow = (OH)Yellow = (OH)

T T O O -- T T O O -- T T OO

vdwvdw

vdwvdw

weak van der Waals bonds between T-O groups weak van der Waals bonds between T-O groups

Clay building blocks

2:1 Clay• Slightly different way to deal with charge on the octahedral layer – put an opposite tetrahedral sheet on it…

• Now, how can we put these building blocks together…

Phyllosilicates

Pyrophyllite:Pyrophyllite: Al Al22 [Si [Si44OO1010] (OH)] (OH)22

T-layer - T-layer - didiocathedral (Alocathedral (Al3+3+) layer - T-layer ) layer - T-layer

T T O O T T -- T T O O T T -- T T O O TT

vdwvdw

vdwvdw

weak van der Waals bonds between T - O - T groups weak van der Waals bonds between T - O - T groups

Yellow = (OH)Yellow = (OH)

Phyllosilicates

Talc:Talc: Mg Mg33 [Si [Si44OO1010] (OH)] (OH)22

T-layer - T-layer - tritriocathedral (Mgocathedral (Mg2+2+) layer - T-layer ) layer - T-layer

T T O O T T -- T T O O T T -- T T O O TT

vdwvdw

vdwvdw

weak van der Waals bonds between T - O - T groups weak van der Waals bonds between T - O - T groups

Yellow = (OH)Yellow = (OH)

Phyllosilicates

Muscovite:Muscovite: KK Al Al22 [Si [Si33AlAlOO1010] (OH)] (OH)2 2 (coupled K - Al(coupled K - AlIVIV))

T-layer - T-layer - didiocathedral (Alocathedral (Al3+3+) layer - T-layer - ) layer - T-layer - KK

T T O O T T KK T T O O T T KK T T O O TT

K between T - O - T groups is stronger than vdwK between T - O - T groups is stronger than vdw

Phyllosilicates

Phlogopite:Phlogopite: K Mg K Mg33 [Si [Si33AlOAlO1010] (OH)] (OH)22

T-layer - T-layer - tritriocathedral (Mgocathedral (Mg2+2+) layer - T-layer - ) layer - T-layer - KK

T T O O T T KK T T O O T T KK T T O O TT

K between T - O - T groups is stronger than vdwK between T - O - T groups is stronger than vdw

A Summary of

Phyllosilicate Structures

Phyllosilicates

Fig 13.84 Klein and Hurlbut Manual of Mineralogy, © John Wiley & Sons

Carbonate Minerals

Calcite Group(hexagonal)      

Dolomite Group(hexagonal)    

AragoniteGroup(orthorhombic)        

mineral formula mineral formula mineral formula

Calcite CaCO3 Dolomite CaMg(CO3)2 Aragonite CaCO3

Magnesite MgCO3 Ankerite Ca(Mg,Fe)(CO3)2

Witherite BaCO3

Siderite, FeCO3 Kutnohorite CaMn(CO3)2 Strontianite SrCO3

Rhodochrosite

MnCO3

Calcite Group

• Variety of minerals varying by cation

• Ca Calcite

• Fe Siderite

• Mn Rhodochrosite

• Zn Smithsonite

• Mg Magnesite

Dolomite Group

• Similar structure to calcite, but Ca ions are in alternating layers from Mg, Fe, Mn, Zn

• Ca(Mg, Fe, Mn, Zn)(CO3)2

– Ca Dolomite– Fe Ankerite– Mn Kutnahorite

Aragonite Group• Polymorph of calcite, but the structure can

incorporate some other, larger, metals more easily (Pb, Ba, Sr)– Ca Aragonite– Pb cerrusite– Sr Strontianite– Ba Witherite

• Aragonite LESS stable than calcite, but common in biological material (shells….)

Calcite vs. Dolomite• dolomite less reactive with HCl calcite has

lower indices of refraction

• calcite more commonly twinned

• dolomite more commonly euhedral

• calcite commonly colourless

• dolomite may be cloudy or stained by iron oxide

• Mg spectroscopic techniques!

• Different symmetry cleavage same, but easily distinguished by XRD