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! SiO44-, CO32-, SO42- anionic groups, respectively Also consider halides (anion is Cl- or F-) and mineralization of silica

Sheet Silicates – aka Phyllosilicates Clays Sheet 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)

Phyllosilicates 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 Brucite: Mg(OH)2 Layers of octahedral Mg in coordination with (OH) Large spacing along c due to weak van der waals bonds c

Phyllosilicates a2 a1 Gibbsite: Al(OH)3 Layers of octahedral Al in coordination with (OH) Al3+ means that only 2/3 of the VI sites may be occupied for charge-balance reasons Brucite-type layers may be called trioctahedral and gibbsite-type dioctahedral

Phyllosilicates T O - T O - T O Kaolinite: Al2 [Si2O5] (OH)4 Yellow = (OH) vdw Kaolinite: Al2 [Si2O5] (OH)4 T-layers and diocathedral (Al3+) layers (OH) at center of T-rings and fill base of VI layer  vdw weak van der Waals bonds between T-O groups

Clay building blocks 1:1 Clay Kaolinite micelles attached with H bonds – many H bonds aggregately strong, do not expend or swell

Phyllosilicates T O - T O - T O Serpentine: Mg3 [Si2O5] (OH)4 Yellow = (OH) vdw Serpentine: Mg3 [Si2O5] (OH)4 T-layers and triocathedral (Mg2+) layers (OH) at center of T-rings and fill base of VI layer  vdw 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 T O T - T O T - T O T Pyrophyllite: Al2 [Si4O10] (OH)2 vdw Yellow = (OH) Pyrophyllite: Al2 [Si4O10] (OH)2 T-layer - diocathedral (Al3+) layer - T-layer vdw weak van der Waals bonds between T - O - T groups

Phyllosilicates T O T - T O T - T O T Talc: Mg3 [Si4O10] (OH)2 vdw Yellow = (OH) Talc: Mg3 [Si4O10] (OH)2 T-layer - triocathedral (Mg2+) layer - T-layer vdw weak van der Waals bonds between T - O - T groups

Phyllosilicates T O T K T O T K T O T Muscovite: K Al2 [Si3AlO10] (OH)2 (coupled K - AlIV) T-layer - diocathedral (Al3+) layer - T-layer - K K between T - O - T groups is stronger than vdw

Phyllosilicates T O T K T O T K T O T Phlogopite: K Mg3 [Si3AlO10] (OH)2 T-layer - triocathedral (Mg2+) layer - T-layer - K K between T - O - T groups is stronger than vdw

Phyllosilicate Structures A Summary of Phyllosilicate Structures Fig 13.84 Klein and Hurlbut Manual of Mineralogy, © John Wiley & Sons

Carbonate Minerals Calcite Group (hexagonal) Dolomite Group (hexagonal)     AragoniteGroup (orthorhombic)         mineral formula Calcite CaCO3 Dolomite CaMg(CO3)2 Aragonite 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