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 (< 1m = <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:– 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)

– 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

• Mixed-layer clays (I/S = illite/smectite layers)

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

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…

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

Carbonate Minerals

Mg Fe

CaCalcite, CaCO3

DolomiteCaMg(CO3)2

AnkeriteCaFe(CO3)2

Siderite, FeCO3Magnesite, MgCO3

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 colorless

• dolomite may be cloudy or stained by iron oxide

• Mg spectroscopic techniques!

• Different symmetry cleavage same, but easily distinguished by XRD

Sulfate Minerals

• More than 100 different minerals, separated into hydrous (with H2O) or anhydrous (without H2O) groups

• Gypsum (CaSO4*2H2O) and anhydrite (CaSO4) are the most common of the sulfate minerals

• Gypsum typically forms in evaporitic basins – a polymorph of anhydrite (-CaSO4) forms when the gypsum is later dehydrated)

Gypsum

• Gypsum formation can demarcate ancient seas that dried up (such as the inland seas of the Michigan basin) or tell us about the history of current seas which have dried up before (such as the Mediterranean Sea)

Halide Minerals

• Minerals contianing halogen elements as dominant anion (Cl- or F- typically)

• Halite (NaCl) and Sylvite (KCl) form in VERY concentrated evaporitic waters – they are extremely soluble in water, indicate more complete evaporation than does gypsum

• Fluorite (CaF2) more typically occurs in veins associated with hydrothermal waters (F- in hydrothermal solutions is typically much higher – leached out of parent minerals such as biotites, pyroxenes, hornblendes or apatite)

Sulfate Minerals II• Barite (BaSO4), Celestite (SrSO4), and Anglesite

(PbSO4) are also important in mining.

• These minerals are DENSE Barite =4.5, Anglesite = 6.3 (feldspars are ~2.5)

Barite, Celestite, Anglesite

• Metals bond with sulfate much more easily, and thus are generally more insoluble – they do not require formation in evaporitic basins

• What do they indicate then?

Ba, Pb, Sr – very low SO42-

Lots of SO42-

Not very much Ba, Sr, Pb

Just silica…• Chert – extremely fine grained quartz

– Forms as nodules in limestone, recrystallization of siliceous fossils– Jasper – variety with hematite inclusions red– Flint – variety containing organic matter darker color

• Chalcedony – microcrystaliine silica (very similar to low quartz, but different – it is yet uncertain how different…) typically shows banding, often colored to form an agate (rock formed of multiple bands of colored chalcedony)

• Jasper – variety colored with inclusion of microcrystsalline oxides (often iron oxides = red)

• Opal – a hydrogel (a solid solution of water in silica) – forms initially as water + silica colloids, then slowly the water diffuses into the silica making it amorphous (no XRD pattern!)– Some evidence opal slowly recrystallizes to chalcedony

Opal - Gemstone

Agates