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! –SiO 4 4-, CO 3 2-, SO 4 2- anionic groups, respectively Also consider halides (anion is Cl - or F - ) and mineralization of silica

Clays Sheet Silicates – aka Phyllosilicates [Si 2 O 5 ] 2- Sheets of tetrahedra Phyllosilicates micas talc clay minerals serpentine

Sheet Silicates – aka Phyllosilicates [Si 2 O 5 ] 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% H 2 O  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 – Al 2 Si 2 O 5 (OH) 4 Illite – K Al 4 (Si,Al) 8 O 20 (OH) 4 Smectites: –Montmorillonite – (Ca, Na) (Al,Mg,Fe) 2 (Si,Al) 4 O 10 (OH) 2 *nH 2 O –Vermicullite - (Ca, Mg) (Al,Mg,Fe) 3 (Si,Al) 4 O 10 (OH) 2 *nH 2 O –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)

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…

Cement Mixture of lime (CaO – made by roasting calcite) and silicates made by sintering limestone and clay Ancient cement was just CaO – mixed with water to form portlandite (Ca(OH) 2 ), which then slowly reacted with CO 2 to reform calcite Modern cement is mixed with water, form several Ca-Si phases that are more durable and don’t shrink as much

Carbonate Minerals MgFe Ca Calcite, CaCO 3 Dolomite CaMg(CO 3 ) 2 Ankerite CaFe(CO 3 ) 2 Siderite, FeCO 3 Magnesite, MgCO 3

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)(CO 3 ) 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 dolomite more commonly euhedral and twinned 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 H 2 O) or anhydrous (without H 2 O) groups Gypsum (CaSO 4 *2H 2 O) and anhydrite (CaSO 4 ) 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 (CaF 2 ) 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 (BaSO 4 ), Celestite (SrSO 4 ), and Anglesite (PbSO 4 ) 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 SO 4 2- Lots of SO 4 2- 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