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Sedimentary Minerals

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!

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Sedimentary Minerals

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  1. 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

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

  3. 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…

  4. Cement • Mixture of lime (CaO – made by roasting calcite) and silicates made by sintering limestone and clay (at 1450°C) • Ancient cement was just CaO – mixed with water to form portlandite (Ca(OH)2), which then slowly reacted with CO2 to reform calcite • Modern cement is mixed with water, form several Ca-Si phases that are more durable and don’t shrink as much

  5. Ca Calcite, CaCO3 Dolomite CaMg(CO3)2 Ankerite CaFe(CO3)2 Siderite, FeCO3 Magnesite, MgCO3 Mg Fe Carbonate Minerals

  6. Calcite Group • Variety of minerals varying by cation • Ca  Calcite • Fe  Siderite • Mn  Rhodochrosite • Zn  Smithsonite • Mg  Magnesite

  7. 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

  8. 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….)

  9. 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

  10. 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 (g-CaSO4) forms when the gypsum is later dehydrated)

  11. Gypsum

  12. 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)

  13. 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)

  14. 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)

  15. Ba, Pb, Sr – very low SO42- 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? Lots of SO42- Not very much Ba, Sr, Pb

  16. 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

  17. Opal - Gemstone

  18. Agates

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