MINERALOGY AND ENVIRONMENTAL SCIENCE

TOPICS THIS LECTURE:

• Adsorption onto mineral surfaces:
  • Charge at a mineral surface, surface energy.
  • Dependence on mineral structure and chemistry
  • Dependence on pH and other factors.
• Adsorption (chem- and phys-) and desorption
• Clay minerals and interlayer substitution
• The physical nature of mineral surfaces:
  • What do they look like?
  • What types of reactive surface sites are important?
  • The role of defects
• Mineral weathering: dissolution and secondary mineral precipitation
  • Silicates
  • Sulfide minerals
• Redox reactions at surfaces, surface catalysis
• The role of organisms in mineral dissolution, mineral precipitation, etc.

What is a mineral surface.

• Termination of the periodic crystal structure creates a surface:

  Expect:

  • Unsatisfied bonds
  • Distorted coordination environments
  • Excess energy
  • Surface restructing (surface layer characterized by surface reconstruction)

• Surfaces are regions where mineral dissolution and growth occur.
• Factors affecting orientations of surfaces?
  • What might the factors be? atomic density, growth rate, surface structure, solution chemistry, etc.
  • Morphology control - slowest growing face.

Ions at mineral surfaces: Adsorption and Desorption!

Association of metals and complexes in solution with mineral surfaces

• Water near mineral surfaces

  Water very near (few nanometers) from a mineral surface is fundamentally different from bulk water:

  - relatively structured for at least a couple of water layers.

  - depends on what the mineral surface looks like

  - water near a Si-O-Si surface dissociates to create a Si-OH + Si-OH surface

  - these H+ in the -OH group are easily lost, resulting in an Si-O- surface (-ve charge) or extra H+ gained resulting in a Si-OH2+ surface (positive).

  *Clearly this will depend on pH!! - at some pH there will be an equal abundance of +ve and -ve species. This is a special pH condition for that mineral. This is called the zero point of charge, or ZPC.

  The ZPC for a mineral is determined by the structure and chemistry of the mineral:

• ZPC for minerals:

  examples:

  Zero point of net proton charge at 25 C and 1 bar [Data from Table 1; Hochella a nd Banfield, 1995)

  • FeO (OH) measured pH PPZC = 9.0-9.7 calc = 9.4
  • Quartz measured pH PPZC = 2.9 calc = 2.91
  • Forsterite measured pH PPZC = 10.5 calc = 9.1
  • Low Albite measured pH PPZC = 6.8 calc = 5.2
  • Kaolinite ~ 4.6
  This is important because the nature of the hydrated surface and reactivity of that surface will depend on the structure of the surface and the solution pH.

Where are ions located and how are they attached? Why does it matter?

• Important in growth and dissolution
  • Chemisorption (specific): formation of a chemical bond between the adsorbing molecule and the surface - orbital overlap.
  • Physisorption (non-specific): electrostatic attraction to the surface - molecule is located in the layer of water adjacent to the surface.
  • Different absorption mechanisms have different effects
• Of other environmental constituents: inorganic and organic contaminants, H+, etc.
• Precipitation versus adsorption: immobilization of contaminant metals in secondary minerals.

Example: Consider a layer silicate: kaolinite

Where is the unsatisfied charge?

Are the surfaces +ve or -vely charged ? This is an important question!

SUMMARY!: Charge at a mineral's surface depends on a number of factors!! Primarily:-

• What is the mineral?
• pH !

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Clay Minerals

Brief overview of clay mineralogy!

- What is a SMECTITE!

-similar in structure to a mica, but containing water and fewer interlayer catins. Organic and inorganic contaminants and the smectite interlayer.. - cation exchange

- other cation exchange minerals (zeolites)

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What does a mineral surface look like?

• Are mineral surfaces flat ? If not, what features are present and how are these features important?
  • Surface steps, kinks, etc.
  • Atomic defects
• What happens at surface sites?
• Are intersections between bulk defects and the surfaces important?
  • dislocations and spiral growth and dissolution

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Mineral Weathering: Why is mineral weathering important?

• sediments
• water
• soils
• environmental contaminants

Weathering reactions involving

• silicate minerals
  • What silicates are important and why ? - crystal chemical controls!

    Olivine versus feldspars: depolymerization of the silicate anion

    - Rupture of Si-O and Al-O: role of H+ in catalysis of bond rupture.

    - Selective removal of atoms from prefered surface sites.

  • non-silicates: e.g., sulfides (and acid mine drainage)- associated with mining of metal sulfides. Weathering of pyrite, which is an important impurity in coal, is also an important problem. - selective removal of atoms from steps and kinks

    - generation of sulfuric acid: 2 FeS2 + 4 H2O + 7.5 O2 -> 4 H2SO4 + Fe2 O3

    - above redox reaction involves oxygen or other electron acceptor...

    - low pH solutions have higher toxic metal concentrations!

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Catalysis and redox reactions at surfaces

Interaction between a species (e.g., an organic molecule) and a mineral surface results in a lowering of the energy needed for a reaction involving the organic to take place. This increases in the probablility that the reaction will take place occurs without change in the structure of the surface itself.

Examples where this is important in the environment:
• degradation of organic contaminants
• photocatalysis of organics
• oxidation and reduction reactions

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Organisms and minerals!

Very important issue in environmental mineralogy!

why?

Here: just consider microbes: one example: Lithotrophs: derive metabolic energy from inorganic compounds (minerals)

redox reactions -> electrons used to make ATP (adenosine triphosphate). ATP is the chemical used to store energy for biosynthesis. Note ADP + P + energy <-> ATP.

"Electron transport phosphorylation": electron transport coupled to ADP->ATP:occurs in respiration and photosynthesis.

• Mineral precipitation by organisms:
  • Fe, Mn, other oxides
  • silicates
  • sulfides
  • carbonates
  • phosphates
  • uranium, gold, selenium, etc.
• Mineral dissolution by organisms:
  • silicates
  • sulfides and acid mine drainage

  Note: Sulfide dissolution and precipitation can be promoted by (different organisms).

• Redox reactions and sedimentary diagenesis.

  Organisms need many elements in their life processes, especially:

  C,O, H, N, P, S, K, Mg, Ca, Fe (Si)

  Overall, the cycles of these and other elements near the Earth's surface are fundamentally impacted by biological activity.