EES 119/219

Lecture 5

Outline

• Sedimentary deposits (continued)

• Aluminum systematics

• Formation of Al deposits

– Bauxite à lateritic deposit

• Gold systematics

– Placer deposits

Aluminum

• Aluminum is the third most abundant element in the Earth's crust and constitutes 7.3% by mass.

• In nature it only exists in very stable combinations with other materials (particularly as silicates and oxides)

• It was not until 1808 that its existence was first established.

• It took many years of painstaking research to "unlock" the metal from its ore and many more to produce a viable, commercial production process

• Consequently, the production of Al shows a much stronger increase over the last century than any other metal (Fig. 1)

Definition of pH

• Concentration of H+ ions in water

– 10 – pH is the concentration of free H ions in water

– H₂O ß à H⁺ + OH^-

– pH = - log [H⁺]

– neutral water has a pH of 7

– pH < 7 à acidic

– pH > 7 à alkaline or basic

Rain water

• Normal rain water is slightly acidic

– 5 < pH <6.5

• Reason: Presence of CO₂ in the atmosphere

– CO₂ + H₂O à H₂CO₃ (carbonic acid)

– H₂CO₃ à H⁺ + HCO₃^-

• Release of SO₂ and NO_x from fossil fuel burning causes acid rain à pH < 5

Weathering of feldspars

2KAlSi₃O₈ + 2(H⁺ + HCO₃^-) + H₂O à Al₂Si₂O₅(OH)₄ +4SiO₂ + 2K⁺ + 2HCO₃^-

· H⁺ ions interact with feldspars, release of K⁺ ions and uptake of H ions in solid, release of K⁺ and SiO₂ into water

à increase in Al concentration in remaining clay mineral, decrease in H⁺ (increase in K⁺ ions) in water

à increase in pH

Distribution of Bauxite deposits (Fig. 2)

Most of the bauxite deposits form surface layers in subtropical areas (Fig. 3)

• Bauxite deposits are at or close to the surface

• Sedimentary deposits with variation in grain size

• Associated with Kaolinite deposits

• Most of the deposits are in subtropical areas

• Age range: Tertiary or younger: < 65 Ma

Changes in pH (Fig. 4): Precipitation

• Rain has pH < 7

à reaction with rock minerals uses up H+, releases cations (K+ etc.)

à change from pH <7 to pH > 7 in deeper layers

Changes in pH - Evaporation

• Upward movement of water

• Deeper waters have pH > 7

• à Surface layers change to pH > 7

Dependence of solubility on pH: Fig. 5

Formation of Kaolinite and Bauxite

• Initial acidic nature of rain will start altering rock forming minerals

• Constituents sensitive to changes in pH will go into solution (i.e. SiO2), while Al2O3 (and Fe2O3) stay behind

à enrichment in Al2O3

à Formation of Kaolinite (China Clay)

à This process occurs in temperate climates with an excess of precipitation (England; NY State)

• Kaolinite still has high amount of Si

• Additional removal of SiO2 occurs during evaporation due to changes in pH

• Kaolinite Al2Si2O5(OH)4

à Bauxite Al2O3.nH2O

• Alternation between wet and dry periods are necessary conditions for the formation of bauxite à Subtropical areas

• Deposits are formed from the top à young deposits

• Removal of ‘unwanted’ material à lateritic process

Placer deposits (Gold)

Production and reserves of noble metals: Fig. 6

Placer deposits

• Consequence of physical weathering and mechanical transport

• Typical densities of minerals (kg/m3)

– Au 19,000

– Pt 17,000

– UO2 8,000

– FeS2 5,000

– FeCr2O4 4,600

– Crustal 2,700

– Sedimentary 2,000

– Tuff 1,000

• Weathering physically breaks down rock

• Transport in flowing water

• When flow rate decreases, grains are deposited

• Deposition depends on density à heaviest materials come out first à differentiation according to density

• Concentration of gold pebbles in sediments

Locations favorable for formation of placer deposits (Fig. 7)

Model for the formation of the Witwatersrand Deposit Fig. 8

Sedimentary deposits

• BIF: Chemical weathering; transport and deposition related to changes in O2 availability

• Bauxite deposits: Chemical weathering and concentration processes related to changes in pH

• Placer deposits: Physical weathering and mechanical differentiation based on density