Weathering and Minerals - 2

Weathering is the physical breakdown (disintegration) and chemical alteration (decomposition) of rocks and minerals at or near the Earth's surface where they contact the atmosphere, water and organic life. Most rocks are composed of several different minerals. Therefore to understand weathering, we need to know how minerals are defined, what they are composed of, and how they interact with the hydrosphere, atmosphere and biosphere.

We will begin with a review of basic chemistry as it applies to the elements that are important in the crust of the Earth - the raw materials of our physical surroundings. The following Table will be referred to throughout this and the following class.

Si and O are the most common elements in the earth's crust

Element	Wt.%	Atom.%	Ionic Radius	R/Ro	Charge
O	46.6	62.6	1.40		-2
Si	27.7	21.2	0.42	0.3	+4
Al	8.1	6.5	0.51	0.36	+3
Fe	5.0	1.9	0.64-0.74	0.46-0.53	+2,+3
Ca	3.6	1.9	0.99	0.71	+2
Na	2.8	2.6	0.97	0.69	+1
K	2.6	1.4	1.33	0.95	+1
Mg	2.1	1.8	0.66	0.47	+2
others	1.5	0.1

The ionic radius is given in Angstroms (1Å = 10^-8 cm), the R/Ro column is the radius ratio calculated by dividing the ionic radius of the cation by that of oxygen.

1. List the major groups of minerals and the basis for the recognition of each.
   • silicates, carbonates, sulfides, sulfates, halides, native elements, oxides

   
   

2. Understand and be able to recognize the physical properties of minerals.
   • color: some specific colors are reliable, remember the generalities streak color much more diagnostic
   • luster is the appearance in reflected light: metallic vs nonmetallic vitreous (glassy), greasy, waxy, brilliant, dull (earthy) (Fig. 2.15)
   • crystal form (Fig. 2.16)
   • cleavage vs fracture (Fig. 2.17, 2.18)
   • hardness: resistance to abrasion; scale of 1-10 (Table 2.6)
   • specific gravity: ratio of a minerals weight to an equal volume of water
   • feel, magnetism, taste, plastic vs flexible, double refraction, reaction with acid (Fig. 2.20)

   
   

3. Most rocks are composed of minerals
   • There are thousands of minerals, but only a few compose the bulk of most rocks.
   • The ultimate source of rocky material is magma (molten rock) from within the earth.
   • Eruption of magma on the surface (lava) forms fine-grained extrusive rocks:
     • Rhyolites (rich in Si, Al and K)
     • Basalts (rich in Mg and Fe)
   • When deeply-buried magmas solidify slowly, they make coarse-grained plutonic rocks:
     • Granite, the coarse-grained equivalent to rhyolite
     • Gabbro, the coarse-grained equivalent to basalt

Chemical Weathering:

1. Solution: ions become dissociated from one another in a liquid
   • water is a great solvent because it is a dipole {Fig. 5.14}
   • most minerals aren't very soluble in pure water
     • however a small amount of acid makes water a much better solvent

     
     

   • H₂O + CO₂ <=> H₂CO₃ <=> H⁺ + HCO₃^-
     
   • note the role here of CO₂ (0.03%), one of the 'greenhouse' gases
     
   • CaCO₃ + H⁺ + HCO₃^- <=> Ca⁺⁺ + 2HCO₃^-

   
   

2. Oxidation: reaction with oxygen to form oxides or hydroxides
   • 4Fe + 3O₂ -> 2Fe₂O₃
   • especially important in the alteration of Fe-Mg silicates
   • oxidation of pyrite to form sulfuric acid is also very important
   • 2FeS₂ + 2H₂O + 7O₂ <=> 2H₂SO₄(aq) + 2FeSO₄(aq)

   
   

3. Hydrolysis: chemical reaction between hydrogen (H⁺) and hydroxyl (OH^-) ions of water and a minerals ions.
   • Cations in minerals are replaced by H⁺ and OH^- radicals
   • actual replacement that liberates soluble salts to the surrounding water
   • Cations and Si go into solution, leaving insoluble aluminum silicate - "clay"
     • 2KAlSi₃O₈ + 2H⁺ + 2HCO₃^- + H₂O <=> Al₂Si₂O₅(OH)₄ + 2K⁺ + 2HCO₃^- + 4SiO₂
   • Clay structure is "sheet-like" - makes small platy grains

Mechanical Weathering:

1. Frost action: repeated freezing and thawing of water in cracks.
   • Water expands about 9% upon freezing
   • may form talus slopes - Fig. 5.5
   • particularly effective in jointed rocks - Fig. 5.4
   • Related process of frost heaving: source of rocks in New England fields
2. Pressure release: differential confinement and release of deep seated pressure
   • exfoliation, sheet jointing - Fig. 5.6, 5.7, 5.8
3. Thermal expansion and contraction: differential change in volume
   • has not been conclusively proven in lab but natural evidence suggests that it may be important
4. Activities of organisms: burrowing, root pressure - Fig. 5.9

Factors Controlling the Rate of Weathering

• Particle size, climate, parent material - Fig. 5.13, 5.14
• World distribution of rainfall important for weathering, because weathering is driven by water delivery to the land
• Streams are main agent for moving materials by either solution or particles
• Stream discharge roughly proportional to surface area for world's continents
  • except Australia and Antarctica: low because of especially low rainfall
• Chemical weathering per unit area is 20-40 metric tons/km²/year and is pretty constant for continents
• Mechanical weathering has more variation (20-300 metric tons/km²/year), because of large variation in slopes. Asia is largest, because of large mountain ranges (e.g. Himalayas).
• Continents with high mean elevations have high mechanical weathering.
• Very high erosion where there are mountains and high rainfall (e.g. Indus from Himalayas and Amazon from Andes: both are in areas of high rainfall and have high sediment load).
• Mean rate of denudation 3-4 cm per thousand years. But this is a misleading figure, since it mostly comes from mountains, which have a small surface area. Mountains are being worn down very quickly. Implies recent geologic process must have made mountains.