Weathering and Minerals - 1

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. Understand and diagram the structure of atoms. (Fig. 2.3)
   • nucleus, protons, neutrons, electrons
   • electrons 'orbit' the nucleus and a predictable number are in each of one to several electron shells

   
   

2. Understand the atomic differences between atoms of different elements.
   • elements are characterized by the number of protons (atomic number) and the number of protons plus neutrons in the nucleus (atomic mass)
   • the number of electrons in an atom is equal to the number of protons

   
   

3. Understand and explain what an isotope is. (Fig. 2.4)
   • varying the number of neutrons in a nucleus (protons remain the same)
   • very important because this leads to the basis of radioactive decay and dating

   
   

4. Understand that most minerals are composed of compounds and recognize some that are not.
   • atoms are joined to one another (bonded) by various kinds of forces ionic, covalent, metallic bonds (Fig. 2.5, 2.6)

   
   

5. Definition of the term "mineral".
   • naturally occurring, inorganic, crystalline solid, with a narrowly defined chemical composition and characteristic physical properties

   
   

6. What are the physical properties of "crystalline solids". (Fig. 2.7)
   • atoms are joined to one another (bonded) by various kinds of forces
   • regular 3-D framework; under ideal conditions this may be reflected by the formation of smooth planar surfaces (crystal faces) and sharp corners and edges (Fig. 2.8)
   • contrast with amorphous (glass) solids

   
   

7. What causes minerals to have variable compositions? (Fig. 2.9)
   • nature is impure; some elements have similar ionic sizes and charges and can substitute

   
   

8. Know that silicates are the most common minerals and why this is so.
   • common ions range from 0.3 to 3.7Å in diameter where (Å = 1x10^-8 cm)
   • no real, absolute sizes, atoms are "fluffy" clouds of electrons
   • the sizes control the geometries that atoms/ions can combine in
     • Al similar in size to Si
     • Mg similar in size to Fe
   • Important because similarly sized atoms often substitute for one another
   • Different ways of packing oxygen (big) around cations (small)
     • 6 oxygens with cation in middle
     • 4 oxygens with cation in middle
     • 3 oxygens with cation in middle

   
   

9. Understand and be able to diagram the structure of the silica tetrahedron. (Fig. 2.10)
   • build some basic structures; Can SiO₂ be a simple linear molecule?
   • charge on tetrahedron is -4, so other cations must pack in to balance the charge to zero:
     • Mg or Fe to make (Mg,Fe)₂SiO₄, a mineral called olivine
     • Olivine crystallizes at a very high temperature (>1350°C)
   • In olivine, silicon tetrahedra are disconnected
   • At lower temperatures, silicon tetrahedra can link in chains
     • making minerals called pyroxenes (single chains)
     • making minerals called amphiboles (double chains)
   • Ca, Na, K combine with silicon (and aluminum) tetrahedra to make feldspars
     • Calcium feldspar (anorthite): CaAl₂Si₂O₈
     • Sodium feldspar (albite): NaAlSi₃O₈
     • Potassium feldspar (microcline): KAlSi₃O₈
   • Silicon tetrahedra alone, sharing all four oxygen atoms, make the mineral quartz SiO₂