Geos 306, Fall 2011
Mineralogy of the Earth's Crust

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• When examining the mineralogy of the Earth's crust we find that there are two mineral groups that are more abundant, and, in that sense, therefore more important, than any of the other mineral groups: silica and feldspars. These two groups will be the subject of the present lecture. Both minerals can be classified as tetrahedral framework structures.
• The consensus is that Earth's crust consists of ~12% silica and 50-60% feldspar, by molar abundances.

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Silica, SiO₂

The silica phase diagram
• The major phases of silica include quartz, cristobalite, tridymite, coesite, and stishovite, but there are about 50 different phases currently known; most are synthetic.

quartz

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cristobalite

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tridymite

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coesite

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• The most studied mineral is quartz because it is the prototype for our understanding of all the silicates.
• Defined as a framework structure, CN(Si)=4. CN(O)=2 according to Pauling’s rules. Compare this to stishovite.
• Framework structures are characterized by strong polyhedral units (usually tetrahedra or octahedra), that are linked together at each corner through flexible Si-O-Si angles. It does not take too much energy to twist the polyhedra around relative to each other, so there are many viable permutations of the structure. It is fairly easy to compress framework structures.
• With application of pressure it adopts a more uniform packing arrangement, then it appears to turn to glass around 25 GPa
• With application of temperature it undergoes a displacive transformation to the hexagonal b form.

This movie shows a polyhedral view of quartz. It starts at 600 C, and continues through the beta-alpha phase transition to room temperature, then pressure is applied to the structure till 25 GPa.

• A common model for the weathering mechanism of quartz, useful for all types of weathering, is the following. H attaches to an O atom. The two bonds from O to Si lengthen, and then one of them breaks.
• Quartz is useful as a source of Si and for silica. It is used in electronics as an oscillator, e.g. quartz watches.
• The structure is made out of helices of tetrahedra that wind parallel to the c-axis. It was once thought that the first DNA molecule grew on the surface of a quartz crystal, using these helixes as a template. We now know that this cannot be not true, because the size of the quartz helix is too small. Some scientists now believe that the structure of clay offers a perfect size template for growing DNA.

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Feldspars

• 3 important phases:

Albite NaAlSi₃O₈
K-spar KAlSi₃O₈
Anorthite CaAl₂Si₂O₈

• Albite has 2 polymorphs,

high albite (high temperature form, disordered Si/Al)
low albite (ordered Al/Si)

• K-spar has 3 polymorphs:

Microcline (ordered Al/Si)
Orthoclase (partially ordered Al/Si)
Sanidine (disordered Al/Si)

• Channels are large in the feldspars so diffusion is rapid and easy.

Movie showing compression

• Weathering related to the bonding of hydrogen to bridging O atoms, just as for quartz
• Sizes of cations: r(K)> r(Na)=r(Ca) means a very limited substitution of K for Na or Ca, but Ca and Na substitute for each other if we have the coupled substitution of CaAl for NaSi.
• The topics of plagioclase-alkali solid solutions and Na-K feldspar exsolution are demonstrated in the two figures below.

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Reading
Wenk & Bulakh, Chapter 19