Geos 306, Fall 2000, Lecture 12
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 important than any of the other mineral groups: silica and feldspars. These two groups will be the subject of today's lecture.
• The Earth's crust consists of ~12% silica and 50-60% feldspar, by molar abundances.

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Silica, SiO₂

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

quartz cristobalite tridymite coesite

• The most studied mineral is quartz because it is the prototype for much silicate knowledge.
• Framework, CN(Si)=4. CN(O)=2. Pauling’s rules.
• With application of pressure it adopts a more uniform packing arrangement, then it turns to glass around 25 GPa
• With application of temperature it undergoes a displacive transformation to the hexagonal b form.

• Weathering mechanism, true for all types of weathering. H attaches to an O atom, then the other bonds lengthen, then break.
• Useful as a sorce of Si and for silica. It is used in quartz watches as an oscillator.

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Feldspars

• 3 important phases:

Albite NaAlSi₃O₈
K-spar KAlSi₃O₈
Anorthite CaAl₂Si₂O₈

• Albite has 2 polymorphs,

high albite (high temperature form, dissordered 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 H bonding to bridging O atoms, just as for quartz
• Sizes of cations: r(K)> r(Na)=r(Ca) means no substitution of K for Na or Ca
but Ca and Na substitute if we have the coupled substitution of CaAl for NaSi.
• Plagioclase, alkali solid solutions
• Na-K feldspar exsolution

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