Mineralogy of the Earth's Crust
When examining the mineralogy of the Earth's crust we find that there are
two mineral groups, silica and feldspars, that are more abundant, and, in that sense, more important,
than any of the other mineral groups.
Both groups of minerals can be classified as tetrahedral framework structures.
- Minerals found with framework structures are usually oxides composed of strong tetrahedral and/or octahedral groups,
where every O atom is bonded to two polyhedra.
Bridgemanite is a framework of octahedra at corners of a cube, while garnets are a framework of tetrahedra and octahedra.
Silica and feldspars are frameworks of SiO4 and AlO4 tetrahedra.
Silicates are the minerals that contain SiO4 tetrahedra.
Those that only contain SiO6 octahedra, those at high-pressure, are not silicates.
They are classified as oxides.
The consensus is that Earth's crust consists of ~12% silica and 50-60% feldspar, by molar
The silica phase diagram
The major phases of silica include
but there are about 50 different phases currently known; most are synthetic.
Opals are hydrated silica, SiO2·nH2O.
opal, composed of spheres of tridymite or cristobalite, held together by water.
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 Paulings 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 cation-anion-cation linkages, in the case of silica, Si-O-Si angles.
These are often called bridging angles and the O atom is called the bridging O atom.
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, quartz adopts a more uniform packing arrangement, bcc,
then it appears to turn to glass around 25 GPa
With application of temperature it undergoes a displacive transformation
to the hexagonal β form.
This movie shows a polyhedral view of quartz. It starts at 600 C, and continues through the β-α
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 weaken and lengthen (tell me why), 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 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 the case, because the pitch of the quartz helix
(length of one complete turn measured parallel to the axix of the helix) is too
small. Some scientists now believe that the structure of clay offers a
perfect size template for growing DNA.
high albite (high temperature form, disordered Si/Al)
low albite (ordered Al/Si)
Microcline (ordered Al/Si)
Orthoclase (partially ordered Al/Si)
Sanidine (disordered Al/Si)
Movie showing compression of albite
Channels are large in the feldspars so diffusion is rapid and easy.
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)5+ for (NaSi)5+.
- The topics of plagioclase-alkali solid solutions and Na-K feldspar exsolution are demonstrated in the two figures below.
Wenk & Bulakh, Chapter 19