Question

Geology question:

1. How does the crystalline structure of a silicate mineral influence its susceptibility to physical and chemical weathering?

Answer 1

Pyroxene, amphibole, magnetite, pyrite, and olivine are most susceptible to oxidation because they have high iron content. The ions released from silicate minerals in the weathering process are sodium, potassium, calcium, iron, and magnesium ions.

The above table shows the relative stability of common minerals under the influence of weathering. Silicate minerals such as Olivine (neso- or ortho-silicate), Anorthite (Tecto-silicate), pyroxenes (single chains ino-silicates), amphiboles (doble-chain ino-silicates) are the least stable, and the rate of weathering is the fastest. Minerals such as hematite (iron-oxide), gibbsite (aluminium hydroxides), quartz (tecto-silicate), and the clay minerals (phyllo-silicates) are the most stable, and are least susceptible to physical and chemical weathering.

The chemical stability of mineral is related to the tendency of a mineral to remain in a given chemical form rather than to react spontaneously to become a different chemical substance. The stability of a specific mineral is determined by the specific set of conditions (e.g. pressure and temperature) under which they formed. Quartz, for example, is the last mineral to crystallize from a silicate magma and has a greater chemical stability under physical conditions at the Earth’s surface than olivine, pyroxene, and feldspar. These minerals crystallize earlier and at higher temperatures and pressure, are therefore least stable and most subject to chemical weathering.