13.6.5: Melting and Weathering Relationships

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13.24.jpg — Figure 13.24: Bowen’s Reaction Series

In Chapter 6, we talked about the melting temperatures of minerals. Figure 13.23 is based on Figure 6.17 (Chapter 6). It depicts Bowen’s reaction series and compares melting temperatures of most common igneous minerals. Quartz has the lowest melting temperature, K^–feldspar the second lowest, followed by muscovite, biotite, amphiboles, pyroxenes, and finally olivine.

Bowen’s reaction series is based on melting temperatures determined by laboratory experiments, but it mirrors the degree to which silicate minerals are polymerized (which is also a reflection of how much silica they contain). Quartz, feldspars, and other framework silicates are highly polymerized, and they melt at the lowest temperatures. Muscovite, biotite, and other sheet silicates are less polymerized, and melt at higher temperatures. Amphiboles (double chain silicates) and pyroxenes (single chain silicates) are still less polymerized, and melt at even higher temperatures. Olivine and other island silicates are not polymerized at all, and melt at the highest temperatures.

Why does the polymerization affect melting temperatures? The answer lies not so much with the nature of the minerals, but with the nature of the melt they create. Magmas, just like minerals, contain polymers when silicon and oxygen form chains in the melt. Magmas richest in silicon and oxygen are more polymerized, and have lower Gibbs free energy. In a sense, magmas that are highly polymerized form at lower temperatures than those that are less polymerized because fewer bonds need to be broken to create the melt. So, silica-rich magmas, and silica-rich minerals, melt at lower temperatures than those that are silica-poor.

In Chapter 7, we pointed out that the order in which silicate minerals weather is opposite the order in which they melt. Those minerals that melt at lowest temperature are most resistant to weathering. This phenomenon, too, is partly a result of how much silica is in the different silicate minerals. Minerals rich in silica are more tightly bonded (and thus more resistant to weathering) because they contain more (SiO₄)^4- anion molecules and because the valence of ionic bonds is generally greater than in minerals poorer in silica. Because of stronger bonds, they are less easily attacked by water and other weathering agents.