Why Are Platinum Group Metals So Rare Compared to Iron?

The geological distribution of elements on Earth's crust is markedly influenced by the formation and cooling processes of the planet. The low abundance of platinum group metals (PGMs) like platinum, palladium, and rhodium is a fascinating case study in how the properties of metals and their interactions with oxides play a crucial role. This article delves into the geological and chemical reasons behind the rarity of PGMs relative to iron, exploring the "like dissolves like" principle, the role of oxygen, and the complex distribution patterns of various metals in the Earth's core and crust.

The Role of Oxides in the Early Earth

In the early stages of Earth's formation, when the planet was molten, stable oxides formed. As the Earth cooled and solidified, these oxides floated to the surface, while other elements and metals remained in the core or subsurface. Oxygen, silicon, and aluminum are the three most common elements in the Earth's crust, and most rocks are composed of these elements. Calculations indicate that over half of the crust is made up of aluminum silicates.

Elements such as calcium, sodium, and potassium also tend to mix with aluminum silicates, which explains the prevalence of these rocks in the crust. This geological process contributes significantly to the distribution of various elements on Earth's surface. Notably, elements that do not react with oxygen, such as gold and platinum, sink to the molten Earth's core, while oxidized elements like uranium and tin are more prevalent. Interestingly, despite the vast reserves of uranium, its oxides are less reactive and more beneficial for nuclear power compared to tin oxides.

The "Like Dissolves Like" Principle and Metal Preferences

A central concept in the distribution of elements is the "like dissolves like" principle, which states that polar molecules form bonds with similar type molecules. Metals have similar preferences. Some metals, like iron, cobalt, and nickel (often referred to as the iron triad), can mix in any proportion, whereas others can only mix to a certain degree, and some, like mercury, do not mix at all. This property of metals affects their distribution in the Earth and influences the abundance of different elements.

Metals in close proximity in the periodic table are more similar in their chemical and physical properties, leading to their tendency to "cling together" in the Earth's mantle. The iron triad, due to their chemical similarities, often form an alloy, known as siderophile elements, in the Earth's core. However, despite their chemical similarities, these elements have vastly different abundances in the universe, with iron being the most abundant due to its production by massive stars through nuclear fusion.

The Geological Distribution of Iron and PGMs

Iron, a siderophile element, is the most abundant metal in the Earth's core, produced in massive stars through the fusion of elements. As heavy metallic iron sank towards the core, lighter iron oxides remained in the crust and mantle. In contrast, PGMs are less reactive, remaining predominantly as metals rather than metal oxides. However, being easily soluble in molten iron, the vast majority of PGMs sank into the core, leaving only trace amounts in the crust.

For instance, the Earth's core is estimated to contain 16 quadrillion tons of gold, enough to cover Earth's surface in a layer 1.5 feet thick. Similarly, there is approximately six times more platinum in the core than gold, despite humans having mined only a fraction of these metals. This distribution is crucial for our understanding of the Earth's internal composition and the unique properties of different elements.

It is important to note that while iron has a dominant role in the Earth's core, the presence of the iron triad is valuable for various industries. Iron's easy extraction and processing make it a workhorse in construction, while its alloys, steels, play a pivotal role in aviation and other industries. Nickel-based superalloys, similarly, are crucial for jet engines, showcasing the multifaceted importance of the iron triad in modern technology.

Conclusion

The rarity of PGMs compared to iron underscores the complex interactions between elements and the geological processes that govern their distribution. While iron's abundant presence in the Earth's core and crust contributes to the rarity of PGMs, their own unique properties and industrial applications make them invaluable. Understanding these dynamics not only enhances our knowledge of planetary geology but also highlights the value of these rare and precious metals in technological and industrial applications.