OXIDES

Just as carbon gives rise to carbides and nitrogen to nitrides, oxygen forms its own predominantly solid compounds, which constitute an essential part of the Earth's crust and the outer layers of the mantle. These oxides, although diverse in properties, share a series of common characteristics that make them especially relevant in the metallurgical field. Some, such as lanthanide oxides or those present in coltan, stand out for their particular characteristics, but in general terms, oxides exhibit features that deserve to be explored in detail.

Hardness is one of their most notable attributes. Although the hardest known compounds belong to the carbon and boron families, oxygen is not far behind. Aluminum trioxide (Al₂O₃), known as alumina in its white ceramic form and as sapphire or ruby ​​in its massive crystalline form, reaches a value of 9 on the Mohs scale, giving it outstanding resistance to wear and abrasion. This property makes it an ideal material for applications where durability is critical.

Another distinctive feature is the high melting point of many mineral oxides. This characteristic not only makes thermal processing difficult but also poses an additional challenge: to separate oxygen from the desired metallic element, it is necessary to introduce a reducing agent, called a "sacrificial element," which allows the oxygen to leave its original compound and form a new one. This principle explains the effectiveness of coke in reducing iron oxide. The oxide with the highest melting point is thorium (ThO₂), which reaches 3390°C and is used in alloys for TIG (Tungsten Inert Gas) electrodes, where thermal resistance is essential.

Regarding their electrical and thermal properties, most oxides act as excellent insulators. This quality makes them useful in applications where heat or electrical conduction must be avoided, such as in ceramic coatings or electronic components.

Corrosion resistance is another notable aspect. Oxides are among the most stable compounds against chemical attack, surpassed only by some fluorides, carbides, and borides, and this only at moderate temperatures. Above 500°C, no non-oxidized compound surpasses oxides in resistance to oxygen, since, by definition, what is already oxidized cannot be further oxidized, except in the presence of highly electronegative agents such as fluorine or chlorine in high oxidation states.

Quartz (SiO₂) is undoubtedly the most emblematic solid oxide. Its internal structure, composed of chains of oxygen and silicon atoms arranged similarly to diamond, gives it exceptional mechanical and chemical properties, notable for its inertness to most reagents.

Other oxides with more complex formulas occur in rocks composed of multiple minerals. Representative examples include soda ash (Na₂O), alumina or corundum (Al₂O₃, depending on its crystal structure), hematite (Fe₂O₃, also known as oligistite), and magnetite (Fe₃O₄). Unlike quartz, many oxides are not binary; that is, they are not limited to a simple combination of metal and oxygen, but rather have considerably more elaborate chemical formulas. Elements such as aluminum, magnesium, iron, vanadium, titanium, and manganese can coexist in varying proportions within minerals like garnet, the composition of which depends on the specific variety. These metals share common characteristics, such as their high reactivity, and belong to the siderophile and lithophile groups according to the Goldschmidt classification, which is essential for understanding their geological distribution.

It is important to note that all elements capable of forming sulfides can also form oxides, although the reverse is not necessarily the case. This observation is relevant in the case of metals such as lead, mercury, and silver, which are usually found in nature combined with sulfur, forming part of the chalcogen group. However, this does not imply that they are literally stainless. In fact, as explained in other sections, stainless steel is not completely immune to oxidation. Chalcogen elements are those that predominate in sulfur minerals, and an illustrative example is silver, whose surface "oxidation" is actually due to the action of gaseous sulfur and ozone, present in industrial and volcanic areas. The well-known "black" or aged silver corresponds to a layer of Ag₂S (silver sulfide) that forms on the metal's surface. Silver oxides are not very stable, which reinforces their noble character, although their prestige has diminished compared to metals such as platinum.

The case of lead is equally revealing. This chalcogen, extracted from galena (PbS), has a blue-gray hue upon exiting the reduction process, which darkens as a surface oxide layer forms. This phenomenon illustrates how even metals considered resistant to oxidation eventually succumb to the action of oxygen.

In short, oxygen represents a constant challenge in the production of reactive metals such as titanium, vanadium, and even tungsten. The latter, despite its remarkable resistance to corrosion thanks to a spontaneously formed passivating oxide layer, requires a complex and energy-intensive process—measured in kilojoules (kJ)—to be isolated from its ore. Thus, oxygen, although essential in the formation of stable compounds, is also one of the main obstacles in the metallurgy of highly reactive elements.

In the industrial field, and particularly in metallurgy, oxides play a fundamental role not because of their metallic content, but because of the intrinsic properties they exhibit as oxygenated compounds. Therefore, this review excludes oxides such as hematite (Fe₂O₃) or magnetite (Fe₃O₄), whose relevance lies in their iron content and not in their properties as oxides per se. Although they can be used as pigments or mineral fillers, their function as oxides is secondary compared to other compounds that are valued for their chemical, thermal, electrical, or mechanical characteristics.

The selection presented here includes only solid oxides that are used directly as such, without considering their role as a source of metals. Both natural and synthetic, binary and non-binary oxides are included, provided that their usefulness derives from their behavior as oxides. This distinction is essential because, unlike carbides or nitrides, oxides usually receive a specific name that reflects the oxidation state of the metallic element involved, given that many of them can form multiple oxides with distinct chemical formulas and divergent properties.

Of the 94 elements in the periodic table that have industrial applications (excluding those so radioactive as to be of no practical use), those whose oxides are particularly valuable for their stability, abundance, or functionality are highlighted. The order followed corresponds to the increasing atomic number of the accompanying element, allowing a systematic view of the spectrum of relevant oxides. Furthermore, the concept of "solubility" is introduced, which is absent in the analysis of carbides and nitrides, since oxides exhibit notable variability in this regard: some are practically insoluble and extremely stable, while others dissolve relatively easily in acidic or basic media, which limits their handling and application.

In short, this classification focuses on oxides that are useful precisely because they are oxides, and not because of the metal they contain. Considered are its most notable physical and chemical properties, its behavior against external agents, and its role in industrial processes where the presence of oxygen is not an obstacle, but a functional advantage.