Are there any sulphides, sulphates, carbonates that cannot be dissolved by aqua regia?

Welcome to this comprehensive article on the solubility of sulfides, sulfates and carbonates in aqua regia. Aqua regia, a mixture of nitric acid (HNO3) and hydrochloric acid (HCl), is known for its powerful and versatile solvent properties. However, there are certain minerals that exhibit exceptional resistance to dissolution in aqua regia. In this article, we will examine the characteristics of these minerals and discuss the reasons for their insolubility. Understanding these exceptions is crucial for researchers, geologists and mineralogists working in the field of earth sciences.

Sulfides

Sulfides are a class of minerals consisting of sulfur (S) combined with one or more metallic elements. They are commonly found in ores and are of great economic importance. While aqua regia is generally effective in dissolving sulfides, there are a few exceptions to this rule. Two notable examples are pyrite (FeS2) and marcasite (FeS2), both of which belong to the pyrite group.
Pyrite and marcasite are highly resistant to dissolution by aqua regia due to their crystal structure and chemical composition. These minerals have a high sulfide ion (S2-) content and the iron (Fe) atoms are strongly bonded to the sulfur atoms. Aqua regia, despite its strong oxidizing properties, is unable to effectively break these bonds. As a result, pyrite and marcasite remain largely insoluble, making them difficult to extract and process.

It should be noted that prolonged exposure to aqua regia may eventually result in partial dissolution of pyrite and marcasite, especially if the solution is heated or subjected to increased pressure. However, their resistance to dissolution under normal conditions makes them valuable indicators of geological processes and environments.

Sulfates

Sulfates are minerals composed of the sulfate ion (SO42-) combined with various metallic elements. They are widespread in nature and can be found in rocks, minerals, and even as evaporite deposits. While many sulfates are readily soluble in aqua regia, there are a few exceptions that exhibit limited solubility or complete resistance to dissolution.
One such example is anhydrite (CaSO4), a common sulfate mineral. Anhydrite is poorly soluble in water, and its solubility in aqua regia is similarly limited. The presence of calcium (Ca) in anhydrite contributes to its insolubility, as calcium compounds generally have low solubility in aqua regia. As a result, anhydrite is largely unaffected by the solvent properties of aqua regia.

Another notable example of a sulfate with limited solubility in aqua regia is celestine (SrSO4). Celestine is a strontium sulfate mineral that is similar to anhydrite in its resistance to dissolution. The strong ionic bonds between the strontium (Sr) and sulfate ions in celestine make it relatively insoluble in aqua regia.

Carbonates

Carbonates are minerals composed of the carbonate ion (CO32-) combined with metallic elements. They are abundant in nature and play an important role in various geological processes, including the formation of caves and the buffering of pH in soils. While aqua regia is known for its ability to dissolve many carbonates, there are exceptions to this rule.
A prominent example of a carbonate mineral that is resistant to dissolution by aqua regia is calcite (CaCO3). Calcite is a common mineral and a major component of limestone and marble. Despite its abundance, calcite has limited solubility in aqua regia. The relatively low solubility of calcite can be attributed to the strong ionic bonds between the calcium (Ca) and carbonate ions.

Another carbonate mineral that is difficult to dissolve in aqua regia is dolomite (CaMg(CO3)2). Dolomite is a calcium-magnesium carbonate and often occurs as a major component in sedimentary rocks. The presence of magnesium (Mg) in dolomite increases its resistance to aqua regia dissolution because magnesium compounds tend to have lower solubility than their calcium counterparts.

Conclusion

Aqua regia, known for its solubility, encounters certain sulfides, sulfates and carbonates that are resistant to its dissolving properties. Pyrite and marcasite among the sulfides, anhydrite and celestine among the sulfates, and calcite and dolomite among the carbonates are notable examples of minerals that are not easily dissolved by aqua regia. The insolubility of these minerals can be attributed to their chemical composition, crystal structure, and the strength of the bonds between their constituent ions.

Understanding the exceptions to aqua regia solubility is critical to several fields of study, including mineralogy, geology, and materials science. Researchers and professionals working with these minerals must be aware of their resistance to dissolution and use alternative methods and solvents for their analysis and extraction.

It is important to note that while aqua regia may not be effective in dissolving these minerals under normal conditions, it may still exhibit partial solubility or dissolution when subjected to prolonged exposure or specific experimental conditions. Therefore, comprehensive research and tailored approaches are required to fully investigate the solubility characteristics of these minerals.
By expanding our knowledge of the solubility properties of minerals in aqua regia, we continue to improve our understanding of the earth’s composition, geologic processes, and the extraction and use of valuable resources. Further research and advances in analytical techniques will help unravel the complexities of mineral dissolution and refine our understanding of these exceptional cases.

FAQs

Are there any sulphides, sulphates, carbonates that cannot be dissolved by aqua regia?

Yes, there are certain sulphides, sulphates, and carbonates that cannot be dissolved by aqua regia. Aqua regia is a highly corrosive mixture of nitric acid and hydrochloric acid, and while it can dissolve many metals and minerals, there are some compounds that are resistant to its action.

Which sulphides are not soluble in aqua regia?

Several sulphides are insoluble in aqua regia, including pyrite (FeS2), galena (PbS), and cinnabar (HgS). These compounds have strong chemical bonds that are not easily broken down by the acids present in aqua regia.

Are there any sulphates that aqua regia cannot dissolve?

Most sulphates are soluble in aqua regia, but there are a few exceptions. For example, barium sulphate (BaSO4) and lead sulphate (PbSO4) are not readily soluble in aqua regia. These compounds have low solubilities and are relatively inert to the acids in aqua regia.

Which carbonates are not soluble in aqua regia?

Aqua regia is generally not effective in dissolving carbonates. Compounds such as calcite (CaCO3), dolomite (CaMg(CO3)2), and magnesite (MgCO3) are insoluble or only slightly soluble in aqua regia. Carbonates require stronger acids, such as hydrochloric acid or sulfuric acid, for effective dissolution.

Why are some sulphides, sulphates, and carbonates resistant to aqua regia?

The resistance of certain sulphides, sulphates, and carbonates to aqua regia can be attributed to their chemical properties. These compounds often have strong ionic or covalent bonds that are not easily broken down by the acids in aqua regia. Additionally, some minerals may form insoluble surface layers or protective coatings that prevent further dissolution.