Sideronatrite from Cabo de Palos (Cartagena, Murcia), a reversible mineral

In the easternmost part of the Murcia region we find the well-known Spanish tourist locality of La Manga del Mar Menor. This inland sea closes to the south by Cabo de Palos, in the municipality of Cartagena.

The hills those define it are found within the Regional Park of Calblanque, Monte de las Cenizas and Peña del Águila. This particular area contains the remains of mining works that began with the Punic period, continued during the Roman Empire and continued until the 20th century.

On one of its highest hills, El Talayón (178 m), we found the “Ferruginosa” mine. This mine has been extensively studied by Morales and Rosell (Morales et al., 2020; Rosell et al. 2017,) due to its particular mineral paragenesis, where chlorides and sulphates stand out.

One of these sulphates is Sideronatrite. This hydrated sodium and iron sulphate, Na2Fe(SO4)2(OH)·3H2O, is found in arid climates. It is found as efflorescences formed by small crystals, of an intense yellow colour, on the walls of the mine galleries. Lanceolated, transparent crystals with a very esthetic orange-yellow colour are observed under magnification.

One of the most interesting characteristics of this mineral species is its reversibility. Sideronatrite transforms into metasideronatrite, losing two water molecules, when the temperature exceeds 35°C to 40°C, in the presence of drying agents or low humidity.

This process is reversible when environmental conditions vary (Ventruti, G., Stasi, F., Scordari, F., 2010). Under normal conditions of humidity and temperature, metasideronatrite rehydrates to re-form sideronatrite.

Furthermore, if the sideronatrite comes into contact with a medium where sulfuric acid has formed (for example, by oxidation of pyrite-marcasite), this species is transformed into Ferrinatrite, according to:

This reaction is also observed with Metasideronatrite.

To increase the chemical interest of these species, if Ferrinatrite is exposed to the humidity on the mine dumps, it changes its structure to Sideronatrite according to:

BIBLIOGRAPHY

• Morales, M., Rosell, J. (2020): “El distrito minero de Cabo de Palos, Cartagena, Región de Murcia, España. Una visita a la mina Ferruginosa”. Paragénesis, vol. 2, núm. 4, pp. 3-33. [online]

• Rosell, J., Martinez, M.D., Morales, M. (2017): “Sideronatrita de las minas de Cabo de Palos”. Paleomina, vol. 8, pp. 46-49.

• Scordari, F., Ventruti, G. (2009): “Sideronatrite, Na2Fe(SO4)2(OH)·3H2O: Crystal structure of the orthorhombic polytype and OD character analysis”. American Mineralogist, vol. 94, pp. 1679-1686. [online]

• Ventruti, G., Stasi, F., Scordari, F. (2010): “Metasideronatrite: Crystal structure and its relation with sideronatrite”. American Mineralogist, vol. 95, pp. 329-334. [online]