Did You Know about the “Rock of the Year”

Rock of the Year is a label aiming to bring rocks that are remarkable due to their geological origin and economic importance into greater public interest. This German initiative started in 2007 initiated by the German Association of Geoscientists (BDG[1]). The awarding committee consisting of specialists that represent branches like geotourism, natural stone economy, raw materials economy, and monument protection and architecture have voted for andesites as rock of the years 2020/2021.

But why andesite, what makes this quite common rock so remarkable as they occur in many places also in Europe (e.g., Germany[2], Greece, Italy, Iceland, Romania)?

Location map of submarine andesites outcrops in the Western-Central Mediterranean Sea

There are several reasons for that starting with the name andesite that is derived from the worlds’ longest mountain range the Andes. Although when the name was proposed by Leopold von Buch 1835, Alexander von Humboldt[3] did not support the suggestion for further distinction of volcanic rocks. It needed one generation more to bring it back to the glossary of rocks. In the 20th century, the British geologist Ian Carmichael came up with the name icelandite when studying the Thingmuli volcano on eastern Iceland in the 1960es[4]. This synonym describes the same rock and is used mainly in the Nordic countries. Both labels hint to the volcanic origin of andesites common in subduction zones, island arcs and orogeny belts. Notable examples of andesites in coastal zones from the south-east of Spain and seamounts in Italy have been included in the MINDeSEA dataset. Andesites are metallotects (prospectivity features) for gold placers connected with the on-land mined deposits in the Miocene Cabo de Gata volcanic field (e.g., Rodalquilar mining district, Western Mediterranean Sea); and polymetallic sulphides in back-arc volcanoes and active hydrothermal vents (Panarea, Thyrrhenian Sea).

Active shallow water hydrothermal Fe-oxyhydroxides-silica chimney formed in andesitic rock substrate. Panarea (Italy)

While the dispute on the nomenclature of rock names for scientific purposes are widely solved there are still several terms for the same rock type existing used by industry or historically grown in the society. MINTELL4EU and EUROLITHOS (among other things) work with aspects related to standardizing of terms.

Another remarkable characteristic is the strength of this compact and dense volcanic hard rock. Andesites come up as a very fine grained to porphyritic texture of predominantly plagioclase and to a lesser extent amphibole, pyroxene, and biotite but also quartz or olivine are possible. The proportion and grain seize of those rock-forming minerals are characterising factors that lead to the name of a rock type. By also using distinguished crystal optical phenomenons the Austrian mineralogist Friedrich Becke[5] realised that andesite is the volcanic pendant to the coarse-grained plutonic rock known as diorite. But the accessory minerals are an important key to the rock forming process and magnetite, apatite, zircon and titanite might occur as accessory minerals as well.

Gold-silica epithermal mineralization in andesites, GeoMinero Museum (IGME, CSIC). Rodalquilar (Spain)

Some curiosities about the rock-forming minerals:

The abundance of a very chemical element can differ significantly leading to a huge number of variants in mineral composition Wildcards representing the different positions in the crystal structure indicate these differences in the variance in the general mineral formulas. Amphibole (A0-1X2Y5Z8O22(OH)2) is an extreme example of this. Mineralogist refer to it as the waste bin of nature as most chemical elements fit to its mineral structure. A huge number of combinations are possible that includes the Li-bearing holmquistite and the clinoholmquistit [6], and the pedrizite-ferropedrizite[7] groups. Pyroxenes that can be expressed by the general formular XY(Z)2O6, where X can be build a great variety of chemical elements into tits structure. Many of them belong to the critical ones such as Mn, or Li on the X site and e.g. Cr, Co, Mn, Sc, T, or V on the Y site. Spodumene is the Li-bearing pyroxene which is among the most prominent sources for this critical metal. Check FRAME to learn more about CRMs in Europe.

As ordinary andesite is very suitable as construction material, it is extracted in quarries (e.g. currently 17 active ones in Germany) and almost exclusively processed into crushed products required for different applications in construction, such as in the concrete and asphalt industries as well as in road, water and railroad construction. However, hydrothermal metamorphic overprints at later stages might change the appearance of the rock significantly. The former grey rock might appear very colourful which make them interesting for architectural purpose. Monumental buildings of the medieval times witness this with masonry columns, mosaic stones in floors and wall claddings, and by sculptures. The “porfido verde antico” from Sparta/Greece that has been used for instance for the Vatican Church of St. Paul is a prominent example that occurs green due to the minerals epidote (pistacite) and chlorite.

Giant andesite formations may be of interest to the mining industry as sulphurisation is a well-known process that took place in many andesites and might have led to ore formation like in the case of Chiles giant Cu-deposits.

[1] https://geoberuf.de/der-bdg/gestein-des-jahres

[2] https://www.bv-miro.org/wp-content/uploads/Andesit_Broschuere-web.pdf

[3] Alexander von Humboldt was a German scientist and pioneer on modern scientific research on South America. He was the first proposing that South America and Africa once belong to each other and graduated 1792 from the School of Mines in Freiberg, Germany

[4] CARMICHAEL, I.S.E., The Petrology of Thingmuli, a Tertiary Volcano in Eastern Iceland. Journal of Petrology, 1964. 5(3): p. 435-460.

[5] The Austrian mineralogist Friedrich Johann Karl Becke developed a method of mineral determination based on their optical properties. He described a crystal-optical phenomenon that is now known as „Beckesche Linie“.

[6] [Li2(Fe2+, Mg)3(Fe3+, Al)2]Si8O22(OH, F, Cl)2 and [Li2(Fe2+, Mg, Mn)3(Fe3+, Al)2]Si8O22(OH, F, Cl)2), respectively

[7] NaLi2[Li(Fe2+, Mg, Mn)2(Fe3+, Al)2]Si8O22(OH, F, Cl)2