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[Joan R.]

If we are thinking on a short list of worldwide famous mineral deposits in Spain, the pyrites from La Rioja-Soria (Northern Spain) stand out among the most significant and spectacular places. Its peculiarity lies in the beauty of the crystals and the richness of its different habitus, which depends on the outcrop area, all in the Cretaceous lutites and sandstones. So, it is well-known the perfection of cubes from Navajún and Valdeperillo (Córnago) and pentagonododecahedral (pyritohedral) crystals and pine cones from Ambasaguas (comprising a wide area). Other less known localities, but not less interesting are the towns of Aguilar del Río Alhama and Cervera del Río Alhama in La Rioja region side and San Pedro Manrique - Valdenegrillos in the Soria province side.


AMBASAGUAS
Ambasaguas is a small town located at 6 km from Muro de Aguas, in La Rioja region. Seeing the large number of sources of crystalline water in Muro de Aguas (waters wall) central square, we could understand the town name’s origin. Nowadays, old Ambasaguas inhabitants resides in Muro de Aguas.
The casual extraction of pyrite by the locals dates from many decades ago. Probably they had been extracting pyrites due to its brilliant faces, magic beliefs or, in the last years, for mineral collectors. They told me that many big pyrites were used as sling projectiles (a sling is a projectile weapon typically used to throw a blunt projectile such as a stone. It is also known as the shepherd's sling) to scare away wild beasts which attacked the herds. It was not until 1966 when the pyrite mining was carried out in an ordered way oriented to get collection specimens. Currently the mine is working under the name "Ambasaguas-II Mine", with the corresponding exploitation rights property of Piritas de Ambasaguas S.L.

The pyrite mineralization is related with the hydrothermal metamorphism that took place in this area. The formation of pyrite crystals may be explained by the reaction of iron from the lutitic sediments (containing iron oxides and chlorite), with the sulfur from sedimentary pyrite thermal decomposition or thermo chemical reduction of the sedimentary sulfates. The pyrite appears in the lutitic levels, always in contact with roof layers of sandstone. This roof levels acted as aquifers during metamorphism transporting hot rich sulphur fluids. The rich pyrite crystal habitus diversity is related to the availability of sulphur in these sediments. (1)
You could see all around of Ambasaguas some galleries and small quarries where, during many years, worldwide mineral collectors have been looking for pyrites. Nowadays you must to ask permission to the owners.
In the below attached image you can see some pyrite habitus that occurs in the mined areas. We would like to emphasize in specimens like pure pyritohedron, cube-pyritohedron combinations, cubes with octahedral faces, deformed pyritohedrons... some of them with beveled edges, with or without striated faces.

The crystals of Ambasaguas and Navajún remain among the most beautiful and perfect pyrite specimens in the world, and thanks to the dedication of the mines owners, we will continue collecting and enjoying their capricious forms.

(1) Rodas M., Alonso-Azcárate, J., Mas J.R., Bottrell S.H. (2002): Los yacimientos de pirita de la Cuenca de Cameros. Zubía, Nº 14, pags. 173-190.
(2) Alonso-Azcárate, J., Rodas, M., Fernández-Díaz, L., Bottrell, S.H., Mas, J.R.,López-Andrés, S. (2001): Causes of variation in crystal morphology in metamorphogenic pyrite deposits of the Cameros Basin (N Spain). Geological Journal, Volume 36, Issue 2 , Pages 159-170.
(3) García, G, Alonso-Azcárate, J. (2003): Piritas de Ambasaguas. Bocamina, Nº 12, pag. 13-45. https://www.bocamina.com/esp/n12/portada.pdf

Mine & locality images: https://mti-minas-rioja.blogspot.com/