Pyrite Crystals from Navajun, Spain

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by Eric Greene

INTRODUCTION

Mina Ampliación a Victoria is a pyrite deposit about 4 kilometers north-northwest of the tiny village of Navajún, in the Cervera district in La Rioja province, Spain. The surrounding area forms the headwaters of the Barranco de la Nava in the Sierra de Alcarama foothills of the Iberian Range. The deposit was discovered in 1965, and, through the years, pyrite from Navajún has been appreciated by and highly sought after by mineral collectors for its unique, highly aesthetic, exquisitely sharp, brightly lustrous, near-perfect single cubic crystals, clusters of interpenetrating crystals, and crystals embedded in matrix. Arguably the finest cubic pyrite crystals in the world come from Navajún (not Logrono, as they are sometimes mislabeled). The degree of perfection of these crystals causes many neophytes to exclaim in amazement and ask if they are manmade. Because of its beauty and perfection, Navajún pyrite has become the icon of Spanish mineralogy since its discovery.


Pyrite crystal on matrix. Crystal 60 mm on edge.

Navajún is located in a region that is better known for its red wines than its pyrite - La Rioja. Other crops such as wheat, barley, and hops thrive in the rich soil of this area. The town sits on a knoll at about 3,300 feet, in the rugged Sierra de Alcarama, which are foothills of the Iberian Range. The original village, which dates to the thirteenth century, was relocated in about 1860 about one mile down the mountain from the original site. Once a bustling village, only 16 people live there today. There are no stores, and the only commercial enterprise is Casa Don Pedro, a bed and breakfast owned and operated by mine owner/operator, Pedro Ansorena Conde.


View of Mina Ampliación a Victoria from the entrance road to the south.

HISTORY

In 1568, the knight Bernardo de Perez de Bargas, described Spanish pyrites as being golden or silvery, forming square "grains" like "dice" in the cliffs. In the 1960s, Pedro Ansorena Garret, the mine owner's father, was the mine engineer for La Victoria, a galena mine originally worked by the Romans for silver and located in Cigudosa (Soria) bordering Navajún about 10 kilometers from the site of the pyrite mine. A mine worker brought him some pyrite cubes he had found in a deep gully exposed by erosion and his interest was piqued. He explored the site and found enough pyrite that he requested an exploration license from the Spanish government in 1965. He originally proposed to name the mine La Victoria, but the authorities objected so he renamed it Mina Ampliación a Victoria, meaning it was an extension of the original Victoria mine. A research permit was approved by the regional mining authority and in 1970 a mining exploitation concession was granted. Ansorena Garret brought specimens from the mine to the University of Madrid in 1965 to inquire about them. Professors told him that though the deposit had no commercial value, the crystals were especially fine and would be of great interest to mineral collectors. In 1976, after overcoming his father's skepticism, Ansorena's son Pedro Ansore Conde brought boxes of specimens to a show in Tarbes, France, where they created a sensation and all the specimens were sold. Pedro returned home and handed his father fistfuls of cash and the elder Ansorena was finally convinced to mine for specimens only. The first report of these specimens appeared in the Mineralogical Record in an article by Wilson in 1976. Many thousands of pyrites have been collected and sold since then, and world-class pieces can now be found in museums and private collections around the globe. Today, specimens are offered for sale at all the major mineral shows, including Tucson, Munich, Sainte-Marie-aux-Mines, Denver, and Changsha. The mine, which is operated only for pyrite specimens, is owned by Piritas de Navajún S.L., which is jointly held by Pedro Ansorena Conde, José Chaver Atanasio, and Néstor López Ogalde.


Pyrite crystals on matrix. 32 cm.

GEOLOGY

The Cameros Basin in Central Spain is an intraplate rift basin that was formed along NW-SE trending extension faults during Late Jurassic to Middle Albian time (125-130 million years ago). In this sedimentary basin was filled with sandstone deposited by rivers. The composition of the sandstone in this basin has been identified as consisting of 4 main petrographic facies. The Cameros Basin sandstones have a long lateral continuity which has resulted in an abundance of outcrops. The section later experienced low grade metamorphism. The most abundant types of rock in the Navajún area are Cretaceous sandstones and marls of the Weald facies. These marls, which typically contain over 50% lime rich clay or mudstone and at least 15% calcium carbonate, are the host rock for the pyrite crystals. Marls range in color from pale tan to medium gray, and in hardness from a pliable clay-like consistency to very hard. The origin of the pyrite crystals at Navajún has been tied to deposits of sulfate minerals found in the metasediments of the Cameros Basin. These veins are hosted by lake evaporites made up of alternating dolomite and gypsum. Analysis of fluid inclusions and geothermal data show that the sulfate veins formed at temperatures of about 225º C, and suggest that the sulfur in the pyrite was the product of reduction by organic matter. Also of local geological interest is the presence of fossil dinosaur tracks in Aguilar, just 12 km from Navajún. Early Cretaceous sediments host prints of three-toed carnivorous dinosaurs. These creatures were from 30 to 32 cm long. The tracks were preserved in muddy plains which were subsequently covered with sediments. In places, where erosion has worn away the top layers, the prints have been well preserved. There is a small dinosaur track museum in Aguilar.


Pyrite crystal cluster on matrix. Largest cube 28mm on edge.

MINERALOGY

Pyrite, with the chemical formula FeS2, contains roughly 47 percent iron and 53 percent sulfur. Pyrite's brassy golden color led to it being called "fool's gold," but the name pyrite comes from the Greek word pur meaning "fire." In prehistoric times, it was struck against flint to create sparks and start fires; in Roman times it was struck against steel for the same purpose. During the Middle Ages, this attribute made it a popular choice in early firearms. Interestingly, pyrite is the only sulfide found at Navajún. The list of other minerals encountered is quite small: green chamosite in nodules; chloritoid as inclusions in pyrite; cookeite as white coatings on cube faces; manganese minerals; and calcite-lined vugs inside fractured pyrite cubes. Limonite pseudomorphs after pyrite, gypsum and jarosite have also been reported. None of these are of any particular interest to collectors. Large septarian nodules to 30 cm and more have recently been recovered in the same layer as the pyrite. Pyrite from Mina Victoria occurs abundantly in large, perfect cubic crystals. The crystal habit is almost exclusively cubic, customarily attaining a geometric perfection found almost nowhere else in the world. Other crystal morphologies have been observed as elongated bars, cubes with small octahedron faces on the corners, and occasionally beveled edges. Pyritohedrons (dodecahedrons with pentagonal faces) have also been reported. The size of the pyrite cubes varies from 1 mm up to 19 cm on edge; 2 to 6 cm cubes are the most typical. The largest crystal found was 19 cm and weighed 9.5 kg. Single cubes are abundant, and groups of crystals from two or three to over 100 cubes have been found. Crystal faces are typically mirror smooth with a brilliant, mirror-like luster. They may also show growth hillocks in swirling patterns or oscillation striations. The color is characteristically a pale, brassy, golden yellow. And although pyrite from many worldwide locations is unstable, Navajún pyrite appears to be perfectly stable. The cause of the formation of such perfect cubes is of great interest to collectors and scientists. This morphology is believed to be the result of the particular temperature, pressure, acidity, and saturation of the fluids from which the crystals formed. These factors determine the speed at which pyrite crystals grow. Studies suggest that smooth cubes are formed relatively slowly at moderate temperatures (just above 250ºC) and moderate supersaturation. As temperatures increase to around 450ºC and as supersaturation increases, crystals form more and more quickly, cubes develop striations, and eventually octahedra, then pyritohedra, develop. Studies also suggest that for cubes to grow symmetrically, the pressure must be low and nondirectional. The cubes form by displacing the host matrix, which was probably quite soft and even porous at the time of growth.


Pyrite cluster on matrix. 42 cm overall (uncleaned specimen).

CRYSTAL INTERGROWTH

Most crystals from Navajún develop as intergrown crystals, not twins. Though not true crystallographic twins, interpenetrating groups of crystals are occasionally found at Mina Victoria. These are typically made up of three to five crystals, with sharp corners sticking out on all sides. They do not separate at the joins the way intergrown crystals do. Most groups have crystals which have come off, which must be reassembled in order to return them to their original positions. Although this practice is generally frowned on by serious collectors, the mine owners plainly disclose to buyers which of their specimens have been reassembled, although this information seldom is given on labels. Many of the complexly intergrown, multi-crystal clusters would simply not be available without such preparation. In addition, cubes on matrix specimens must sometimes be glued back into position as well. In both cases, Ansorena Conde maintains an exceptional degree of honesty about the process, and refuses to indulge in creating fanciful sculptures out of crystals that did not originally grow together. A specially formulated cyanoacrylate resin, which is said to support up to 300 kilograms per cubic centimeter, is used to hold crystals in their original positions. Note that the practice of reconstructing clusters is very similar to that used and tolerated for other mineral specimens, most notably Herkimer diamond clusters and Sweet Home Mine rhodochrosites. Even the Smithsonian Institution accepts this practice; it displays reconstructed Navajún pyrite specimens in their collection without reference to reassembly and prints photos of them on posters as well.


Pyrite cluster with intergrown crystals on matrix. Largest cube 28mm on edge.

THE MINE

At Mina Victoria, pyrite crystals are exposed in three different layers in a cut that is 40 to 50 meters deep. Layer one is the lowest level, and layer 3 the highest. The thickness of these layers ranges from 1 meter to 3 meters, with an average of about 2.5 meters. The layers strike east-west, with a dip of 15º south. Within each layer there are differences in both crystal morphology and the degree of oxidation of the pyrite. Layer one has been the focus of most mining to date, as it has produced the most spectacular specimens. Layer two was at one time worked using horizontal open galleries with concrete pillars for support, but is not currently in production because of safety concerns. Specimens from this layer are generally of lower quality than those in the first layer, though some good matrix specimens were found. Layer three remains in reserve. Mina Victoria is one of the few Spanish mines that is quarried exclusively for mineral specimens. During my visit in July 2013, it was being worked using an excavator and backhoe to dig up and remove the waste rock between the layers. A loader and dump truck were used to haul away the debris. The two workers who were present when I visited the mine were collecting using hand tools only in a recently exposed area of the first layer. A small hammer, a pry bar, and several thin chisels were all they required to extract outstanding matrix specimens, groups, and cubes. No explosives are used as this would destroy the fragile crystals and crystal groups.


The three exposed layers in the mine.

ACCESS

The prospect is owned by Piritas de Navajún S.L., and is only open to collectors by prior arrangement. The property is patrolled and marked "No Trespassing." Authorized visits can be arranged through the website www.piritasdenavajún.com.


The author kneeling on the top of a bed of pyrite crysals in matrix that is over 12 feet thick.

ABOUT THE AUTHOR

Eric S. Greene is president of TreasureMountainMining.com, an Internet mineral dealership specializing in fine mineral specimens. He is also an avid field collector and member of the Keene, NH Mineral Club.


Pedro Ansorena Garret shares breakfast with the author at Pedro's Casa dos Pedro Inn

ACKNOWLEDGEMENTS

I am indebted to Pedro Ansorena Conde for his hospitality during my stay, for sharing his knowledge about the history, geology, and mineralogy of the mine, and for his helpful review of this article. I am also grateful to Jeanne Greene and Helene Grogan for reviewing the article, and to Adam Caron, Geneve Rege, and Holly Roberts of Treasure Mountain Mining for photographing the specimens.

REFERENCES

Alonso-Azcárate, J., L. Rodas, L. Fernández-Díaz, S.H. Bottrell, J. R. Mas, and S. López-Andrés. 2001. Causes of variation in crystal morphology in metamorphogenic pyrite deposits of the Cameros Basin (N Spain). Geological Journal 36 (2) 2: 159-170.

Alonso-Azca?rate, J., S.H. Bottrell, and J. Tritlla. 2001. Sulfur redox reactions and formation of native sulfur veins during low grade metamorphism of gypsum evaporites, Cameros Basin (NE Spain). Chemical Geology 174 (4): 389-402

Arribas, J. A. Alonso, R. Mas, A. Tortosa, M. Rodas, J.F. Barrenechea, J. Alonso-Azcárate, and R. Artigas. 2003. Sandstone Petrography of Continental Depositional Sequences of an Intraplate Rift Basin: Western Cameros Basin (North Spain). Journal of Sedimentary Research 73 (2): 309-327

Callem, J. 1981. La Pyrita de Logrono y Soria. Mineralogises de Catalunya 1 (11): 6-9.

Calvo, M. and E. Sevillano. 1989. Pyrite crystals from Soria and la Rioja provinces, Spain. Mineralogical Record 20: 451-456.

Calderón, S. 1910. Los Minerales de España. Junta para Ampliación de Estudios e Investigaciones Científicas, Madrid, Vol. 1: 134-135.

Cuesta, E. and G. Garcia. 1995. Navajún, naturaleza cubista. Bocamina 1: 30-37.

Gibbons, W, and T. Moreno, editors. 2002. The Geology of Spain. Southampton, U.K.: The Geological Society of London.

Hurlbut, Jr. C.S.; Klein, C. 1985. Manual of Mineralogy, after James D. Dana, 21st edition, revised. John Wiley & Sons, New York.

Lodders, K., Klingelhofer, G., and Kremser, D. T. 1998. Chloritoid inclusions in pyrite from Navajún , Spain. Canadian Mineralogist 36: 137-145

Murowchick, J. and H.L. Barnes. 1987. Effects of temperature and degree of supersaturation on pyrite morphology. American Mineralogist 72: 1241-1250

Piritas de Navajún. http://www.piritasdenavajún.com. September, 2013.

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