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El Hoyazo is a garnet placer deposit near Níjar in Almería, Spain.
A placer is a deposit of sand or gravel that contains valuable minerals, at El Hoyazo the most significant mineral is garnet in the form of almandine (Fe3Al2Si3O12).
The garnet rich sands are located to the south of El Hoyazo volcanic complex and are the only economic deposit of garnet in Spain. An eruption about 12 million years ago produced a dacite dome (a pipe and surrounding block lava) which was subsequently covered in a reef deposit. Erosion has resulted in a depression through the limestone into the softer volcanic rock, which is cut by a gorge creating an alluvial fan that forms the placer deposit. Garnets are found loose in layers in the sand of the many small gullies.
The garnet placer deposit has been exploited commercially for use as industrial abrasives for which this mineral variety is especially suited because of its high specific gravity and hardness. These qualities make a high quality abrasive used in sandpaper, abrasive wheels and polishing/lapping grains. Garnet exploitation at El Hoyazo reached a peak in 1933 and continued intermittently until the end of the 1990's.
The origin of the garnets is shrouded in controversy. Two different mechanisms have been proposed to explain the origin of the garnets, but a study in 2000 produced evidence that contradicted both theories, leaving open the possibility of an alternative origin. Although garnet is the most significant mineral in the placer deposit, many others are present. These include potassium feldspar, corundum, zircon, quartz, graphite and magnetite.
Take exit 481 (Níjar) on the N340-E15 motorway. Parking is available at N 36deg 57.070' W 002deg 11.133' just off the main road. This requires a 30 minute walk to the volcanic complex. Alternatively much closer parking is available at N 36deg 57.259' W 002deg 10.094' and N 36deg 57.248' W 002deg 10.467'. However these require access by dirt track. The crater-like depression forms a large area (about 0.7km2) that is interesting to explore, with many gullies and hills.
As well as the various mineral deposits, there is also remains of the fossil reef that use to overlay the volcanic rock. The co-ordinates for the earthcache are a point near the centre of El Hoyazo.
As the placer deposit has been exploited commercially I'm sure it would be permissible to collect a small sample of garnets.
To log this cache post a photograph of the characteristic U-shaped valley at he south west end of the El Hoyazo Volcanic Complex (u-shape.jpg).
More Information: A technical paper that is the source of a lot of the information above can be found at: www.episodes.org/backissues/234/266-269%20Maria.pdf
Información en español: www.ua.es/ice/betica/Hoyazo_Nijar/nijar_informa.htm
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Coordinates are in the WGS84 datum