During an excursion to Eraldo cave, a vertical natural shaft intercepted by a tunnel in the mine workings of Barega (Iglesias), a white mineral covering the walls and the terraces in the main pit has been discovered and sampled. The subsequent analysis by X-ray powder diffraction has shown with absolute certainty that the sampled material is composed of the clay mineral Halloysite-10Å (Al2Si2O5(OH)4 · 2H2O). Halloysite belongs to the Kaolinite group of minerals and is known both in the dehydrated form Al2Si2O5(OH)4 called Halloysite-7Å and in the hydrated form, also known as Endellite, found in the Eraldo cave. The occurrence of this mineral in caves is unusual. Up to now it has been described from the following localities: Mbobomkulu Cave (South Africa), Mishima-Fuketsu Cave (Japan), Faggeto Tondo cave (Umbria, Italy), Carlsbad Cavern and Lechuguilla Cave (New Mexico, USA), two granite caves in the Sierra Nevada (USA), in a mine cave in Kirghizistan and in some Belgian, North French and Czech palaeokarsts. In all these places Halloysite appears to be formed by low temperature weathering of silicate rocks and/or sediments with the possible local involvement of acid solutions rich in H2SO4. If this type of mechanism is responsible also for the Halloysite of Eraldo cave this could indicate a production of sulphuric acid by oxidation of sulphides in a sub-aerial environment.
Ritrovamento di Halloysite nella Grotta Eraldo (Barega, Iglesias, Sardegna sud-occidentale)
FRAU, FRANCO;CANNAS, CARLA
2008-01-01
Abstract
During an excursion to Eraldo cave, a vertical natural shaft intercepted by a tunnel in the mine workings of Barega (Iglesias), a white mineral covering the walls and the terraces in the main pit has been discovered and sampled. The subsequent analysis by X-ray powder diffraction has shown with absolute certainty that the sampled material is composed of the clay mineral Halloysite-10Å (Al2Si2O5(OH)4 · 2H2O). Halloysite belongs to the Kaolinite group of minerals and is known both in the dehydrated form Al2Si2O5(OH)4 called Halloysite-7Å and in the hydrated form, also known as Endellite, found in the Eraldo cave. The occurrence of this mineral in caves is unusual. Up to now it has been described from the following localities: Mbobomkulu Cave (South Africa), Mishima-Fuketsu Cave (Japan), Faggeto Tondo cave (Umbria, Italy), Carlsbad Cavern and Lechuguilla Cave (New Mexico, USA), two granite caves in the Sierra Nevada (USA), in a mine cave in Kirghizistan and in some Belgian, North French and Czech palaeokarsts. In all these places Halloysite appears to be formed by low temperature weathering of silicate rocks and/or sediments with the possible local involvement of acid solutions rich in H2SO4. If this type of mechanism is responsible also for the Halloysite of Eraldo cave this could indicate a production of sulphuric acid by oxidation of sulphides in a sub-aerial environment.
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