Hydrothermal Calcite

Wonderful geometries in calcite crystals

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This image is a close-up view of large calcite crystals. The presence of the coloured bands is due to twinning: what  appears to the naked eye to be compact crystals, 

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 when seen under a microscope reveals internal layers in which the atoms are distributed differently from those of the encasing crystal, and which therefore refract light differently, creating variable interference colours.

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These calcites were found in a sedimentary formation that dates to approximately 240 million years ago, but they can also be found in other “vessels”.

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In fact, this calcite does not have sedimentary origins but hydrothermal: once the rock was formed, hot water with a high mineral salt content flowed over the fractures and cavities, filling them with minerals such as calcite.

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This calcite was formed together with galena and blende (or sphalerite),minerals that were extracted at Col Piombin up until the start of the last century 

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for the production of zinc, lead and iron. References: http://en.wikipedia.org/wiki/Sphalerite

http://en.wikipedia.org/wiki/Galena

 

 

Name: Hydrothermal Calcite
Classification: Hydrothermal mineral
Mineral composition: Calcite, Dolomite, Galena, Blende, Pyrite, other
Fossils: none
  Location: Col Piombin in the vicinity of Passo Giau (46°28’36.2″N 12°03’42.8″E)
Formation: Contrin formation
Era:
Upper Anisic (approximately 242 million years ago)
Depositional environment*:

Piattaforma carbonatica (ambiente di mare basso sotto costa)

*) The environment refers to the encasing rock and not the calcite that is the subject of the photo