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Garnetite from the Miocene of Japan. (~4.0 cm across at its widest)

This is a coarsely-crystalline, garnet-dominated, crystalline metamorphic rock called garnetite. Garnet is a group of silicate minerals. These are andradite garnets, a calcium-iron variety of garnet (ideally Ca3Fe2Si3O12). When tilted at certain angles in the light, many of the crystals have a rainbow iridescence (click on the photo to zoom in and look around - iridescence does show up in this picture in places). Mineral collectors call this material "rainbow garnet". Studies have shown that the iridescence is caused by thin film, two-beam interference and light diffraction (see Hainschwang & Notari, 2006).

Available geologic information indicates that this is a garnetite skarn. Skarns are contact metamorphic rocks, formed by heating and chemical alteration from a nearby igneous intrusion. In this case, the Omine Granite intruded the area during the Middle Miocene and altered the surrounding country rocks.

Locality: garnet vein near the abandoned Kouse Magnetite Mine, near the town of Tenkawa, Toshino District, Nara Prefecture, southern Honshu Island, southern Japan

See also: <a href="https://www.flickr.com/photos/finnchaga/37659507116">www.flickr.com/photos/finnchaga/37659507116</a> and <a href="https://www.flickr.com/photos/finnchaga/37674616662">www.flickr.com/photos/finnchaga/37674616662</a>

Reference cited:

Hainschwang & Notari (2006) - The cause of iridescence in rainbow andradite from Nara, Japan. Gems & Gemology 42: 248-258. (<a href="https://www.gia.edu/doc/WN06A4.pdf" rel="nofollow">www.gia.edu/doc/WN06A4.pdf</a>)
Date
Source Garnetite (apparently a skarn) (Middle Miocene; garnet vein near Kouse Magnetite Mine, near Tenkawa, Honshu Island, Japan) 3
Author James St. John

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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/33020706364 (archive). It was reviewed on 4 November 2019 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0.

4 November 2019

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current14:51, 4 November 2019Thumbnail for version as of 14:51, 4 November 20192,065 × 2,115 (2.8 MB)Ainz Ooal Gown (talk | contribs)Transferred from Flickr via #flickr2commons

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