File:Unakite (epidotic granite) (Buena Vista, Virginia, USA) 3.jpg

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English: Unakite is a distinctive rock consisting of greenish epidote (Ca2(Al,Fe)3(SiO4)3(OH)), salmon-colored potassium feldspar (KAlSi3O8), and gray quartz (SiO2). This rock is granite that has been metasomatized, which refers to metamorphism by chemical alteration. In this case, the original plagioclase feldspar and/or mafic mineral content of the granite has been altered to epidote. Epidote is a low-temperature metamorphic mineral. It usually forms along fractures or faults in granite. Hydrothermal fluids (hot water) in the fractures will alter the adjacent country rocks by selectively changing the plagioclase and mafic minerals.

Epidote-rich rocks can form by regional metamorphism of shale. From low to higher grade metamorphism, many shales will alter to chlorite schist to epidote-rich rocks to actinolite-rich rocks. These are the greenschists.

Epidote can also occur in pegmatites, which are very coarsely-crystalline, intrusive igneous rocks. Incompatible and rare elements get concentrated near the end of pegmatite crystallization - epidote is a "garbage can" mineral that often forms in such settings. If the epidote ends up having thorium, which is radioactive, surrounding quartz will usually be altered to smoky quartz.

Tumble-polished unakite is considered a semiprecious stone.

Locality: unrecorded/undisclosed site at or near the town of Buena Vista, Rockbridge County, Virginia, USA
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Source https://www.flickr.com/photos/47445767@N05/51719518353/
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@N05/51719518353. It was reviewed on 27 December 2021 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0.

27 December 2021

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current17:51, 27 December 2021Thumbnail for version as of 17:51, 27 December 20214,000 × 3,000 (6.06 MB)Ser Amantio di Nicolao (talk | contribs)Uploaded a work by James St. John from https://www.flickr.com/photos/47445767@N05/51719518353/ with UploadWizard

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