File:Moonstone (iridescent peristerite-oligoclase feldspar) (Chupa Pegmatite Field, Mesoproterozoic, 1.75 to 2.10 Ga; at or near Chupa Bay, Karelia, Russia) 2.jpg

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English: “Moonstone” (iridescent peristerite-oligoclase feldspar) from the Precambrian of Russia. (field of view 5.6 cm across)

The term “moonstone” has been used to refer to various materials, but properly refers to iridescent whitish-gray feldspar. The sample shown here consists of peristerite and oligoclase feldspar. Oligoclase is a sodium-rich plagioclase feldspar. Peristerite consists of closely-intergrown varieties of albite (= very sodium-rich plagioclase feldspar) with slightly varying sodium and calcium contents. Light gets split along the boundaries between the intergrown feldspars, resulting in bluish-white iridescence.

This is a portion of a single large crystal derived from a Precambrian-aged pegmatitic granite intrusion in the White Sea area of northwestern Russia. Pegmatitic granites have huge crystals, formed by cooling of water-rich magmas.

Geologic & age: Chupa Pegmatite Field, late Mesoproterozoic, 1.75-2.10 Ga

Locality: at or near Chupa Bay (at or near the town of Chupa), near the western coast of the White Sea, northwestern Karelia, far-northwestern Russia
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Source https://www.flickr.com/photos/47445767@N05/14936848270/
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@N05/14936848270. It was reviewed on 8 October 2020 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0.

8 October 2020

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current14:56, 8 October 2020Thumbnail for version as of 14:56, 8 October 20203,644 × 2,428 (3.55 MB)Ser Amantio di Nicolao (talk | contribs)Uploaded a work by James St. John from https://www.flickr.com/photos/47445767@N05/14936848270/ with UploadWizard

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