File:Thinolite tufa (Quaternary; Pyramid Lake or Winnemucca Lake, north of Nixon, Nevada, USA) 4.jpg
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English: Lacustrine thinolite tufa from the Quaternary of Nevada, USA.
Calcareous tufa is a scarce, calcitic, finely- to coarsely-crystalline textured, chemical sedimentary rock. It principally forms around cold springs having water relatively rich in dissolved calcium carbonate. Calcareous tufa is often characterized as a lightweight, porous, friable precursor to travertine (another calcitic, chemical sedimentary rock). The tufa specimen seen here is from the shoreline area of a western Nevada lake. It precipitated chemically and/or microbially from lacustrine water during the Pleistocene Ice Age. It is coarsely crystalline - such specimens have been referred to as thinolite, or thinolitic tufa. Thinolite is considered a synonym of glendonite, a "variety" of calcite (CaCO3). This material started out as a crystalline mass of ikaite, a hydrous calcium carbonate mineral (CaCO3·6H2O). Ikaite only forms in near-freezing water (~0º to 7º C) of high alkalinity, in organic-rich sediments at the sediment-water interface. At warmer temperatures, ikaite is not stable, and the mineral loses its water content and converts to calcite (anhydrous calcium carbonate). During the ikaite-calcite conversion, the original crystal structure of the ikaite may be retained. Calcite masses that retain ikaite crystal shapes are called glendonite (or, in this case, thinolite). The terms "glendonite" and "thinolite" do not refer to minerals; rather, they refers to the pseudomorph of calcite-after-ikaite. Pseudomorphs ("false-forms") are minerals that have replaced a previous mineral, but have retained the original crystal form. The presence of glendonite in a succession of rocks is diagnostic evidence for the presence of cold water temperatures and proximity to glaciers. Many ancient successions known to be deposited in glacial settings have glendonite. Locality: undisclosed locality about 25 miles ~north of the town of Nixon (this probably refers to the shoreline area of Pyramid Lake or possibly Winnemucca Lake), southern Washoe County, western Nevada, USA Info. at: pubs.usgs.gov/circ/2004/1267/pdf/Tufas%20of%20Pyramid%20L... |
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| Source | https://www.flickr.com/photos/47445767@N05/51460227216/ |
| 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@N05/51460227216. It was reviewed on 13 September 2021 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
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| current | 17:55, 13 September 2021 | | 3,525 × 2,948 (5.21 MB) | Ser Amantio di Nicolao (talk | contribs) | Uploaded a work by James St. John from https://www.flickr.com/photos/47445767@N05/51460227216/ with UploadWizard |
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