magnesite - white vitreous magnesite - hand/display specimen
Magnesite, magnesium carbonate, is usually earthy or granular, but it can be massive vitreous or crystalline and can occur in the form of nodules up to several feet in diameter. This particular magnesite is the cryptocrystalline vitreous form. When vitreous magnesite was first discovered in Muddy Valley, Nevada in the late 1800s, it was thought to be kaolin and plans were made to mine it as such. In 1915, before mining could begin, a specimen sent to the U.S. Geological Survey was identified as magnesite.
Hard kaolin looks much like this variety of magnesite. Both break with a sharp-edged fracture. Magnesite, when powdered by taps of a geologist’s hammer, will effervesce with hydrochloric acid. There is no reaction with kaolin. An alternate test is to scrape the magnesite on a streak plate to produce a powder. A drop of hydrochloric acid will produce the effervescence, but it can be subtle.
These specimens were collected at the Windous magnesite deposit, near Windous Butte, roughly 20 miles west of Ely, Nevada. The deposit, apparently hydrothermally altered volcanic ash, was mined in 1940-41 by Westvaco Chlorine Products Corp. The mine is inactive, but specimens from this deposit have been dyed and sold as turquoise.
The field photo shows a large magnesite nodule that had been extracted when the mine was still active.
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