magnesite - teaching hand specimen of light tan porous magnesite - Windous magnesite deposit, Westvaco Mine

Magnesite - MgCO₃

Magnesite, magnesium carbonate, is usually earthy or granular, but it can be massive vitreous or crystalline. This particular earthy magnesite shows a somewhat conchoidal fracture. When first discovered in Muddy Valley, Nevada in the late 1800s, magnesite 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. That mine is still in operation.

Hard kaolin looks much like the more vitreous variety of magnesite. Both break with a sharp-edged conchoidal 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. This tan magnesite is porous and will stick to the tongue. It readily effervesces with HCl because of its high porosity.

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 vitreous specimens from this deposit have been dyed and sold as turquoise.

Compare this magnesite with the very light gray to white earthy magnesite we collect from the Kramer Hills near Boron, California.

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