VOLKOVSKITE: THE BEST BUY IN DENVER

 The only show that I have attended (briefly) since Tucson 2020  was the Colorado Mineral and Fossil Fall Show in mid-September.  A couple of Postings about the Show were made on September 20 and October 5.  However, I have saved the best purchase from Denver until last—volkovksite! Of course, prior to the Show I had never heard of this hydrated calcium potassium borate—so I thought!  That was a wrong assumption since the Sept.-Oct. 2016 edition of Rocks and Minerals published an article Exceptional Volkovskite Crystals from Zechstein Evaporites at the Boubly Mine, Cleveland, England.  I read the article, I am certain, four years ago; however, for a guy who has trouble remembering where he laid the truck keys I forgave myself for not remembering an article on a mineral with an unpronounceable name.

At any rate I am pleased that our college mineralogy class did not study this borate since the chemical formula is a booger to remember (and to understand): KCa₄[B₅O₈OH]₄[B(OH)₃]₂Cl · 4H₂O.  And to make matters worse the “pure” mineral, at first glance, looks like muscovite, and with impurities resembles (to non-mineralogists) the potassium chloride sylvite.

Boron (B) is a tough one for me to understand, perhaps because elemental boron does not occur naturally on earth (evidently in some meteorites though).  It usually appears in some sort of a compound with oxygen (B + O) and often as BO₃ with an oxidation state of 3-.  The common borate minerals, for example kernite, colemanite, and borax are evaporites and are not very colorful.  Other borates are more collectable such as londonite and rhodizite. Volkovskite is also a marine evaporite and not very spectacular in “looks”; however, it is rare and uncommon in the mineral record and therefore becomes collectable (at least to me).

The discovery and subsequent history of volkovskite is interesting and full of intrigue.  It was first discovered and named in 1961 from small crystals found in insoluble residues from wells drilled into salt domes (Permian) in the USSR.  In those days of the “Cold War” not much information was exchanged between scientists of the USA and the USSR and it was not until several years later that the Type Locality was revealed as a locality in the former Soviet hemisphere of Kazakhstan.  In the late 1980 Canadian scientists discovered “similar appearing” crystals in the Salt Springs evaporite deposits of New Brunswick (Carboniferous).  However, the Canadians could not borrow enough type material  from the Russians to make an adequate identification, so their material was designated at the neotype.  In the early 2000s the Boulby Mine on/near the northeast coast of England started producing volkovskite in substantial quantities (for an uncommon mineral) “that sets s new standard for the species” (Genis and others, 2016).

The Boulby Mine is one of the deepest mines in Europe and is a major producer of potash ore (mostly sylvite [KCl]) that is used in making agricultural fertilizer (50% of the UKs), and in rock salt (mostly halite).  The sequence producing the potash and salt, known as the Zechstein Evaporite Cycle, is Permian in age and is associated with various drying and flooding cycles of the Zechstein Sea, an epicontinental sea associated with the supercontinent Pangaea.  The Mine also produces a number of other rare to uncommon borate minerals.

Map showing Boubly, England along the coasts and the surrounding area.  Public Domain map courtesy of Lencer, own work.
 

OK, Boubly/Cleveleand, England--unless you are intimately associated with the UK--do you really know anything about the area?  Count me as someone who has been to Cleveland, Ohio, but knows zip about the UK Cleveland.  No, the US Cleveland was not named after the UK variety.  The Cleveland area lies on the coast of the North Sea where rocks of Jurassic age form high cliffs along the Sea.  In past times these rocks produced "alum" (hydrous potassium aluminum sulfate) and iron and scars from the mines are still viable.

The Cliffs at Boubly, about 666 feet of Jurassic rocks are the highest points on the East Coast.  Photo Public Domain courtesy of Helen Wilkenson.

The most visible landmark near Cleveland is Roseberry Topping coming in at ~1049 feet in elevation.  It is part of the New York Moors National Park that is the largest Heather Moorland in the UK.

   

The moorland of New York.  Photo Public Domain courtesy of Mr R Jordan.

  

Roseberry Topping composed of Jurassic rocks.  Photo Public Domain and ChrisO. 

In the United States volkovskite has been discovered in the Louann Salt in a 12,000-foot-deep well drilled into a salt some in Alabama.  The  salt was formed in an evaporite basin that formed in the Jurassic as the South America Plate was rifting away from the North American Plate.

Volkovskite occurs as transparent, colorless (some are light pink), micaceous-like plates (perfect platy cleavage as in micas).  It is soft at ~2.5 (Mohs), displays a white streak, has a vitreous luster. and is brittle with splintery fragments if broken. 

Crystals of generally colorless micaceous volkovskite with red to reddish brown hilgardite.  Arrows on top point to crystals of hilgardite. Width FOV ~4.5 cm.

Notice the micaceous crystals of colorless volkovskite with hilgardite.  Width FOV ~1.4 cm.

This is an interesting photomicrograph showing several stacked platy crystals of volkovskite with streaks of red ?hilgardite.  Width FOV ~1.1 cm.

Volkovskite from the Boulby often contains, or is intermixed with, the hydrous calcium borate hilgardite [ Ca₂B₅O₉Cl · H₂O].  At times, this red to reddish brown or colorless mineral appears as inclusions between the volkovskite platy layers.  In other instances, hilgardite appears along the edges of volkovskite crystals.

Hilgardite is harder than volkovskite (~5. 0 Mohs), has a vitreous luster, is transparent and leaves a white streak.  It is found in a greater number of localities than volkovskite, but all are associated with marine evaporites.  The Type Locality is from the Bayou Choctaw Salt Dome in Louisiana (Hurlbut and Taylor, 1937). Like many other uncommon borate minerals hilgardite was first noted by mineralogists studying insoluble residues from salt dome core samples.

REFERENCES CITED

Genis, J., M.D. Freier, D.I. Green, and T.E. Cotterell, 2016, Exceptional volkovskite crystals from Zechstein Evaporites at the Boubly Mine, Cleveland, England: Rocks and Minerals, vol. 91, no. 5. 

Hurlbut, C. S., and R. E. Taylor,1937, Hilgardite, a new mineral species from Choctaw Salt Dome, Louisiana: American Mineralogist. Vol. 22.