Monday, November 12, 2012

BLUE-PURPLE HALITE

HALITE FROM DELAWARE BASIN, NEW MEXICO. CUBE ~2.5 X 2.5 CM
I have always associated the mineral halite (NaCl) with white-colored table salt, the shimmering small crystals on the “Bonneville Salt Flats” in western Utah, the salt anticlines and salt valleys in the Paradox Basin of the Four Corners, and the massive subsurface deposits left behind by the evaporating Permian cratonic seas.  These thick subsurface beds of Permian age are abundant in my native Kansas and, in my youth, could be observed by descending into one of the mines and picking up a few crystals.  Although mines at Hutchinson and Kanopolis are still producing “rock salt” for a variety of uses, getting a free visitor trip into the diggings is virtually impossible. 
HALITE CRYSTAL FROM PERMIAN SALT, KANSAS. CUBE ~ 2.5 X 3.0. 
But even if you cannot descend into a mine there are numerous opportunities to observe natural halite on the surface--the above mentioned Bonneville Salt Flats, for example, and certainly the vast Searles Lake region in California where nice pink crystals are exposed over an area of about 20 mi2.
HALITE CRYSTALS FROM SEARLES LAKE, CALIFORNIA.  WIDTH ~9 CM.
We all realize that halite easily dissolves in ordinary water, H2O.  This process is “good” in some cases, for example in flavoring our foods.  But in other instances, the results are badly degrading the environment---“road salt” runoff certainly contaminates local streams and in some cases even ground water.  The solubility of halite is also responsible for the large number of sinkholes in western Kansas (and many other states).  Although Kansas is not known as a “cave state” where carbonate rocks dissolve to form voids, many people are surprised about the subsurface solution of Permian-age halite and gypsum with resulting collapse of overlying rock layers. A couple of examples include the Ashland Basin in southwestern Kansas, perhaps 10-12 miles long, and representing a series of coalescing sinkholes.  The Basin lies in the High Plains Physiographic Region and that upland is perhaps 400-500 higher than the bottom of the sinkhole.  In Clark County, also in southwestern Kansas, the circular Big Basin is a collapse structure about one mile in diameter and dissected and drained by the Cimarron River.
GOOGLE MAP VIEW OF BIG BASIN, A SINKHOLE LOCATED IN SOUTHWESTERN KANSAS.  THE CIRCULAR STRUCTURE IS ABOUT ONE MILE IN DIAMETER.  NOTE ROAD DISSECTING THE FEATURE.
In central Kansas, where I spent my youth wandering hills of the Dakota Formation (Cretaceous), I was fascinated with the amount of halite leaching out of the rocks.  The Jamestown Wildlife Refuge is a major stop for migrating waterfowl in the Central Flyway, and a tremendous place to “bird watch”.  The refuge is a large marsh with a series of salt water springs and seeps issuing from the upper part of the Dakota.  As the water evaporates halite crystals are constantly being produced (and re-dissolved) along water’s edge.
I grew up on the Saline River below its contact with the Dakota and the water was highly charged with sodium chloride and could not be used for direct irrigation of plants and crops (several thousand milligrams of chloride per liter).  The Saline is a fairly long river at ~400 miles (entirely in Kansas) but is actually quite small in size---except during the numerous floods!  There are at least two “Salt Creeks” flowing into the Saline and French explorers noted the “briny” water as early as 1724.

So halite, a somewhat interesting mineral, is similar to many of the other evaporates--they seem rather dull as a collectable mineral. Halite comes in a variety of colors, mostly light in nature, with the tint commonly due to small amounts of impurities.  However, at a recent show my eyes about popped out when I discovered a dealer with several specimens of blue to purple halite, a really bright-colored halite! At first I thought perhaps this was simply a crystal constructed from a halite-saturated solution with food coloring added in.  But, I was assured the crystal was natural.  I had really never seen blue to purple halite before but thought it would look nice in my collection for a couple of dollars.  After returning home I begin a literature search to try and locate information about this halite and came across several specimens listed for sale in the 50 to 500 dollar range.  That aspect made me feel good about my frugal purchase!  

There are some rather famous collecting localities for colored halite in Poland; however, it appears that all blue to purple halite collected in this country comes from potash mines in the Delaware Basin of southeastern New Mexico: the upper Permian McNutt member of the Salado Formation Bickham, (2012).  As for origin of the blue to purple color, many/most geologists believe the coloration is due to gamma-ray bombardment of halite with the rays coming from radioactive potassium-40 found in associated minerals (like sylvite: KCl and isomorphous with halite).  K-40 is rare but does occur in some instances.  The gamma rays then disrupt the lattice structure of the halite and force the displaced electrons to reflect the blue to purple wavelengths from the visible light.  I am not enough of a mineralogist to vote yay or nay on this thought but it sounds good to me.  Bickham (2012) has some other ideas that seem worth exploring.  Whatever the reason for the bright color, these specimens make very nice displays and certainly generate many questions from visitors.

REFERENCES CITED

Bickham, M., 201, Chemical Analysis of Blue Halite [abs]: Geological Society of America Abstracts with Programs. V. 44, no. 1.

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