Wednesday, November 6, 2013


In my previous post I spotlighted one of the zeolite minerals found in the basalts of the Midcontinent Rift System (MRS)—thomsonite (NaCa2[Al5Si5O20]-6H2O).  The zeolites are a group of aluminosilicate minerals that are industrially important and used in a variety of products from cat litter to elements used in the reprocessing of nuclear material.

Thomsonite is one of the few zeolites available for use as semi-gemstones; tumbled or polished small nuggets are utilized in pendants and bracelets.  However, most rockhounds are interested in zeolites due to their well-developed crystals (and often very reasonable prices).

Gem-type thomsonite is best known from the MRS while one of the prime places to collect crystalline thomsonite is from North Table Mountain near Golden, Colorado.   Millions of people are  familiar with North and South Table Mountains due to Golden’s most famous export—Coors Beer.  The brewery, established in 1873, is located between the Mountains and has used a stylized projecting point on South Table Mountain as part of its logo.  Castle Rock, as the point is known, is a large hunk of volcanic rock.

Distant view of Golden City. View from the foothills west of the city, looking east over the broad basaltic tables, flanking Clear Creek upon either side. Jefferson County, Colorado. 1872 photo by W.H. Jackson from USGS Library.

United States Geological and Geographical Survey of the Territories (Hayden Survey). "Table Rock", Golden City. Colorado Territory, July 28, 1869. From USGS Library (H.W. Elliott).

Persons of my age, mostly male Baby Boomers, fondly remember Coors beer for a number of reasons, not the least of which is that only beer drinkers in the western U.S. could sample the brew—those “back east” had to settle for Hamms or Bud or Falstaff, or any number of others.  However, although Budweiser was the King of Beers, Coors was thought to be the “best” since it was brewed with fresh Rocky Mountain Spring Water, and it was off limits to those “easterners”!  Later in life I took a look at the “fresh water” and noted that Clear Creek flowed through some nasty looking mining areas. However, the “big thing” about Coors was its availability only in the West (including my home state of Kansas) due to some arcane law about a lack of permits associated with alcohol distribution.  Remember the cult classic movie Smoky and the Bandit where Burt Reynolds (the bandit) and Jerry Reed (Cletus, his buddy) transported 400 cases of Coors from Texas to Georgia all the time being tormented by Sherriff Buford T. Justice (Jackie Gleason)?  While attending graduate school in South Dakota I hauled back cases of Coors and made a small profit selling them to fellow students (later found out this was considered bootlegging—is that a felony?).  The return trip featured “Green Death,” Heilemann’s premium Old Style Beer, transported back to Kansas.  Again, later in life, I worked in La Crosse, Wisconsin, where Old Style was brewed and home of The World’s Largest Six Pack..

The World’s Largest Six Pack. Photo courtesy of The Pour Pub. 


Botryoidal thomsonite (T) with plates of analcime (A) situated on a bed of chabazite (C) crystals within a vug in the shoshonite matrix.  Width of vug ~5.5 cm.
OK, back to zeolites and thomsonite.  North and South Table Mountains are formed from the Denver Formation with capping and intercalated shoshonite lava flows (non-explosive, potash-rich basaltic rocks).  The sedimentary rocks of the Denver Formation span the Cretaceous-Tertiary boundary as dinosaurs have been described in the lower part while Tertiary mammals have been found in upper beds.  During the early Paleocene the lava was extruded from several nearby volcanic plugs (Ralston) where the only evidence that remains is the roots of this volcano. There were at least four different flows over a 1 my. period. The zeolites that formed in vugs of the shoshonite are secondary minerals. (above information from Drewes (2008). 

Photomicrograph of crystalline chabazite with plates of analcime (A).  Largest analcime width ~1.5 mm.
Www.MinDat.Com lists 16 different zeolite minerals known from North Table Mountain, including thomsonite-Ca.  In the last post I noted the calcium of thomsonite-Ca is at times replaced by strontium so the mineral becomes thomsonite-Sr.  The specimen I have from North Table Mountain has light tan (stained?) botryoidal thomsonite-Ca (I presume the calcium variety) setting on nice crystals of chabazite [CaAl2Si4O12-6H2O] with smaller plates of analcime [Na2(Al2Si4O12)-2H2O].  At least that is my novice interpretation of the specimen!  

Photomicrograph of botryoidal thomsonite.
So, thomsonite can be collected as semi-gemstones from the MRS or as crystals from many localities including North Table Mountain.  I threw in the Coors beer bit as a bit of nostalgia with apologies to some overseas blog followers, or also to readers who have not seen Smoky and the Bandit:
 Cledus: Hey Bandit. Me an' Fred's (a dog) got a question.
 Bandit: What you an' Fred want?
 Cledus: How come we doin' this?
 Bandit: Well why not?
 Cledus: Well they said it couldn't be done.
 Bandit: Well that's the reason, son!
 Cledus: That's good with Fred. 

So, that is sort of my motto--why you doin' this?  Well why not?

Drewes, H., 2008, Table Mountain Shoshonite Porphyry Lava Flows and their Vents, Golden, Colorado: U.S. Geological Survey Scientific Investigations Report 2006–5242.