ZEOLITE: MORDENITE

I have sort of become fascinated with zeolite group minerals during the last several months and have tried to argument my modest collection by making a few strategic frugal purchases.  Some zeolites such as chabazite and stilbite seem to be everywhere including “nature-oriented gift shops.”  Many of the other zeolites are more difficult to acquire at a reasonable price.

At one of the shops this summer I saw a thumbnail of mordenite, looked at it with my hand lens, and immediately marked it for purchase.  Mordenite is one of those minerals forming crystals that are long and slender---acicular, hair-like sprays.  These individual crystals are quite fragile and need protection and a “lack of touching.”  The specimen came from the “Siletz River Basalt Flows, Oregon.”

 

Mordenite.  Width ~1 cm.

Mordenite photomicrograph.  Width ~6 mm.

 Mordenite is a hydrated calcium sodium potassium aluminum silicate [(Ca, Na₂,K₂)Al₂Si₁₀O₂₄-7H₂O] that has a silky luster, transparent to translucent diaphaneity, and a hardness of 3 or 4 (Mohs).  It usually is colorless (clear) to white but sometimes has a pinkish cast (rarely yellowish cast).

Mordenite can occur in several different environments including as an “alteration product of pyroclastic sediment and sedimentary rocks, as well as lava flows. These occurrences range from low temperature diagenesis of vitric tuff in lacustrine sediment [where the silica glass reacts with pore water], to thermal aureoles surrounding volcanic vents, and to active hydrothermal systems” (Deer and others, 2004).  Mordenite has been synthesized, like many zeolites, and is commercially valuable for use in catalytic and separation processes in the petrochemical industry.

I cannot locate the exact figures for mordenite but the USGS (www.minerals.usgs.gov) noted that in 2011 natural zeolites were mined by 10 companies in the U.S with New Mexico being the leading producer.  The most commonly mined zeolites were chabazite, clinoptilolite and mordenite.  These minerals were mined from sedimentary rocks—“associated with the alteration of volcanic tuffs in alkaline lake deposits and open hydrologic systems” (easier to mine these rocks rather than basalts).  Production of all zeolites in 2011 was ~62,000 metric tonnes.

The zeolites of the Siletz River volcanics are interesting as they formed in vugs, cracks and fractures of theolitic, basaltic pillow lava.  The mordenite in my collection came from one of the vugs in these basalts.  Keith and Staples (1985) believed the zeolites were the result of alteration of basaltic glass.  Furthermore, they “suggested that zeolitization occurred during a low-temperature (60°–70°C) submarine hydrothermal event, or by reactions of cold (∼10°C) meteoric water with basalt over a long time.” 

OK, theolitic basalts are a very common type of silica-rich basalt dominated by the minerals clinopyroxene and plagioclase feldspar.  Pillow lavas are basalts extruded “under water” and are characterized by pillow-shaped masses.  Mostly we think of pillow lavas as being extruded under oceanic waters; however, I have seen pillow lava that was formed as volcanos in central Utah dumped their lava into Pleistocene Lake Bonneville. 

Besides the hair-like crystals, the mordenite is interesting due to the nature of the Siletz River Volcanics.  Found in the Coastal Range of Oregon, the Paleocene to Eocene volcanic rocks belong to a terrane geologists refer to as Siletzia.  These volcanics erupted from some sort of a spreading center (similar to the current Mid-Atlantic Ridge) situated between oceanic tectonic plates (hence the pillow lavas).  This oceanic plate containing the riding volcanics (known to geologists as the Farallon Plate) was moving eastward (current direction), bumping into what is now North America.  The Farallon Plate (more dense) was drawn under (or subducted) the North American Plate (lighter density).  However, the Siletz River volcanics were quite massive and not all were subducted and so were accreted (stuck on to) the North American Plate.

It gets better, and especially for readers here in the Mountain West.  Schmandt and Humphreys (2011) believed the Siletz River volcanics were so massive that they sort of jammed the subduction zone, an event that had regional implications.  At ca. 53 Ma the Laramide Orogeny (building of the Rocky Mountains) had an abrupt shift.  Up to then the Orogeny was one of compressional tectonics (lots of thrust faulting and uplift) caused by the North American Plate overriding the Farallon Plate.  But now the subduction zone is jammed by the Siletz River volcanics and the compression sort of stops.  As the compression waned the Mountain West. also seemed to experience a hiatus of volcanism. Somewhere ca. 40 Ma more changes were in the air as the area begin to experience extensional tectonics (stretching of the crust) and a great outpouring of explosive volcanism (Mid-Tertiary Ignimbrite Event).  The Basin and Range area is the result of extension as is the Arkansas River around Buena Vista and Leadville (see previous blogs).  The volcanism can be seen in the great calderas around Silverton and near LaGarita (see previous blog). 

Wow, this is interesting stuff but also might confuse readers without some sort of a geology background.  Sorry for all of the jargon but I tried to make an interesting story.  I think it amazing that a little ole piece of mineral with some pretty crystals can lead to a tale involving global tectonics.

REFERENCES CITED

Deer, A., R. Howie, W. S.Wise,  and J. Zussman, 2004, Rock Forming Minerals. v. 4B. Framework Silicates: Silica Minerals, Feldspathoids and the Zeolites, The Geological Society, London.

Keith, T. E. C. and L. W. Staples, 1985, Zeolites in Eocene Basaltic Pillow Lavas of the Siletz River Volcanics, Central Coast Range, Oregon:  Clay and Clay Minerals, v. 33, no. 2.

Schmandt, B. and E. Humphreys, 2011, Seismically Imaged Relict Slab from the 55 Ma Siletzia Accretion to the Northwest United States: Geology, v. 39, no. 2.

TABLE MOUNTAIN, ZEOLITE, and COORS BEER

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?

   

REFERENCES CITED

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