Mordenite photomicrograph. Width ~6 mm.
Thursday, November 7, 2013
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 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.
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.