PALYGORSKITE AND SEPIOLITE: BIRDS OF A FEATHER

 Get the facts first, then you can distort them as you please.              Mark Twain

Special Signed O. Ikibus Pegasus Emblem Meerschaum Pipe

Designed and sold by Royal Meerschaum

 I recently acquired a randomly selected collection of micromounts that I thought would be fun to explore.  However, I almost got myself in trouble with one of the mounts—palygorskite collected from the “Ohio Pit Mine, N. Ishpeming, Michigan in 1976 by acs.”  I thought, oh great, another Michigan specimen adding to my small collection from the UP.  I “sort of” knew that palygorskite was a clay mineral and assumed my specimen would “sort of” look like other clay minerals.  Now, I am far from experienced in working with clay minerals and about the only thing I remember from a sedimentary geology course was using deflocculates (?), maybe a water softener like Calgon (?), to break down mud rocks (composed of clay minerals). But that process is really stretching my brain cells.  In later life I simply gave the department mineralogist some of the shale/mud rock and he ran a sample through the X-Ray Powder Diffractometer and the gizmo spit out a bunch of squiggly lines on paper and he then was able to help with identification.  In other words, I don’t know much about the identification clay minerals.  But one clay mineral I did come to recognize was bentonite since the extensive oil industry in western Kansas used bentonite in their drilling muds.  These muds are used to keep the drill bit cool, bring drill cuttings to the surface for examination, and keep formation fluids from entering the drill stem.  I recognized it as bentonite since the sack containing dry clay said “bentonite.”  Bentonite is also scattered around in the Cretaceous rock exposures in western Kansas as altered volcanic ash.  During the Cretaceous, volcanoes to the west spewed out voluminous amounts of ash that settled into the marine waters of the Western Interior Seaway and slowly settled to the floor and accumulated in layers.  These layers are quite useful to stratigraphers since each layer (ranging in thickness from millimeters to tens of centimeters) has a unique chemical signature and can be traced over hundreds of square mines (chronostratigraphic units). 

 

Imagine my surprise when I opened the mount box and discovered that not all clay minerals, including palygorskite, are dull, clay-like fragments where crystals are not visible. The Michigan specimen is composed of lath shaped crystals bundled together in thin sheets in which the individual fibers are not visible with ordinary light microscopes. I immediately grabbed my Mineralogy of Michigan book (Heinrich revised by Robinson, 2004) and zipped to the palygorskite entry expecting to learn much about a mineral that I knew little about. Surprise! What I found was palygorskite is “a relatively uncommon fibrous mineral...Reported by Morris at the Ohio mines…near Michigamme.”   

 

Palygorkskite from Michigan.  Width FOV ~9 mm.

 

Palygorskite [(Mg,Al)₂Si₄O₁₀(OH)-4H₂O] is a  hydrated magnesium aluminum (sometimes the magnesium partially replaced by iron) phyllosilicate (sheet-like silicates; examples mica minerals, clay minerals) that has an earthy luster with pastel colors—white, gray, gray-green, brown-white, yellow.  It is quite soft at ~2.0 (Mohs).

 

Palygorskite is an interesting mineral due to its large number of depositional modes.  Authigenic palygorskite is formed by direct precipitation from solution, or from the development of the mineral by transformation of precursor minerals.  The direct precipitation process implies that a new mineral structure crystallizes, so that any prior mineral structure is not inherited. The latter process usually includes the dissolution of the precursor minerals (no inherited mineral structure) and then precipitation. With palygorskite one formation model indicates the mineral originates from aluminous precursors while a second model points to palygorskite as a result of direct precipitation from solution.  (above information from Pozo and Calvo, 2018).

 

So, what environments produce palygorskite?  It is common for magnesium clays to form in evaporitic depositional environments from aluminum detrital precursors (probably other aluminum-rich clay minerals) and are important constituents of weathering profiles and soils developed in basic and ultrabasic rocks, especially in arid and semi-arid climates (Pozo and Calvo, 2018), for example, evaporites in Senegal and Iran.

 

Thiry and Pletsch (2011) noted how palygorskite clays have developed in deep oceans at various times in geological history but especially in the Middle Cretaceous Atlantic. These authigenic clays formed when hypersaline bottom waters had similar chemistry as continental evaporitic environments.  These minerals seem to be the result of direct precipitation by transformation of inherited mg-rich clay minerals on the seafloor.

 

Xie and others (2013) described the pedogenic formation of authigenic palygorskite in the Red Loess Plateau in China (in other words relating to soil).  Here palygorskite forms: 1) from the weathering of preexisting smectite (a clay mineral); and 2) by a direct dolomite precipitation from pore fluids, associated with dolomite (magnesium calcite carbonate) crystallization. Palygorskite is a response mineral to change of climate and soil environment. 

 

The largest deposit of palygorskite in the United States is located along a trend extending from Meigs, Georgia, to Quincy, Florida.  Here the mineral formed in brackish water lagoons and tidal environments associated with a marine channel extending through southern Georgia (today’s geography) connecting the Gulf of Mexico with the Atlantic Ocean. The channel was bordered on the south by the “island of Florida.” This arrangement ended when the “Florida island” became connected to the mainland in the Miocene.  It appears that palygorskite found in Early Miocene sediments/sedimentary rocks is the result of alteration of the clay mineral montmorillonite (Weaver and Beck, 1977).

 

But what about my specimen of palygorskite collected from Precambrian iron rocks?  That is a really great question, and I have few answers.  Almost all palygorskite seems associated with sedimentary environments of some sort, both continental and marine.  However, there are two localities in Michigan that have produced palygorskite in a Precambrian iron mine environment: 1) the Imperial and Webster Mines (AKA Ohio Mines) in Baraga County; and 2) the Wakefield iron pit in Gogebic County.  Both of these areas are in the Upper Peninsula (Yooper Land).  

 

The Ohio Mine produced iron from both underground (Imperial) and surface (Webster) workings.  The pay zone was  the ~1.85 Ga  Bijiki Iron Formation Member of the Michigamme Formation which accumulated near the end of the Earth's initial phosphogenic episode (~2.2 and 1.8 Ga). It seems likely that the iron minerals of the Bijiki  precipitated out of surface waters and settled into a low-energy environment below wave base. It is fine grained, thin bedded, and cherty and contains black slate, varicolored argillite-siltstone, iron carbonate, and iron oxides.  The iron has usually weather to hematite and goethite and has been mined for iron ore (Ojakankas, 1994).

 

After about three days of looking through articles and maps I cannot locate information about the formation of the rare palygorskite. It is obviously a secondary mineral but when did it accumulate?  Where did it come from?  What was the depositional environment?   I don’t have the slightest idea!  Possibilities include 1) weathering of the precursor clay mineral smectite; 2) some association with Mg-rich brines associated with layered basalts; 3) precipitation with the iron minerals in a low-energy environment below wave base; or a really wild one; 4) on a tidal flat.  I got that idea from a couple of different sources.  A web site, The Diggings (www.thediggings.com) noted that in 1933 Ford Motor Company and E. A. Wetmore shipped out high phosphorus ore from the Imperial Mines. Hiat and others (2015) described a tidal flat containing phosphorus from the 1.85 Ga Michigamme Formation.  No mention of palygorskite but the tidal flat rocks could have been the source rock.  Wild guess since I am not a geochemist or mineralogist but an ole guy trying to learn! But then again since it is a secondary mineral and perhaps it formed much later than deposition/formation of the host iron rocks.

 

The palygorskite mined in Georgia is usually combined with smectite (a group of clay minerals commonly known as bentonite) and locally are known as attapulgite clays, and have important industrial uses: drilling muds, animal food, paints, tape-joint suspensions, wild fire suppressants, and a number of other uses. At one time it was used as an anti-diarrheal medicine such as Kaopectate ™.  In the Mesoamerican Mayan civilization palygorskite was use as a paint/textile pigment called Maya blue.

 

Sepiolite [Mg₄(Si₆O₁₅)(OH)₂-6H₂O] is a hydrated magnesium phyllosilicate that has an earthy, dull luster with pastel colors—white, gray, gray-green, brown-white, yellow.  It is quite soft at ~2.0 (Mohs) when fresh but hardens with exposure and heat. In other words, it is very similar to palygorskite except it usually lacks the aluminum. One major difference is that sepiolite has “serpentine” as its precursor.

 

Fibrous acicular crystals of sepiolite from Mexico.  Width FOV ~8 mm.

There are two major forms of sepiolite: 1) the fibrous type, similar to palygorskite lathes, but sometimes matted together into larger “pieces” and is often called Mountain Leather; 2) consolidated nodular masses of earthy particulates.  The first type is a novelty mineral often found in rockhound collections (my specimen is from Mina Cerro de Mercado, Durango, Mexico). The nodules are much more interesting as the original name for sepiolate was meerschaum.  Now any male college student of the 1960s would recognize meerschaum as the bowl of a tobacco pipe (finally you understand about the photo at the top).  The bowl was carved by artisans, allowed to harden in the heat, and then attached to a “plastic” stem.  Preferably the pipe was filled with Cherry Blend tobacco and smoked in a coffee shop while dressed in a tweed sport coat with leather elbow patches. As an undergraduate student I had the pipe (the bowl was covered with leather) and carried the Cherry Blend tobacco.  However, that particular tobacco had a great aroma in an open room but tasted like crap (at least in my mouth).  So, I just sort of carried the pipe around in my shirt pocket trying to look cool.  Before heading out to have a frothy adult beverage I left the pipe behind as there were no coffee shops in western Kansas and the Horseshoe Lounge had a large population of oil field workers and cowpokes who preferred Red Man chaw, unfiltered Camels, and snus/snooze. It was fascinating, however, to visit a tobacco/pipe store and see all of the beautiful carved meerschaum pipes. 

 

Personally, this has been a fascinating research project and I learned much.  It also reinforced thoughts that to even partially understand clay minerals: 1) one must have access to a XRD (Xray Diffraction) and/or SEM (Scanning Electron Microscope); and 2) have a good/great understanding of chemistry.  Guess that sorts of leaves me out! 

 

I apologize for all of the geology jargon in this post--it did not start out that way.  Sometimes minerals just need much explanation for me to understand.  This is one of those times!

Writing blog posts is like capturing birds without killing them. Sometimes you end up with nothing but a mouthful of feathers.                    apologies to Tom Waits

REFERENCES CITED

Heinricj, E.W., updated and revised by George W. Robison, 2004, Mineralogy of Michigan: A.E. Seaman Mineral Museum, Houghton Michigan.

Hiatt, E. E., P. K. Pufahl,  C.T. Edwards, 2015, Sedimentary phosphate and associated fossil bacteria in a Paleoproterozoic tidal flat in the 1.85 Ga Michigamme Formation, Michigan, USA: Sedimentary Geology, vol. 319.

Ojakankas, R.W., 1994, Sedimentology and provenance of the Early Proterozoic Michigamme Formation and Goodrich Quartzite, northern Michigan, regional stratigraphic implications and suggested correlations: United States Geological Survey, Bulletin 1904-R.

Pozo, M, andJ.P. Calvo, 2018, An overview of authigenic magnesian clays: Minerals vol. 8, no. 11.

Qiaoqin Xie, Tianhu Chen, Hui Zhoua, Xiaochun Xua, Huifang Xu, Junfeng Jic, Huayu Lu, William Balsame, 2013, Mechanism of palygorskite formation in the Red Clay Formation on the Chinese Loess Plateau, northwest China: Geoderma, vol. 192.

Thiry, M., T. Pletsch, 2011, Developments in palygorskite-sepiolite research in Developments in Clay Science, Elsevier B.D.

Weaver, C.E. and K.C. Beck, 1977, Mioceneof the S.E. United States: a model for chemical sedimentation in a peri-marine environment: Sedimentary Geology: vol. 17, issue 1-2.