Mineral deposit in the US

Two companies are involved in the industrial extraction and processing of gellant-grade attapulgite clay within the same Attapulgus deposit: Active Minerals International, LLC, and BASF Corp. In 2008, BASF acquired the assets of Zemex Attapulgite, leaving only two gellant-grade producers. Active Minerals operates a dedicated factory to produce the patented product Actigel 208 and built a new state-of-the-art production process in early 2009 involving portable plant processing at the mine site.^[11]

Properties

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Attapulgite clays are a composite of smectite and palygorskite. Smectites are expanding lattice clays, of which bentonite is a commonly known generic name for smectite clays. The palygorskite component is an acicular bristle-like crystalline form that does not swell or expand. Attapulgite forms gel structures in fresh and salt water by establishing a lattice structure of particles connected through hydrogen bonds.

Attapulgite, unlike some bentonite (sodium-rich montmorillonites), can gel in seawater,^[12] forming gel structures in salt water and is used in special saltwater drilling mud for drilling formations contaminated with salt. Palygorskite particles can be considered as charged particles with zones of positive and negative charges. The bonding of these alternating charges allows them to form gel suspensions in salt and fresh water.

Stabilization of nanopalygorskite suspensions was improved using mechanical dispersion (magnetic stirring, high-speed shearing and ultrasonication) and polyelectrolytes (carboxymethyl cellulose, alginate, sodium polyphosphate, and poly(sodium acrylate)) at different pH.^[13] Surface energy and nanoroughness were studied in a palygorskite sample.^[14]

Potential toxicity

Studies have shown that Palygorskite may be carcinogenic to humans. Much like asbestos and some fibrous zeolites, Palygorskite can be found in asbestiform habits.^[15]

Studies thus far on the possibility of Palygorskite being a carcinogen has been mixed. Some studies show that cytotoxicity in rats, mice, livestock, hamsters, and even humans have caused malignant mesothelioma. In rats specifically, studies have ranged from 2.5-94% mesothelioma rates. Differences in palygorskite fiber length and purity (i.e., presence of other carcinogenic mineral fibers) may have been responsible for the disparate results observed in those experiments.^[16]

Specifically in Nevada, there is a strong link between Palygorskite and mesothelioma. In 2011, medical Geologist Brenda Buck of The University of Nevada Las Vegas (UNLV) was looking for arsenic minerals in Nellis Dunes. What she found was fibrous Palgorskite in her sample.^[17] Further research found that more women and children than men had higher rates of malignant mesothelioma; with the ratio being as high as 3:1. Palygorskite samples were taken from 4 different locations in southern Nevada, and scanned by electron microscopy (SEM). The results showed that Palygorskite fibrous physical features similar to those of asbestos minerals.^[16][18]

Medical use

Attapulgite is used widely in medicine. Taken by mouth, it physically binds to acids and toxic substances in the stomach and digestive tract. Also, as an antidiarrheal, it was believed to work by adsorbing the diarrheal pathogen. For this reason, it has been used in several antidiarrheal medications, including Diar-Aid, Diarrest, Diasorb, Diatabs, Diatrol, Donnagel, Kaopek, K-Pek, Parepectolin, and Rheaban.^[19] It has been used for decades to treat diarrhea.

Until 2003, Kaopectate marketed in the US also contained attapulgite. However, at that time, the U.S. Food and Drug Administration retroactively rejected medical studies showing its efficacy, calling them insufficient.^[20][21] The manufacturer also settled with the State of California over toxic levels of lead in the attapulgite component. Part of this settlement was a reformulation to remove attapulgite in the liquid version in the US.^[22]

Kaopectate's U.S. formula was changed to bismuth subsalicylate (pink bismuth). The next year (2004), an additional change in labeling was made; from then on, Kaopectate was no longer recommended for children under 12 years old.^[23] Nevertheless, Kaopectate with attapulgite is still available in Canada and elsewhere. Until the early 1990s, Kaopectate used the similar clay product kaolinite with pectin (hence the name).

Construction

Palygorskite can be added to lime mortar with metakaolin for period-correct restoration of mortar at cultural heritage sites.^[24]

In human culture

Palygorskite is known to have been a key constituent of the pigment called Maya blue, which was used notably by the pre-Columbian Maya civilization of Mesoamerica on ceramics, sculptures, murals, and (most probably) Maya textiles. The clay mineral was also used by the Maya as a curative for certain illnesses, and evidence shows it was also added to pottery temper.

A Maya region source for palygorskite was unknown until the 1960s, when one was found at a cenote on the Yucatán Peninsula near the modern township of Sacalum, Yucatán. A second possible site was more recently (2005) identified, near Ticul, Yucatán.^[25]

The Maya blue synthetic pigment was also manufactured in other Mesoamerican regions and used by other Mesoamerican cultures, such as the Aztecs of central Mexico. The blue coloration seen on Maya and Aztec codices, and early colonial-era manuscripts and maps, is largely produced by the organic-inorganic mixture of añil leaves and palygorskite, with smaller amounts of other mineral additives.^[26] Human sacrificial victims in postclassic Mesoamerica were frequently daubed with this blue pigmentation.^[27]

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  Electron diffractogram of palygorskite (Wiersma 1970).
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  Electron micrograph of palygorskite (Wiersma 1970).
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  Palygorskite variant Pilolite, "mountain leather", with "modulated layers" of growth, feeling like flexible leather, Seaton, Devon, UK, before 2011.
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  Rows of colorless calcite crystals held together by layers of papery palygorskite. From Metaline Falls, Washington, USA, 2013.
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  Palygorskite. Estonian Museum of Natural History, 2015.