Palygorskite

Palygorskite
A sample of palygorskite
General
Category	Phyllosilicate
Formula (repeating unit)	(Mg,Al)2Si4O10(OH)·4(H2O)
Strunz classification	09.EE.20
Crystal symmetry	Monoclinic prismatic H-M symbol: (2/m) Space group: C 2/m
Unit cell	a = 12.78 Å, b = 17.86 Å, c = 5.24 Å; β = 95.78°; Z=4
Identification
Color	White, grayish, yellowish, gray-green
Crystal habit	Lath shaped crystals, fibrous and compact masses
Crystal system	Monoclinic
Cleavage	Distinct/good, good on {110}
Tenacity	Tough
Mohs scale hardness	2 - 2.5
Luster	Waxy, earthy
Diaphaneity	Translucent
Specific gravity	1 - 2.6
Optical properties	Biaxial (-)
Refractive index	nα = 1.522 - 1.528 nβ = 1.530 - 1.546 nγ = 1.533 - 1.548
Birefringence	δ = 0.011 - 0.020
Pleochroism	X= pale yellow Y=Z= pale yellow-green
References

Palygorskite or attapulgite is a magnesium aluminium phyllosilicate with formula (Mg,Al)₂Si₄O₁₀(OH)·4(H₂O) that occurs in a type of clay soil common to the Southeastern United States. It is one of the types of fuller's earth. Some smaller deposits of this mineral can be found in Mexico, where its use is tied to the manufacture of Maya blue in pre-Columbian times.

Name

Palygorskite was first described in 1862 for a deposit at Palygorskaya on the Popovka River, Middle Urals, Permskaya Oblast, Russia.

The synonym attapulgite is derived from the U.S. town of Attapulgus, in the extreme southwest corner of Georgia, where the mineral is abundant and surface-mined.

Mining and usage

Mineral deposit in Georgia

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.

Properties of the Georgia deposit

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 bentonite, forms 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.

Attapulgite clays found in the Meigs-Quincy district are bundles of palygorskite clay particles between 2 and 3 μm long and below 3 nm in diameter. The bundles are surrounded by a matrix of smectite clays that are slightly swellable. Dry-process grades contain up to 25% nonattapulgite material in the form of carbonates and other mineral inclusions. Processing of the clays consist of drying and grinding the crude clay to specific particle size distributions with specific ranges of gel viscosity measured by a variety of means depending on the end use.

Gel-grade, dry-processed attapulgites are used in a very wide range of applications for suspension, reinforcement, and binding properties. Paints, sealants, adhesives, tape-joint compound, catalysts, suspension fertilizers, wild-fire suppressants, foundry coatings, animal feed suspensions, and molecular sieve binders are just a few uses of dry-process attapulgite.

Seven to 10% attapulgite clay mixed with the eutectic salt, sodium sulfate decahydrate (Glaubers salt), keeps anhydrous crystals suspended in the solution, where they hydrate during phase transition and hence contribute to the heat absorbed and released when Glaubers salt is used for heat storage.

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 bacteria or germ that may be causing the diarrhea. 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. 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. Kaopectate's U.S. formula was changed to bismuth subsalicylate (pink bismuth). The next year (2004), an additional change in labelling was made; from then on, Kaopectate was no longer recommended for children under 12 years old. 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.

Prehistoric use

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.

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. Human sacrificial victims in postclassic Mesoamerica were frequently daubed with this blue pigmentation.

External links

• International Chemical Safety Card 1321
• attapulgite clay patent