Like probably most of you, I guess I never would have heard of palygorskite before, had I not been dealing with a long manuscript about the structure and properties of just this mineral earlier this week. There can be funny coincidences...
Palygorskite, or attapulgite, as it has also been called, is a clay mineral, belonging to a large group of clays known as Fuller's Earth. Technically speaking, it's a magnesium aluminium phyllosilicate with the formula (Mg,Al)2Si4O10(OH)·4(H2O).
This formula looks very complicated, but actually, silicate minerals are quite fascinating stuff. Essentially they are built up of small tetrahedra of silicon atoms surrounded by four oxygen atoms, and octahedra of metal atoms (magnesium and aluminium, in this case) in the centre of six oxygen atoms. Like building bricks from a kid's construction kit, these tetrahedra and octahedra can be combined in a huge number of regular patters, giving rise to the enormous botany of different silicate minerals. In phyllosilicates, the silicon tetrahedra and metal octahedra form planar sheets (hence the name), which then build up layered structures.
Palygorskite has such a layered structure, but actually, it is very uncongested. It does not consist of continuous sheets, but of long bars of layers of metal octahedra sandwiched between silicate tetrahedra. These bars, then, are arranged parallel to each other in a kind of checkerboard pattern, with large, channel-like empty spaces in between.
In this cookie model, the chocolate filling represents the metal octahedra, which are sandwiched between the biscuit, standing for the silicate tetrahedra. You see a cross section through the parallel arrangement of long bars. A more technical illustration is shown in figure below, which has been adapted from the paper On the unusual stability of Maya blue paint: Molecular dynamics simulations, by Ettore Fois, Aldo Gamba, and Antonio Tilocca, Microporous and Mesoporous Materials 57 (2003) 263-272:
The cross section through the basic building bar is marked by the grey rectangle. This basic building block is repeated in a checkerboard pattern in the plane of the figure, and just continues along the axis normal to the figure plane. You can see the silicate tetrahedra, with oxygen atoms at the vertices, and the metal atoms - the large dark-grey spheres surrounded by six oxygen atoms each.
The empty, tunnel-like spaces between the bars are not completely empty, however. Usually they are filled with water molecules. A few of these molecules can be spotted in the illustration above. But if other molecules are around, they can enter the tunnels, and physically bind very efficiently to the silicate framework. For this reason, palygorskite has often been used in medicine: The mineral is not absorbed by the body, but binds acids and toxic substances in the stomach and digestive tract. Thus, it acts as an antidiarrheal medication, for example.
The same thing happens when palygorskite is brought together with Indigo, a dye that can be be extracted, for example, from a plant named Añil, which is native to tropical America.
Indigo molecules fill the channels of the palygorskite structure in a random pattern, as shown in the illustration, which has also been taken from the paper of Ettore Fois, Aldo Gamba, and Antonio Tilocca. The silicate hull shelters the dye molecules, and thus creates the enormous persistence of the pigment and its robustness against harsh climatic conditions, alkali and acid treatment and organic solvents.
How to create this indigo-clay pigment is what the ancient Maya had discovered - that's Maya Blue.
- Mineralogical details about palygorskite can be found at the databases mindat.org
and webmineral.com. Webmineral.com has also a very nice JavaApplet to explore interactively the unit cell of palygorskite.
- The peculiar crystal structure of the palygorskite mineral was described for the first time in The structural scheme of attapulgite by W. F. Bradley, American Mineralogist 25 (1940) 405. Bradley used the classical x-ray diffraction technique, and you can have a look at the PDF file of the paper.
- Since the 1940s, mineralogists have applied the whole arsenal of methods provided by modern condensed matter physics to shed light on the structure of palygorskite and its role in the properties of Maya Blue. A very recent review is Pre-columbian nanotechnology: reconciling the mysteries of the maya blue pigment by G. Chiari, R. Giustetto, J. Druzik, E. Doehne and G. Ricchiardi, Applied Physics A: Materials Science & Processing 90 (2008) 3-7 (subscription required).
- For more about the archaeological aspects of Maya Blue, see the entry The Palygorskite and Indigo Mix of Maya Blue at about.com:archaeology as a starting point.