Using Crystallography to Test Materials for Asbestos

Asbestos

Asbestos is a naturally occurring group of silicate minerals known for their fibrous form and unique physical and chemical properties. Asbestos fibers are heat resistant, chemically inert (with some exceptions being reactive with acids), non-volatile, and insoluble in water and organic solvents. The fibers have a tensile strength greater than steel which can be spun and woven into thermally and electrically insulating materials. As a result, asbestos has been used in numerous applications since ancient times and has been used in over 5,000 modern day products including shingles, ceiling tiles, floor tiles, paper products, cement, textiles, and automobile parts. ^[1]

Figure \(\PageIndex{1}\): Asbestos Fibers

Six specific minerals are classified as asbestos: members of the Amphibole group which include amosite, crocidolite, tremolite, actinolite, and anthophylite, and from the Serpentine group, chrysotile. Chrysotile, also known as white asbestos, makes up 95% of asbestos found in the United States ^[2]. Its chemical formula is Mg₃(Si₂O₅)(OH)₄, although occasionally magnesium ions can be replaced with iron or other metals.

The Environmental Protection Agency identifies asbestos as an environmental hazard and carcinogen. When materials which contain asbestos are damaged or deteriorate, the microscopic asbestos fibers may become airborne and can be inhaled into the lungs. The fibers become embedded into lung tissue and over time can lead to serious health problems including lung cancer, mesothelioma, and asbestosis. Care must be taken in the removal and disposal of asbestos containing materials to maintain air quality and prevent release of airborne fibers. ^[3]

Crystal Structures and Unit Cells

One method of identifying asbestos fibers in materials is by analyzing the crystal structure of a material. Techniques such as transmission electron microscopy (TEM) and X-ray diffraction allow for identification of asbestos by producing a unique pattern of refraction that results from the crystal lattice of the mineral.^[4]

A material’s crystal structure consists of a repeating pattern of atoms arranged into a lattice that exhibits order and symmetry. The lattice is composed of an array of points which periodically repeat within a three-dimensional framework. These lattice points are the points in space where particles are free to vibrate within a crystal. ^[5]

The smallest repeating units into which a crystal lattice can be divided is known as a unit cell. Figure \(\PageIndex{2}\) below shows the unit cells of some common salts

Figure \(\PageIndex{2}\): Unit cells of some common salts

A unit cell must meet the following criteria:

• The unit cell is the simplest repeating unit in the crystal.
• Opposite faces of a unit cell are parallel.
• The edge of the unit cell connects equivalent points

There are a total of 14 possible unit cells into which crystals can be divided, known as Bravais unit cells, shown below in Figure \(\PageIndex{3}\).

Figure \(\PageIndex{3}\): The 14 possible Bravais unit cells

Determining the unit cell for a substance can be difficult without experience. For more complex materials containing numerous atoms or polyatomic ions, it can also be difficult to determine the number of each atom within a unit cell. In general though, the formula for determining the number of an atom or ion within a unit cell is

\[N=N_{\text{body}}\text{ + }\dfrac{N_{\text{face}}}{\text{2}}\text{ + }\dfrac{N_{\text{edge}}}{\text{4}}\text{ + }\dfrac{N_{\text{corner}}}{\text{8}}\]

The formula accounts for the sharing of atoms at the interfaces between unit cells. Atoms along a face are being shared among two unit cells, atoms along an edge are shared by four unit cells, and atoms which fall on a corner are shared by eight unit cells. N_body represents the number of an atom found within the body of the unit cell.

Crystal Structure of Asbestos

In the case of asbestos, the crystal structure is primarily influenced by the fact that it belongs to a family of minerals known as silicates. Silicates contain a tetrahedron structure of silica, SiO₄. The central silicon atom surrounded by four oxygen atoms creates a pyramid like structure. The silica tetrahedrons can arrange into either a double layer, as seen in the amphibole class of asbestos, or into an extensive sheet as in the serpentine class (chrysotile), as shown in Figure \(\PageIndex{4}\).

Figure \(\PageIndex{4}\): Crystal structures of the two classes of asbestos

The tetrahedral silica layer is only one part of the structure of asbestos. The remainder of the crystal structure is a second octahedral layer, which consists of a metal cation (in the case of chrysotile, magnesium) surrounded by six oxygen atoms. A unit cell of chrysotile contains one tetrahedral layer linked to one octahedral layer. The resulting unit cell may be either monoclinic or orthothrombic.
Interestingly, the layering of tetrahedral and octahedral layers is what allows asbestos to form fibers. The oxygen spacing within each layer is different, and to accommodate for this difference the sheets curve, forming the asbestos fibers ^[6]

Figure \(\PageIndex{5}\): Unit Cell of Chrysotile. Oxygen atoms shown in red, silicon atoms in blue, and magnesium cations in green. Note the octahedral units layers over the tetrahedral units

Figure \(\PageIndex{6}\) below demonstrates the layered crystal lattice of the amphibole class of asbestos. The silica tetrahedreals are shown in dark blue and the cation octahedrals are shown in shades of light blue and green. Yellow and gray balls indicate other ions and hydroxide groups that may be present within the crystal.

Figure \(\PageIndex{6}\): Crystal structure of amphibole, showing the layering of silica tetrahedrals and cation octahedrals

This crystal lattice structure will produce a unique X ray diffraction pattern that allows for the identification of asbestos fibers within a sample. Proper identification of asbestos containing products provides for safer disposal and clean-up of potentially harmful materials.

From ChemPRIME: 10.2: Lattices and Unit Cells

References

1. Barbalace, R. C. “Asbestos, its Chemical and Physical Properties.” Oct. 2004. http://environmentalchemistry.com/yogi/environmental/asbestosproperties2004.html

2. “Chrysotile”. en.Wikipedia.org/wiki/Chrysotile#cite_note-5

3. “Asbestos: Basic Information”. Environmental Protection Agency. http://www.epa.gov/asbestos/pubs/help.html

4. “Analytical Methods.” ALS Laboratory Group. http://www.asbestos-laboratory.com/analytical_methods.asp

5. “Unit Cells: The Simplest Repeating Unit in a Crystal.” Bodner Research Group.
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch13/unitcell.php
6. Asbestos: Selected Cancers. Institute of Medicine (US) Committee on Asbestos: Selected Health Effects. National Academies Press. 2006. http://www.ncbi.nlm.nih.gov/books/NBK20335/

Contributors and Attributions

• Ed Vitz (Kutztown University), John W. Moore (UW-Madison), Justin Shorb (Hope College), Xavier Prat-Resina (University of Minnesota Rochester), Tim Wendorff, and Adam Hahn.