Asbestos - The Raw Material
Excursion into Mineralogy
Asbestos minerals are so-called silicates, and these are the main constituents of all crystalline rocks.
Let us take a look at the chemical composition of our earth's crust: It consists of
- 46% oxygen and approx.
- 28% silicon.
In addition there are aluminium, iron, magnesium and calcium. Everything else are "traces". It is obvious that the combination of silicon and oxygen is quite popular in nature - as there is enough of both.
If you group 4 oxygen atoms around a silicon atom, you get a tetrahedron (a triangular pyramid). The chemical formula would be SiO4 - which is not possible because of the annoying "stoichiometry".
Oxygen has 2 so-called valence electrons, which don't want to stay alone. The oxygen ion therefore carries 2 negative charges (O2-), Si mostly 4 positive charges (Si4+), which then lacks in the valence band 4 electrons to be happy. Since molecules hold together due to different charges of the atoms (more precisely: ions), nature strives for charge neutrality. However, SiO4 is not neutral but has 4 negative charges left - the spelling (SiO4)4- would be correct. What now?
Another important part is the octahedron: in the centre there is usually aluminium or magnesium. This time 6 oxygens or OH groups (hydroxyl ions) are grouped around the center. The octahedron has 8 triangular surfaces and looks like 2 square pyramids that have grown together at their base. Suppose Mg2+ sits in the middle, around it we have 4 times O2- and 2 times (OH)-. This doesn't work at all! 2 times plus compared to 10 times minus. 8 negative charges have to be balanced. This doesn't work with octahedra or "only" 6 corners!
A single tetrahedron would not be charge neutral and rather unstable. There are silicates which consist of single tetrahedra like olivine (important for asbestos -> see below). Olivine "neutralizes" the charges by other metal ions around it. However, most silicates cross-link in such a way that tetrahedra share corner oxygens. Almost all combinations are possible, as long as the total oxygen content is reduced or the silicon content is increased. Nature is quite creative in this respect. In addition to solitary tetrahedra, group silicates, chain silicates, ribbon silicates, layer silicates and finally scaffold silicates always crosslink to form a regular lattice.
In the 3D lattice the sum formula* SiO2 or the better known name for this tectosilicate is quartz.
*the chemical formula for a smallest unit or "unit cell"
So: The most important component of silicate minerals and rocks is the SiO4 tetrahedron.
The combination of octahedrons and tetrahedra is very popular with the so-called phyllosilicates. They form layers which lay on each other like a stack of paper. Either a tetrahedron layer and an octahedron layer form a unit (called: T-O) or 2 T-layers build a sandwich (T-O-T) with an O-layer in the middle.
Let's see if it works now with the charge: The example in the picture is the layered silicate talc - or talcum (baby powder!).
The formula of the unit cell is Mg3 Si4O10 (OH)2. We have 3 times 2 plus (i.e. positive charges) and 4 times 4 plus, making a total of 22 times plus. Then we have 10 times 2 minus (negative charges) and 2 times 1 minus, also 22, but minus. Charge balance - fits!
Famous phyllosilicates are the two minerals of the mica group Biotite and Muscovite. The latter means "Moscow glass" in Russian, because it was used as window glass earlier because of its large, thin and crystal clear crystals. In the SEM picture (right) you can see the leafy structure of the muscovite.
In your cosmetics and on especially smooth paper for inkjet printers you will find the clay mineral Kaolinite - English: China Clay: The famous raw material for porcelain.
Talcum powder or in its solid form: soapstone, is not only popular with sculptors, but also as a technical application in baby powder, etc..
Why all the history?
To understand what asbestos minerals are and why they can or do what they do, you have to look at the blueprint:
Let's stick with phyllosilicates for now. Here there are very important groups in connection with asbestos 2:
- the pyrophyllite-talk group and
- the Kaolinite Serpentine Group.
The pyrophyllite talc group are so-called T-O-T layer silicates, i.e. 2 tetrahedron layers each at the top and bottom and an octahedron layer in the middle. In pyrophyllite the octahedrons contain aluminium and in talc it is magnesium. The tetrahedron layer is always the same: SiO4.
Let's assume that we take one tetrahedron layer from each of the two minerals! Then we build 2 T-O layer silicates. The mineral with aluminium - phyrophyllite - becomes kaolinite. Same chemical stock, but different ratio. The magnesium-mineral talc becomes serpentine.
| Pyrophyllite (T-O-T) | Kaolinite (T-O) |
|---|---|
| Al2 [(OH)2 Si4O10] | Al2 [(OH)4 Si2O5] |
| Talc (T-O-T) | Serpentine (T-O) |
|---|---|
| Mg3 [(OH)2 Si4O10] | Mg3 [(OH)4 Si2O5] |
Alone: Nobody takes a tetrahedron layer away from a talc - just like that. This is not how serpentine is produced. The picture is only intended to explain the really simple difference in structure and the similarity of the chemical composition.
Picture: The structures of the pyrohyllite talc group and the kaolinite serpentine group are shown. Why the serpentine structure is crooked is explained in a moment.
Have you ever been to Bad Harzburg? Go there - it's beautiful, not only because of the mineralogy! Take a look at the typical Harzburgite (picture left): A pitch-black rock - mantle peridotite - with black Klinopyroxen (that's what it's called...) and thin light green veins - serpentine, which was formed by the transformation of olivine and clinopyroxene.
Serpentine is produced by the alteration of pyroxenes (e.g. enstatite Mg2Si2O6) or olivine or forsterite (Mg2SiO4) under the absorption of water. These are typical minerals of the earth's mantle and therefore love very high temperatures of around 1800°C (melting temperature of forsterite approx. 1900°C). If temperature and pressure drop substantially, these minerals do not feel well anymore. For them it is simply too cold, too humid and ... well ... too little pressure. If they then come into contact with water, they can no longer resist and first transform into serpentine antigorite and finally below 260°C into chrysotile.
Everything "only" chemistry!
| Forsterite + water -> Serpentine + Brucite (Magnesiumhydroxide) |
|---|
| 2 Mg2 [SiO4] + 3 H2O -> Mg3 [(OH)4 Si2 O5] + Mg(OH)2 |
The special thing about serpentine is that unfortunately the T-layer and the O-layer do not fit exactly together. The T-layer is too small. The whole package is wavy or rolls up in the worst case.
Here we go: our asbestos. The mineral is called chrysotile. The "white asbestos" or "fibre serpentine".
To see in the picture: Chrysotile fibers in cross section. Each of the "tree rings" consists of a T and an O layer. Together they are exactly 7 angstroms thick, i.e. 0.7 nanometers (7 x 10-10 m) or 0.7 billionths of a meter. The diameter of a single fiber is not larger than 40 nm, i.e. 40 billionths of a meter. Pretty thin!
The stupid thing is: The chrysotile variant of serpentine is the one that is stable on our planet's surface and does not weather or decay further. The other variant Antigorite (picture below) is rather stable at high temperatures. It can also be found in nature, but not so often.
Do you get the picture?
With antigorite, the planes are also wavy, but change regularly the upper and lower surface, so that the layers won't roll up.
Have you been following the news lately?
Take another look at the difference between talc and serpentine. Exactly: in asbestos only one T-layer is missing, everything else, especially the chemical composition (modal) is the same. Only a little Si and O less.
But it is not that simple! Talc loves high pressure and high temperatures to form, serpentine rather the opposite. Remember: The Mg O-layer and the SiO4 T-layer do not fit together exactly. Talc simply needs more energy to compensate this distortion with an additional T-layer and to hold the O-layer in the middle. The serpentine does not have spend the extra enegy.
Nevertheless: the chemistry is right, you could say! It can therefore happen again and again that a few asbestos fibres mix with the talc. Exceptions confirm the rule, especially in nature.
The formerly highly acclaimed properties of Chrysotil are obvious: Nearly infinite (by human standards) chemical resistance, high fire resistance, long, wafer-thin fibres that are easy to weave and process into textiles and the high cost availability have made this asbestos form so popular.
"Asbestosos" is ancient Greek for "imperishable". In New Greek, however, asbestos simply means "limestone", which is geologically completely wrong. This does not mean, however, that New Greek is wrong... Only if a Greek points to a rock during your next Greek holiday and says "asbestos", don't run away right away!
As a building material in fire protection at least one thing is certain: If a building in which asbestos was used burns down - the asbestos remains and looks like new!
However, it has only recently become clear or accepted that these very properties are also a curse:
Due to their size (a thousand times thinner than a human hair), the fibres penetrate into the smallest vesicles. They can penetrate into the tissue. They remain stable and are neither dissolved nor transported out of the body. They are even so fine that they can penetrate into the cell nuclei and manipulate the DNA there - cancer can develop.
However, this does not only work with chrysotile, but also with the other typical asbestos minerals: the amphiboles crocidolite ("blue asbestos"), Grunerite ("brown asbestos") and some other minerals from the amphibol family (tremolite, actinolite, anthophyllite).
Although these minerals do not roll up like chrysotile, they form the SiO4 band structures described above and grow needle-like. They are not fine hairs but ultrafine, stubborn needles. They are not as flexible as chrysotile fibers, but they are stiffer and break more easily.
They have one thing in common: they are fibers that are so fine that the finest fibers consist of only a few atomic layers. The rings of the chrysotile snails are easy to count - and each layer is only a few millionths of a millimetre thick.