Chrysotile


Chrysotile is a fibrous mineral that belongs to the serpentine mineral group. Other members of the group are lizardite and antigorite. All serpentine group minerals share the same chemical composition (Mg3Si2O5(OH)4) but they have different crystal structures. Chrysotile is the only one among them with a fibrous habit.

Chrysotile occurs as cross-cutting veins in serpentinized rocks. Serpentinization is a hydrothermal (temperature below 350 °C1) metamorphic process that affects magnesium-rich igneous rocks like peridotite and pyroxenite. These are rocks that contain lots of olivine and pyroxene. These minerals are altered to serpentine group minerals plus magnetite. Magnetite forms because ultramafic igneous rocks contain lots of iron but serpentine group minerals contain no iron at all. So the iron just has to form its own phase. This is the reason why serpentine minerals often seem to be weakly magnetic. They are not but they usually contain lots of small magnetite crystals.

Chrysotile (serpentinite as a rock sample) from the Sayan Mountains in Siberia. Chrysotile is visibly composed of silky fibers although the fibers are actually a lot smaller in width. These fibers that we see here are aggregates of many parallel chrysotile tubes. Width of sample is 8 cm.

Most serpentinites form in the oceanic crust which is heated from below and percolated by ocean water. Obduction of such rocks accounts for serpentinites squeezed between continental blocks. Such former pieces of oceanic lithosphere which are now part of the continental lithosphere are known as ophiolite complexes.

Chrysotile aggregate is composed of many parallel long and very thin rolled tubes of chrysotile sheets. Each tube is about 20 nanometers thick. It is far too thin to be seen with the unaided eye. The fibers that we see are actually aggregates of many parallel rolled tubes. The length of the tubes is variable but may be up to several centimeters. Chrysotile forms rolled tubes because their crystal structure is composed of two layers, one of them being slightly smaller. This difference is compensated by rolling the sheet so that the shorter sheet remains inside.

Chrysotile asbestos and associated health hazards

Chrysotile is the most commonly used asbestos mineral although its golden days are clearly over because of health concerns. Asbestos minerals do not form a single mineralogical whole. Most of them are amphiboles. Chrysotile is the only non-amphibole asbestiform mineral. This is why it perhaps deserves to be treated separately from other asbestos minerals. Grouping them all together is a dubious practice, at best, that simply demonstrates a lack of scientific literacy.

The health risks associated with asbestos are very real but one should be able to make a clear difference between serpentine and amphibole groups which are most likely not equally hazardous. Another aspect that seems to be poorly understood is that pulmonary disease may develop as a result of intensive and long-term exposure to asbestos fibers in the air. Chrysotile or white asbestos as it is frequently called is likely not as hazardous substance as it is usually imagined to be.

Veins of chrysotile (white asbestos) cutting through serpentinized igneous rock. Width of the sample from Canada is 4 cm.

The reason why people are so afraid of it is in my opinion their ignorance. They simply don’t know what it is — have never seen, have no personal experience. Perhaps they even don’t know that this is a natural material. The situation is somewhat similar to nuclear energy which is also very dangerous stuff if handled the wrong way. However, in my opinion it is really the only serious alternative to fossil fuels at the moment. Unfortunately, the scientifically illiterate public just do not understand what are atoms and how on earth they can be used to make electricity. They can not see nor sense radiation and are therefore frightened and don’t want to know anything more about it. I guess most people have no idea that we are all the time exposed to radiation. The same seems to be true with particulate mineral matter. Our bodies are under constant biological and mineralogical attack but we are not that easily knocked out. Our immune system and repair mechanisms are effective and can cope with most of the dangers. It is extremely unlikely that we have not developed an adequate defense against low-level exposure to mineral dust, fibrous or not.

I already mentioned fossil fuels and nuclear energy. There is one more thing that I would like to say about it. The current situation in the world proves that there really are no serious alternatives to fossil fuels. We have postponed or cancelled lots of nuclear energy projects in many countries after the Fukushima accident and as a result our appetite for shale gas, tar sand, oil shale, and coal are going straight up. I don’t want to think what it means to our climate. I have actually largely given up worrying about that because firstly, I can not do anything about it and secondly, I will be dead before the situation gets really bad. It was ironic and desperate remark. I do actually feel responsibility but to my grandchildren who might ask why did you (I mean the mankind living today) do it I simply say that I am not powerful enough to alter the course we as a human race have chosen to follow. I wrote my blog to educate people but only a handful of them read that. And those who read already knew it all. People that should read and learn are unfortunately not doing it. So my impact is almost nil anyway and I guess that blogging won’t count as an excuse, unfortunately.

Properties and uses of chrysotile

Chrysotile has been extensively used in the past because it has many useful properties. It is a good thermal and electrical insulator. It also absorbs sound and is chemically inert. Chrysotile is fire-resistant and absorbs mechanical energy. Its fibers are flexible with enough tensile strength to be woven. It is perhaps needless to say that it was once considered to be an almost ideal material for hundreds of versatile industrial applications.

Lots of chrysotile veins in serpentinite from Tuva near Mongolia. Width of sample is 9 cm.

This is one of asbestos minerals but not serpentine. It is an amphibole group mineral crocidolite which is also known as blue asbestos. The connecting property shared by all asbestos minerals is fibrous habit which is also nicely demonstrated by this sample from South Africa. Width of sample is 3 cm.

References

1. Baronnet, Alain J. (2007). Chrysotile rock. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 4. 151-153.


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