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SERPENTINITE AND ASBESTOS
July 2010 suddenly saw a lot of discussion about a rock type called serpentinite.
Geologists in California
were incensed that the state wanted to do away with its official state rock
(see article in the New York Times).
Geologists and notably petrologists elsewhere were delighted that a rock was
making the limelight. Serpentinite is a rock type that can contain one of the six
naturally occurring minerals used as source material for asbestos construction materials,
now a known health hazard in certain conditions.
The rock
The rock is the result of a process called serpentinization,
which replaces ferromagnesian minerals such as olivine by
serpentine. Roughly speaking, it's magmatic ocean floor rock which has undergone low-temperature and
low-pressure metamorphism, typical of ocean floor conditions. (See
this NOAA page for some more information.)
Gem stone quality serpentinite can also be known as jade.
Its minerals
Serpentines - general formula (Mg,Fe)3Si2O5(OH)4 - is the group name for
the following minerals:
antigorite;
chrysotile;
lizardite.
Serpentine means "snake-like", which may reflect the appearance of the rock or refer to the
qualities it was rumored to have as protection against snake bites. The following lines are from a poem attributed to
Orpheus and said to have been written in the 4th century A.D.
(see this page at Central Michigan University):
No more the trailing serpent's tooth to fear.
Let him who by the dragon's fang hath bled,
On the dire wound Serpentine powdered spread,
And in the stone his sure reliance place,
For wounds inflicted by the reptile race.
Chrystotile, or white asbestos
Chrysotile is the mineral known as white asbestos, and the most widely used asbestos mineral.
Serpentine crystals are leaf-like (phyllosilicates), just like mica minerals. The crystal
structure is trioctahedral, a combination of tetrahedral and octahedral sheets, which creates a mismatch.
In chrysotile, the sheets are rolled into long hollow fibers to accommodate for this mismatch
in the crystal lattice.
The other asbestos minerals
There are five other asbestos minerals:
amosite (green, brown);
crocidolite (blue);
tremolite;
actinolite;
anthophyllite.
Amosite and crocidolite are most used, after chrysotile. Crocidolite is mainly mined in South Africa.
Other asbestos minerals are part of the amphibole mineral group. Their general formula for
amfiboles is A2B2(Si,Al)8O22(OH)2.
A is usually Mg, Fe, Ca and Na. B stands for Mg, divalent Fe, Al, and trivalent Fe.
Amphiboles are inosilicates, which means that the oxygen-silicon tetrahedrons in it
are linked in long chains, leading to elegant needle-like crystals.
The health hazards
Asbestos was once used widely for its properties of high resistance
against temperature and acids, high strength, sound insulation and wear resistance.
It is now known that prolonged exposure to a high number of asbestos fibers in air is a
health hazard. It can lead to various forms of lung disease, notably in combination with smoking.
(See this page at the US EPA, and
this page at the UK HSE.)
Blue and brown asbestos pose the greatest health risk, greater than that of chrysotile,
the asbestos mineral occurring in serpentinite.
The health hazards of asbestos are very different than those of substances like cyanide, carbon monoxide,
osmium tetroxide or concentrated nitric acid and its fumes.
In 2001, Dutch scientist Mirna Hensen concluded an award-winning study into
the safe limits and the impact of environmental conditions on the hazard of asbestos fibers in soil.
In her experiments, notably the presence of even a small percentage of moisture in soil lowered the
emission and hence the health risk of asbestos fibers in soil considerably. Wind speed also played a role.
Disclaimer: Angelina Souren and SmarterScience cannot be held liable for any decisions made on the basis of information
given on this web page.
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