9. Amphibole asbestos varieties
All of the asbestos minerals other than chrysotile are members of the amphibole family of silicate minerals. The amphibole minerals all have essentially the same crystal structure, that of a double chain of linked SiO4 tetrahedra with cations and anions between the chains. The general formula of the amphibole group is sometimes given as AX2Y5Z8022(OH)2
We will now outline the rules for amphibole composition. They are quite complicated and you need to understand a little chemistry to make sense of the information.
- The A site may be occupied with a large monovalent ion such as Na or K or it may be empty.
- The X site is generally occupied by two divalent ions such as Ca, Mg or Fe but it may also be occupied by a monovalent ion such as Na. In this latter case the electric charge balance of the mineral will normally be maintained by further substitution of a trivalent ion for a divalent ion elsewhere in the structure.
- The Y group may be occupied by five divalent ions such as Mg, Fe or Mn, trivalent ions such as Fe3+ or Al, or quadrivalent ions such as Ti4+ with charge balance being maintained as before by other substitutions. The maximum level of trivalent ion substitution within the Y group is 2.
- The Z group contains eight Si+ ions with a maximum limit of 2 substitutions of Al3+ being permitted, with parallel substitutions elsewhere in the A group or Y group.
For example:
A |
X |
Y |
Z |
|
Na2 |
Mg3 Al2 |
Si8 |
is an amphibole called glaucophane |
|
Na |
Ca2 |
Mg4Al |
Si6Al2 |
is called pargasite |
Fe22+ |
Fe52+ |
Si8 |
is called grunerite |
As we warned, this all sounds complicated and there is an enormous range of possible amphibole compositions. Fortunately only a few specific types have been found as asbestiform minerals.
These are amosite (proper name grunerite), crocidolite, anthophyllite, tremolite and actinolite.
The most common asbestos amphibole is amosite, with the formula Fe7Si8O22(OH)2. Amosite is the commercial name derived from the acronym of Asbestos Mines of South Africa, the producers of the mineral. The mineral should more properly be called grunerite. It is also known casually as brown asbestos. It occurs in veins in metamorphoric iron-rich rocks only in South Africa. It is coarser and stronger than chrysotile and forms more needle-like fibres when processed. In the raw state it is dark brown or black but when processed it is grey-brown, or white if heavily milled.
Crocidolite is the correct name for the amphibole asbestos commonly known as blue asbestos. Its chemical composition is Na2Fe32+Fe23+Si8O22(OH)2 and the presence of sodium instead of iron in the X group and the smaller proportion of iron distinguish it chemically from amosite. The primary source of crocidolite was South Africa but it was also produced in Wittenoom, Australia, or in a slightly different variety in Bolivia. It is blue-black in the raw state but when processed it forms fine fluffy fibres with a distinctive smoky blue colour. When found in its non-asbestiform form it is correctly called riebeckite.
Figure 9 Crocidolite
Anthophyllite is a coarse white asbestos variety that was produced in Finland until the 1960s. Its chemical formula is Mg7Si8O22(OH)2 - it was not in widespread use and can only be found very occasionally in commercial products or laggings.
Tremolite and Actinolite are the magnesium and iron analogues of calcic amphiboles. Their formulae are Ca2Mg5Si8O22(OH)2 for tremolite and Ca2Fe5Si8O22(OH)2 for actinolite. Both are quite rare as commercial asbestos minerals although quite common as normal crystalline minerals. Actinolite is green in colour. It is found in association with some of the other South African amphibole asbestos (where it is known as prieskaite) and it may be present as a contaminant in products manufactured with asbestos from that area.
Figure 10 Actinolite
Tremolite asbestos was also produced in moderate quantities in Taiwan and in Korea, Pakistan, India and Italy. It consists of white silky fibres similar at first sight to chrysotile, although on closer examination it has a needle-like form. Tremolite has not had a widespread use but it can be found in commercial products.
Tremolite and actinolite may occur as a trace contaminant in other mineral products like talc or vermiculite, and also in chrysotile asbestos.
While chrysotile can only occur in one crystal form (fine fibrils or bundles of fibrils), all of the amphibole minerals may occur in a variety of different forms ranging from coarse crystals of prismatic or columnar shape to needle-like crystals and alternatively in some cases to fine fibrous aggregates as in conventional asbestos. So an amphibole such as tremolite from one geological source may look like asbestos while from other sources it is clearly not asbestos nor would it break down to produce long thin fibres like asbestos. This distinction is fairly easy to make when examining bulk samples of materials of the extreme types (fibres or coarse crystals) but it is much less obvious when dealing with intermediate forms (such as needle-like crystals) or when finely powdered materials require identification. In fact, other than morphology (size and shape) there are no easily measurable properties that distinguish between powders of asbestos and non-asbestos varieties of the same mineral.
There is currently some concern in the USA about non-asbestos elongated mineral particles (EMP's) that have a needle-like (acicular) or prismatic crystal structure, and whether these materials may also present a health risk. The available epidemiological evidence shows no indication that these materials cause mesothelioma, although the evidence for the absence of a lung cancer risk is less clear. In the past IOM scientists carried out a number of experiments to try to understand whether there was a risk associated with the non-asbestiform forms of tremolite6. The figures show an asbestoform and a non-asbestiform tremolite.
Figure 11 An asbestiform and a non-asbestiform tremolite
These studies showed that the three "asbestiform" tremolites produced mesotheliomas in almost all animals. A brittle type of fibrous tremolite, which produced a sample with relatively few asbestiform fibers produced tumors in 70% the experimental animals. Two samples of nonfibrous tremolite produced dust samples containing numerous EMP's. Both these samples produced relatively few tumors, although one had more long "fibres'' than did the brittle tremolite that produced 70% of tumors. Clearly, this study provided some reassurance that non-asbestos EMP's do not create a serious risk to health.
The US National Institute for Occupational Safety and Health (NIOSH) has published a "Roadmap" for research that it considers necessary to deal with the potential risks from EMPs 7.
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