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Asbestos Minerals
Chemical composition
The most commonly encountered types of asbestos and their formulae are shown in the table below.
| Mineral |
Formula |
| Actinolite | Ca2(Mg,Fe)5(Si8O22)(OH)2 |
| Amosite | (Fe,Mg)7(Si8O22)(OH)2 |
| Anthophyllite | (Mg,Fe)7(Si8O22)(OH)2 |
| Chrysotile | Mg3(Si2O5)(OH)4 |
| Crocidolite | Na2Fe"3Fe"‘2Si8O22(OH)2 |
| Tremolite | Ca2Mg5(Si8O22)(OH)2 |
n.b. Fluoride (F-) may substitute in part for hydroxyl (OH-)
Crystallography
The term asbestos covers peculiarly elongated crystalline forms of silicate minerals belonging to several different families. Actinolite, tremolite, and crocidolite (fibrous riebeckite) are monoclinic amphiboles; amosite and anthophyllite are orthorhombic amphiboles; and chrysotile is fibrous crystalline serpentine.
Physical properties
All of the asbestos minerals are characterised by their fibrous nature to various degrees. These fibres are strong and flexible and resistant to heat. Some of these minerals, particularly amosite, crocidolite, and chrysotile, can be spun and woven into textiles, and to some extent value depends upon length of fibre.
| Type |
Colour |
Hardness |
Specific Gravity |
Refractive Indices
(approx.) |
| alpha |
beta |
gamma |
| Actinolite |
Green to greyish-green |
5.0 - 6.0 |
3.0 - 3.2 |
1.610 |
1.630 |
1.640 |
| Anthophyllite |
Brownish grey to green |
5.5 - 6.0 |
2.9 - 3.2 |
1.596 |
1.605 |
1.615 |
| Chrysotile |
Greenish |
2.5 |
2.5 |
1.540 |
- |
1.550 |
| Crocidolite |
Blue |
5.0 |
3.2 - 3.3 |
1.698 |
1.699 |
1.706 |
| Tremolite |
White to dark grey |
5.0 - 6.0 |
2.9 - 3.2 |
1.600 |
1.610 |
1.620 |
Asbestos occurs as ‘cross-fibre’ (fibres more or less normal to vein walls) and as ‘slip-fibre’ (fibres more or less parallel to the vein walls). ‘Mass fibre’ is an aggregate of unoriented fibres.
Occurence and uses
Asbestos minerals are thought to have been formed throughout the agency of waters percolating through rocks comprised largely of olivine, pyroxene, or amphibole. Asbestos is used in making many fire-proof materials such as boards, sheets, tiles, packing, insulating compositions and laggings, fire-proof textiles, brake and clutch linings, filters and fillers. The main consumption was in making asbestos-cement sheets, tiles etc.
Industrial properties of asbestos
|
Chrysotile |
Crocidolite |
Amosite |
| Fibre length (usual maximum) |
35 - 50 mm |
37 - 75 mm |
175 mm |
| Tensile strength |
High |
Higher than chrysotile |
Good |
| Fineness of fibre |
Very Fine |
Fine |
Fine |
| Resistance to heat |
Good
(becomes brittle) |
Poor
(fuses to glass) |
Good
(becomes brittle) |
| Resistance to acids, alkalis and brine |
Poor |
Good |
Good |
| Electrical insulation |
Fair to good |
Good |
- |
| Heat insulation |
Good |
Good
(moderate heat) |
Good |
| Spinnability |
Excellent |
Fair |
Fair |
|
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