Innovation In Structural Investigation

GBG Structural Services


Materials Testing and Structural Investigations

Tests


Materials


Structures

Asbestos Minerals

Chemical composition


The most commonly encountered types of asbestos and their formulae are shown in the table below.

Mineral Formula
ActinoliteCa2(Mg,Fe)5(Si8O22)(OH)2
Amosite(Fe,Mg)7(Si8O22)(OH)2
Anthophyllite(Mg,Fe)7(Si8O22)(OH)2
ChrysotileMg3(Si2O5)(OH)4
CrocidoliteNa2Fe"3Fe"‘2Si8O22(OH)2
TremoliteCa2Mg5(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

Click here for latest news items from our newsletter
Site map
© 2004 GBG, All Rights Reserved.