Document Vj39j4yRVxmznmDKNwN8x4vyp

ASBESTOS 219 In respect of the basic features of the body's reaction to asbestos fibres there is nothing to choose between them and particles of silica. The par ticular differences, which are undoubtedly observed, arise from different ' physical form; difference in solubility; difference in size, and the possible % difference in carcinogenic properties. The term asbestos is vague and industrially may refer to any of a series ;-of complex silicates. It belongs to a group of crystalline magnesium silicates, which include talc (steatite) MgjH,,(SIOj)4> Meerschaum Mg4H,(SlO,),.HtO, i possessing varying degrees of resistance to heat and to chemical agents. It is best as recommended by Spencer (1937) to preface the word asbestos with the particular mineral from which it is derived, for example serpentine ^asbestos and amphibole asbestos (hornblende). Unfortunately, more t words than one are used in this industry for the same natural product. The common forms, with country of main origin, used in the various ^.industries with which our present theme is concerned are: (1) Serpentine asbestos (Canadian) (chrysotile*; anriant). Chrysotile is the most used of the fibrous silicates. Empirical formula: H,Mg,SI,0, [(=3Mg0.H4(S.01)i). As mined tho following is the analytical composition of an averago sample j-of the naturally found mineral (serpentine: hydrous magnesium silicate): per cent SIO, 43 Al,0, FeO Fe,0, MgO H.O 0 52 10 41-36 13-79 v(2) Amphiboles Empirical formulae: ijCajMg^SiOj)* Tremolite (Italian) l,Ca#(MgFe),(SiOI1)4 Asbestos (amphibole asbestos) jCaMg,(SlO,)4 Actinolite '^N3Fe''Fe,'"(Si03)e Crocidolite (Blue asbestos--South Africa, Australia) (3) Amosite asbestos (South Africa) (ferro-anthophyllite). Empirical formula: Hj(M6Fo),(SiOa)a. Their commercial value is due to the long fine, infusible, non-conducting bres in which ihcsc silicates exist and can be worked and it is this char- which motivates the characteristic changes in the lungs when inhaled yer considerable periods. In the mining of serpentine and of the asbestos om it as a mass of fine, silky crystals In Canada, Cartier (1949) found that ifcould divide the 3,242 workers of whom 40 per cent had been in the i for 10-40 years into two groups, one exposed to the dust of serpentine, hich remained free from asbestosis and the other, exposed in grinding, .-Not to be contused with hfennwilire which || in Uemurphmu mixture of (forgienre) end PoSiO