GEOLOGICAL DISTRIBUTION
Asbestos originates for the most part from rocks consisting largely of olivine, such as peridotites or dunites, or from rocks consisting largely of pyroxene. Hence it is only in altered rocks of this nature that asbestos of the common types is to be expected. Two distinct types of alteration are common: Alteration of the olivine or pyroxene to serpentine, with development of chrysotile in places; and alteration of olivine or pyroxene to amphibole with development of anthophyllite (a variety of amphibole) and related forms. Both types of alteration are well represented in the great North American belt of asbestos-bearing peridotites that extends from central Alabama along the Piedmont Plateau to the Gaspé Peninsula of Quebec, a distance of more than 1,600 miles. In the northern part of the belt, in Vermont and Quebec, the alteration has been to serpentine with chrysotile asbestos in places, while in the southern part the alteration has been largely to amphibolite with development of anthophyllite asbestos and talc.
Chrysotile asbestos (Quebec type), as represented in most deposits, is formed by serpentinization, with subsequent prismatic crystallization in cracks, the veins thus formed representing a recrystallization along the walls and thus being replacement veins rather than fissure veins. Contact metamorphism evidently plays an important part in the process, for in most regions intrusive dikes are associated with the deposits and evidently had a definite influence on the development of the commercial product. As serpentinization is a deep-seated process, chrysotile deposits may occur at considerable depth.
Anthophyllite asbestos (Georgia type) results from alteration of peridotites or pyroxenites to amphibole, giving the fibrous anthophyllite and related forms. This type of asbestos does not occur in veins but constitutes the major part of the rock mass.
While the modes of alteration noted above account for most of the asbestos deposits known, there are two important exceptions—the crocidolite, or blue asbestos, of South Africa, and the chrysotile deposits of Arizona, both of which occur in sedimentary rocks. Crocidolite is interbedded in jasper and ironstone, and the Arizona chrysotile has resulted from the alteration of cherty limestone influenced to some extent by the action of diabase intrusions.
As pointed out in the discussion of uses, for various select purposes anthophyllite cannot be employed as a substitute for chrysotile of spinning grade. As the uses of anthophyllite are thus restricted, and as the supply seems to be ample for many years to come, the problem of supply centers about the deposits of high-grade chrysotile.
Deposits of serpentinized basic rocks are by no means rare, and even the development of a fibrous structure is common, but in all chrysotile-bearing deposits only a small part of the serpentine is fibrous, and of the fibrous part only a small percentage can be utilized as high-grade asbestos. The value of deposits of most ores depends largely on the percentage of metal and of impurities present, and little attention need be given to the physical character of an ore. The value of asbestos, however, depends not only on purity of composition, but on very definite physical properties, such as length of fiber, flexibility, and tensile strength. Such properties result from a combination of favorable conditions of crystallization, conditions that are at present little understood.
A large body of serpentinized basic rock bears, therefore, no certain promise of an asbestos supply, though it may offer encouragement for prospecting. High-grade fiber is evidently formed under a peculiar combination of geologic conditions which involve the presence of certain primary rock types, their alteration to secondary minerals and recrystallization of these in veins, such recrystallization being induced by a combination of metamorphic agencies. Although it is quite probable, therefore, that new fields will be discovered, there is no probability of an abundant supply.
An important point to be considered in connection with future supplies is that for most uses of asbestos there is little or no scrap recovery; that which is once used is for the most part gone beyond recall. It is wise, therefore, to maintain a conservative view of the asbestos reserve, for although there is evidence of a supply in the Canadian deposits and elsewhere to last for many years, the probability is that deposits of high-grade material are neither numerous nor extensive.
The total production and estimated reserves of spinning asbestos in the important producing countries are shown in [Figure 13].
Fig. 13.—Total production and estimated reserves of spinning asbestos.