TALC 



623 



Mt. Sinai School of Medicine in New York City has 

 shown a positive correlation between incidence of 

 lung disease and working with or near asbestos 

 without safety precautions (New York Academy of 

 Sciences, 1965). Because many of the talc deposits 

 mined in the United States contain asbestiform 

 tremolite and anthophyllite, emission-control stand- 

 ards will undoubtedly be established for mining and 

 milling them. The tremolite and anthophyllite con- 

 tent of talc used as a cosmetic or for food process- 

 ing, such as polishing rice or dusting salami, also 

 will probably be carefully monitored, as will that 

 in talc used as a carrier for pesticide dusts. 



GEOLOGIC ENVIRONMENT 



Talc and soapstone deposits of commercial im- 

 portance occur mainly in metamorphosed dolomite 

 and altered ultramafic igneous rocks. These rocks 

 and contained talc deposits are present in meta- 

 morphosed terranes throughout the world. In the 

 United States, talc is found in the older Appala- 

 chians and Piedmont from New England to Ala- 

 bama in the east and in California, Nevada, Wash- 

 ington, and Montana in the west (Chidester and 

 Worthington, 1962). Important deposits also occur 

 in Texas, but most of the Central United States, 

 from the Appalachians to the Rocky Mountains, lack 

 a convenient source of industrial talc. 



TALC FROM REGIONALLY METAMORPHOSED 

 DOLOMITE 



Dolomites with ascociated siliceous sediments can 

 be metamorphosed to form talc. In many deposits 

 derived from sedimentary dolomite and silica, talc 

 forms as a final retrograde hydrothermal phase 

 from earlier magnesian silicates such as tremolite, 

 forsterite, and diopside. These three minerals re- 

 sult from high-grade metamorphism with the addi- 

 tion of silica and limited water. Retrograde altera- 

 tion of the earlier minerals to talc and serpentine 

 is accomplished hydrothermally with the release of 

 calcite and CO2. 



Most of the major talc deposits in the United 

 States are in metamorphosed dolomite; this type 

 accounts for more than half the total production. 

 The largest producers in the country are mines on 

 such deposits in St. Lawrence County, N.Y., where 

 talc, tremolite, and serpentine rocks occur in Pre- 

 cambrian carbonate and siliceous sedimentary rocks. 

 The talc zones are stratigraphic units that have 

 been tectonically swelled in axial parts of folds and 

 at places are pinched out on fold limbs. Shearing has 

 played a major role in the development of talc schist 

 zones. 



Similar talc deposits are in the Murphy Marble 

 belt of North Carolina, the Dillon-Ennis district of 

 southwestern Montana, and the Inyo and Panamint 

 Ranges of California. The deposits in North Carolina 

 are lens shaped and follow a zone of quartzose dolo- 

 mitic marble. Some of the North Carolina talc is 

 exceptionally white, and locally massive varieties 

 are sawed for steel-marking crayons. The deposits 

 in Montana are rather large but require consider- 

 able selective mining and sorting because of inter- 

 layered unaltered wallrock. The Montana deposits 

 and those in the Inyo and Panamint Ranges of Cali- 

 fornia provide some rock of steatite or near-steatite 

 grade. 



Deposits in the Chatsworth district of Georgia and 

 the Allamoore district of Texas, also formed from 

 metasedimentary rocks, are not so closely related to 

 dolomitic parent rocks. These deposits are associated 

 with phyllites, schists, and quartzites, and required 

 extensive metasomatism for their development. 



The Winterboro deposits in Talladega County, 

 Ala., occur in or as residual deposits derived from 

 Cambrian and Ordovician dolomite that is relatively 

 unmetamorphosed. The talc is virtually unfoliated 

 and preserves the crystallinity of the parent rock 

 (Wedow and Sweeney, 1968). These deposits should 

 be studied further because of their low metamorphic 

 grade. Possibly they result from prograde meta- 

 morphism, with talc being the highest grade min- 

 eral produced, in contrast to other deposits in which 

 retrograde metamorphism was important. 



TALC DEPOSITS FORMED FROM ULTRAMAFIC 

 AND MAFIC IGNEOUS ROCKS 



Talc deposits associated with serpentinized ultra- 

 mafic rocks occur in regionally metamorphosed and 

 folded sedimentary and volcanic rocks such as those 

 in the older Appalachian Mountains and Piedmont 

 in a belt from New England to Alabama (Larrabee, 

 1966, 1971). These deposits generally occur as rinds 

 on serpentine bodies but in places have completely 

 replaced them. Some deposits occur where sodic 

 pegmatites intrude serpentine. Steatitization, the 

 formation of talc-rich-rock, was accomplished by 

 either of two processes — the metasomatic reaction 

 between serpentinite and introduced CO2 or a meta- 

 morphic reaction between serpentinite and siliceous 

 country rock. These two processes, though independ- 

 ent, can work together. Generally, the deposits 

 formed mainly by CO2 metasomatism have an abun- 

 dance of talc-carbonate rock; and where metamor- 

 phic reaction was important, talc rock is dominant. 

 The largest producing talc mines in altered ultra- 

 mafic rocks are in Vermont; many similar but 



