MICA 



419 



evenly distributed throughout, but in other pegma- 

 tites the minerals are segregated into certain layers 

 or parts of the body called zones. At places, these 

 zones can be mined selectively to recover the desired 

 minerals by hand sorting and are, therefore, im- 

 portant economically. In general, zones are succes- 

 sive shells, complete or incomplete, that commonly 

 reflect the shape or structure of the whole pegma- 

 tite body. Where best developed, the zones are con- 

 centric about an innermost zone, or core, and ac- 

 cording to accepted practice, the zones from the 

 outermost one inward to the core are called border 

 zone, wall zone, intermediate zones, and core. 



Border zones are generally thin fine-grained sel- 

 vages of quartz-mica-feldspar pegmatite; wall zones 

 are of similar composition but are coarser grained. 

 Intermediate zones may contain mixtures of several 

 minerals or virtually all one mineral, usually feld- 

 spar. Most cores are a single mineral — commonly 

 quartz or potash feldspar. The best quality and 

 greatest concentration of sheet muscovite are com- 

 monly in the wall zone; less important concentra- 

 tions are in intermediate zones and the core margin. 

 In poorly zoned pegmatite, muscovite is scattered 

 throughout the rock. 



The principal mica pegmatite districts in the 

 United States are in New England (Cameron and 

 others, 1954), the Southeastern Piedmont from Vir- 

 ginia to Alabama (Jahns and others, 1952), the 

 Blue Ridge Mountains in North Carolina (Lesure, 

 1968), the Black Hills of South Dakota (Page and 

 others, 1953), and northern New Mexico (Jahns, 

 1946). Less important districts are in Idaho and 

 Montana (Stoll, 1950), Colorado and Wyoming 

 (Hanley and others, 1950), Nevada (Olson and 

 Hinrichs, 1960), and California (Lesure, 1966). In 

 India, the most important mica pegmatite districts 

 are in Bihar, Madras, and Rajasthan (Skow, 1962; 

 Rajgarhia, 1951). In Brazil, the important mica 

 pegmatite districts are in Minas Gerais (Pecora 

 and others, 1950). Important mica pegmatite dis- 

 tricts are also known in Argentina, parts of Africa, 

 the Soviet Union, and China (Skow, 1962). 



The origin of pegmatites has been studied by 

 hundreds of workers for more than 100 years. 

 Jahns (1955) gives an extensive bibliography of 

 these studies. Recent reports (Jahns and Tuttle, 

 1963; Jahns and Burnham, 1969) stress the origin 

 of granitic pegmatite from crystallization of gra- 

 nitic magma in place in virtually a closed system 

 with little or no wallrock alteration. In many dis- 

 tricts, like the Black Hills of South Dakota, the 



spatial relationship between pegmatite and large 

 masses of granite indicates the same or a similar 

 source. In other areas, like parts of the Blue Ridge 

 Mountains of North Carolina, no close relationship 

 to granitic masses is evident, but the mica pegma- 

 tites are restricted to areas where regional meta- 

 morphism has reached at least kyanite or sillimanite 

 grade. 



PHLOGOPITB DEPOSITS 



According to Hoadley (1960, p. 45), the occur- 

 rences and distribution of phlogopite deposits are 

 everywhere similar. Known deposits are in areas of 

 metamorphosed sedimentary rocks intruded by 

 masses of pegmatite-rich granitic rocks. The phlogo- 

 pite deposits commonly are in veins or irregular 

 pockets in diopsidic pyroxenite interlayered with 

 or cutting marble or gneiss. The pyroxenite may be 

 normal intrusive mafic igneous rock or a contact 

 metamorphic alteration of crystalline carbonate by 

 solutions from granitic magma. The phlogopite de- 

 posits may also have formed by contact metamorph- 

 ism at about the same time as the surrounding 

 pyroxenite. The principal resources of phlogopite 

 are in the Malagasy Republic (Murdock, 1963, p. 

 124-129) ; minor deposits are found in Canada 

 (Hoadley, 1960), Mexico (Larrabee, 1946), Ceylon, 

 and the Soviet Union (Skow, 1962). The United 

 States has no commercial phlogopite deposits. 



SCRAP- AND FLAKE-MICA DEPOSITS 



The original source of scrap mica was the waste 

 from sheet-mica mining and preparation, hence the 

 name scrap. About 1910, however, demand for scrap 

 had increased so much that mica deposits contain- 

 ing poor quality sheet or smaller sizes were being 

 mined for just the scrap content. Today in Spruce 

 Pine, N.C., which produces nearly half the scrap 

 mica mined in the United States, deeply weathered 

 alaskite bodies containing; 10-20 percent muscovite 

 are mined for fine-grained scrap mica or flake mica. 

 In such deposits, depths to unweathered rock may 

 be 50-100 feet for areas of several acres. Some de- 

 posits are mined for clay, and the mica is recovered 

 as a byproduct; some are mined for mica, and the 

 clay generally is lost in the washing process. Un- 

 weathered alaskite is mined for feldspar, and mica 

 is recovered as a byproduct. 



Flake mica can also be recovered from muscovite 

 schist, a rock found in many metamorphic terranes. 

 Such deposits may be a few tens or hundreds of 

 feet thick and many miles long. Mica content can 

 range from a few percent to at least 90 percent. 



