LITHIUM, CESIUM, AND RUBIDIUM 



367 



though after 1938 a sizable supply came from brine 

 at Searles Lake, Calif. In 1966, however, production 

 started from brine at Silver Peak, Nev., and this 

 deposit seems to have become the world's largest 

 source of lithium. Certain clays are also known to be 

 enriched in lithium, but no attempt has ever been 

 made to mine them. 



LITHIUM DEPOSITS IN PEGMATITES 



GEOLOGY 



Pegmatites containing lithium minerals can be 

 divided into two categories: (1) deposits of spodu- 

 mene in which there is so little change in the com- 

 position or texture of the rock from one place to 

 another that it can be said to be virtually homoge- 

 neous; and (2) deposits of spodumene or other 

 lithium minerals that are segregated into a series 

 of zones of different composition or texture, in places 

 accompanied by replacement bodies and fracture- 

 filling units. 



The Foote Mineral Co.'s deposit at Kings Moun- 

 tain, N. C, stands out as the best example of a 

 homogeneous lithium-rich pegmatite. Kesler (1961) 

 showed that the mineral content, in weight percent, 

 is : spodumene, 20 percent ; quartz, 32 percent ; albite, 

 27 percent; microcline, 14 percent; muscovite, 6 

 percent; and other minerals, 1 percent. Crystals of 

 spodumene and microcline are as much as 3 feet 

 long, but most are less than 1 foot long. The other 

 minerals are all finer grained. The deposit consists 

 of many irregularly shaped, interfingering bodies, 

 some large and some small ; yet, the only noticeable 

 inhomogeneity is the absence of spodumene and 

 microcline within a few inches of some contacts, 

 where a narrow wall zone consists of quartz, albite, 

 and muscovite. 



Many spodumene-rich pegmatites have a more 

 conspicuous wall zone of this same kind and are 

 assigned to the zoned category, though actually they 

 may differ only slightly from the homogeneous peg- 

 matites. The Beecher No. 2 pegmatite in the Black 

 Hills, S. Dak., has a well-defined wall zone barren 

 of lithium minerals, but otherwise it resembles the 

 homogeneous pegmatites at Kings Mountain in its 

 composition and texture, and also in its structural 

 irregularities. It is rightfully called a zoned pegma- 

 tite, but it can be regarded as being gradational 

 between the homogeneous pegmatites and the com- 

 plexly zoned pegmatites. 



Some lithium pegmatites contain many zones, of 

 which one or several may contain lithium minerals. 

 The Bob Ingersoll No. 1 pegmatite in the Black 

 Hills has been a major source of lepidolite from its 

 innermost zone and contains amblygonite in another 



zone; yet it carries no lithium minerals in the four 

 remaining zones. On the other hand, the nearby 

 Etta pegmatite contains spodumene throughout the 

 three largest zones; only the narrow wall zone and 

 the small quartz core are barren. 



MINERALOGY 



Spodumene is by far the most abundant lithium 

 mineral of pegmatites. Other important minerals 

 are lepidolite and petalite. Additional minerals from 

 which lithium has been obtained are amblygonite 

 and eucryptite. Table 72 shows the chemical formu- 

 las and LiaO contents of these minerals. 



Table 72. — Composition, in weight percent, of lithium min- 

 erals mined from pegmatites 



^ Formula for polylithionite end member. Natural lepidolite haa various 

 proportions of muscovite molecule. 



'For Bikita, Rhodesia (Cooper, D. G., 1964, p. 460) 



Spodumene is invariably accompanied by quartz, 

 and, except in the inner zones of a few zoned peg- 

 matites, it is also accompanied by feldspar. Ordi- 

 narily spodumene constitutes between 20 and 25 

 percent of the rock mined, whether in a homoge- 

 neous pegmatite or in a zone of a zoned pegmatite. 

 All deposits that have been successfully mined carry 

 at least 1.0 percent LijO. 



Lepidolite (lithium-rich mica) ranks as an im- 

 portant source of lithium mainly because of its 

 abundance at Bikita, Rhodesia, but it also has been 

 mined at several other localities. It occurs mainly in 

 the central part of zoned pegmatites, where its chief 

 associates are albite and quartz. 



Petalite has been found in very few places, but 

 where it does occur, it commonly is abundant. Vir- 

 tually no petalite has been found in the United 

 States. Descriptions of such foreign deposits as the 

 one at Bikita, Rhodesia (Cooper, 1964), show that 

 the petalite is mainly in intermediate zones of zoned 

 pegmatites, where it is associated with quartz, feld- 

 spar, and spodumene. Eucryptite is common as an 

 alteration product of spodumene at Bikita, though 

 very rare elsewhere. 



Amblygonite is widely distributed in pegmatites, 

 but rarely if ever does it form as much as 10 percent 

 of any sizable body of rock. It has been mined chiefly 

 from very coarse grained parts of certain zoned peg- 

 matites that yielded crystals large enough to be 

 concentrated by hand. Its large lithium content made 



