242 



UNITED STATES MINERAL RESOURCES 



and greisens. Germanium is appreciably concentrated 

 in sulfide minerals, particularly sphalerite and wurtz- 

 ite, which commonly contain 0.01-0.05 percent ger- 

 manium and have been reported to contain as much 

 as 0.5 percent germanium. Enargite (CusAsSi) com- 

 monly contains 0.01-0.1 percent germanium. Ger- 

 manium is absorbed by the organic matter in plants 

 and becomes concentrated in coal beds, especially in 

 the uppermost and lowermost layers in a coal se- 

 quence. In weathering, germanium apparently enters 

 the clay minerals and is slightly concentrated in 

 shales. 



Indium is not closely coordinated with any major 

 element, and its distribution in nature is less well 

 known. It has some association with tin, for which 

 it apparently can proxy. It is a rare element, and 

 notable concentrations are mainly limited to occur- 

 rences in sulfide minerals, especially sphalerite, 

 which commonly contains 10-20 ppm and has been 

 reported to contain as much as 1 percent indium in 

 some specimens. Some copper-bearing minerals, par- 

 ticularly chalcopyrite and tetrahedrite, also have 

 indium contents ranging from a few parts per mil- 

 lion to 1,500 ppm. 



ABUNDANCE 



As shown in table 47, the estimated abundance of 

 these three elements in rocks of the continental crust 

 ranges from 18 ppm for gallium to 0.14 ppm for 

 indium. Gallium is thus slightly less abundant than 

 cobalt and considerably more abundant than various 

 much better known metals such as tin, molybdenum, 

 cadmium, and silver. Germanium, with an average 

 abundance of 1.5 ppm, is as abundant as molybde- 

 num and tin. Indium is slightly more abundant than 

 silver. The fact that none of the three elements 

 forms its own minerals, except in a few rare deposits, 

 is largely a function of the chemical nature of the 

 elements that permits them to substitute in trace 

 amounts for other elements in a variety of common 

 minerals. 



Gallium in igneous rocks tends to occur in larger 

 amounts in the more aluminous rocks, and within 

 individual intrusions it is more concentrated in the 

 later formed aluminous minerals. Highest concen- 

 trations are found in pegmatites, where gallium is 

 most abundant in muscovite. Gallium tends to follow 

 aluminum in the processes of weathering, although 

 it may be slightly more mobile in normal soil-profile 

 development. In the development of bauxite, how- 

 ever, the relative concentration of gallium increases 

 with respect to aluminum. In sedimentary rocks only 

 shales and clays tend to accumulate gallium. In 

 metamorphic rocks there does appear to be a sig- 



nificant migration of gallium, although data are 

 sparse. Gallium does not enter the biologic cycle to 

 a significant extent, but it is taken up by some 

 plants, where it is a minor component, presumably 

 in an organic complex. 



Germanium in igneous rocks is slightly more abun- 

 dant in the silicic and intermediate rocks and is 

 especially more abundant in the rocks developed in 

 the late pegmatitic and pneumatolytic stages. In 

 such rocks fluorine apparently provides a mecha- 

 nism to transport and concentrate germanium into 

 such minerals as topaz and cassiterite. In the weath- 

 ering cycle, germanium is not notably concentrated 

 in most processes but may be partly removed by 

 surface waters. The bulk of the germanium content 

 remains and is most likely in the clay minerals. The 

 germanium content in living plants never exceeds 

 10 ppm in their ash, which is reported to be the 

 toxic level in plants. The germanium content of coals 

 ranges from a few parts per million to several hun- 

 dred parts per million and is irregularly distributed 

 laterally within individual coal beds and vertically 

 within coal seams. Highest germanium contents are 

 typically found in either the uppermost or lower- 

 most layers. The germanium content is predomi- 

 nantly bound to the organic matter, coals with lowest 

 ash content having the highest germanium content. 

 Fly ash from coal combustion may contain as much 

 as several percent germanium. 



Very sparse data are available to indicate the dis- 

 tribution of indium. In igneous rocks it is slightly 

 more abundant in intermediate and silicic rocks than 

 in basaltic or alkalic rocks. It is notably concen- 

 trated only in pneumatolytic and hydrothermal 

 stages. In sedimentary rocks it may be slightly con- 

 centrated in clays and shales. Data are insufficient 

 to categorize its occurrence in metamorphic rocks, 

 although slight enhancement of the indium content 

 is suggested. Indium is not reported to be concen- 

 trated in plant ash or in coal. 



ORE MINERALS 



Gallium, germanium, and indium are concentrated 

 in sulfide minerals, especially the zinc sulfide sphal- 

 erite. Table 48 lists the principal ore minerals that 

 have been reported to contain appreciable amounts 

 of any of the three elements, and where data exist, 

 it gives some idea of both the range and the typical 

 contents of these elements. The table also lists the 

 rare minerals containing any of the three elements 

 as essential components. 



Of these rare minerals, most have been found 

 mainly in studies of a single deposit, that at Tsumeb 

 in South West Africa. 



