ZIRCONIUM AND HAFNIUM 



717 



amount of the zircon it consumes, domestic resources 

 of zircon are large and could be utilized in greater 

 quantities if foreign supplies were not available. 

 World resources of zircon are sufficient to meet the 

 demand in the foreseeable future. A better under- 

 standing of the zirconium and hafnium resources of 

 the world can be obtained by an examination of the 

 geologic occurrence of these elements. 



GEOLOGIC ENVIRONMENT 



GENERAL GEOCHEMICAL AND 

 MINERALOGIC CONSIDERATIONS 



Zirconium and hafnium have long been recognized 

 as a classic example of a pair of geochemically co- 

 herent elements ; that is, they invariably occur to- 

 gether in rocks and minerals. Their close association 

 is explained by their nearly identical charge, ionic 

 radii, electronegativity, and ionic potentials (table 

 158). 



Table 158. — Selected data on zirconium, hafnium, and geo- 

 chemically associated elements 



[Data from Green, 1959, table 2] 



Both elements are markedly lithophile (have a 

 strong affinity for oxygen) and are enriched strong- 

 ly in the sialic crust of the earth. Because of its 

 physical and crystallochemical properties, Zr^+ has 

 restricted entry into the lattice sites of the major 

 rock-forming minerals, hence, relatively few miner- 

 als account for the occurrence of most of the zir- 

 conium of the earth's outer crust. Approximately 20 

 well-defined minerals, all of which are classified as 

 silicates or oxides, contain zirconium as an essential 

 constituent. In addition, a large number of minerals 

 contain minor or trace amounts of zirconium in solid 

 solution (Vlasov, 1966a, b; Frondel, 1957). From a 

 quantitative standpoint the mineralogical distribu- 

 tion of zirconium in the earth's outer crust can be 

 almost entirely accounted for by only three minerals : 

 (1) zircon, ZrSi04; (2) baddeleyite, ZrO?; and (3) 

 eudialyte, Na.,(CaFe)2ZrSi60i7(OH,Gl)2. Because of 

 its widespread occurrence as an accessory mineral 

 in igneous and metamorphic rocks and its f-oncentra- 

 tion in sedimentary rocks (Poldervaart, 1955, 1956; 



Mertie, 1958; Ronov and others, 1961; Saxena, 1966; 

 Marshall, 1967) , zircon is the principal source of 

 zirconium and hafnium. 



Baddeleyite occurs in economically important de- 

 posits in association with feldspathoidal igneous 

 rocks in the Po§os de Caldas Plateau, Minas Gerais, 

 Brazil (Guimaraes, 1948; Franco and Loewenstein, 

 1948) . The Pogos de Caldas zirconium ore concen- 

 trates consisting of baddeleyite and intimately as- 

 sociated zircon (locally called caldasite) contain 0.1- 

 2.0 percent uranium, averaging about 0.5 percent 

 (Tolbert, 1966). 



Eudialyte, although considered a relatively rare 

 mineral, occurs in large quantities in some types of 

 potassium- and sodium-rich igneous rocks. It is an 

 essential mineral in a variety of nepheline syenites 

 and nepheline syenite pegmatites of the agpaitic 

 type. (Nepheline syenites in which the molecular 

 ratio KaO + NaaO: AI2O3 is greater than 1 are named 

 agpaitic; nepheline syenites in which the ratio is 1 

 or less are named miaskitic.) Agpaitic nepheline sye- 

 nites are known from the Ilimaussaq intrusion in 

 Greenland and the Kola Peninsula in the U.S.S.R. 

 (Gerasimovskii, 1956). In addition to containing 

 deposits of eudialyte, the agpaitic rocks of the 

 Ilimaussaq intrusion contain potential low-grade 

 uranium deposits (Sorensen, 1970a, b) and niobium 

 mineralization (Hansen, 1968). 



ABUNDANCE AND DISTRIBUTION IN ROCKS 

 OF THE CRUST 



Estimates from recent compilations of the crustal 

 abundance of zirconium and hafnium and of average 

 zirconium and hafnium contents in some of the 

 major rock types of the upper crust are given in 

 table 159. It should be recognized that there are 

 problems inherent in making such estimates of the 

 abundances of elements (Fleischer and Chao, 1960) . 



Table 159. — Abundances of zirconium and hafnium in ter- 

 restrial rocks, in parts per million 



[Data from Brooks, 1969, 1970; Chao and Fleischer, 1960; Degenhardt, 

 1957; Horn and Adams, 1966; Taylor, 1965; Turekian and Wedepohl, 1961] 



Rock type Zr 



Crust 165 



Ultramafic 40 



Basalt: 



Tholeiitic 50-100 



Alkaline 250 



Syenite 500 



Granodiorite 140 



Granite 180 



Shale - 160 



Graywacke 140 



Sandstone 220 



Limestone 20 



Pelagic clays 150 



