RUNEB. LARSEN,M IREILLE POLVE& GUNNARJUVE NGU-BULL 436, 2000 - PAGE 57 Granite pegmatite quartz from Evje-Iveland: trace element chemistry and implications for the formation of high-purity quartz RUNE B. LARSEN, MIREILLEPOLVE&GUNNARJUVE Larsen, R.B ., Polve, M., & Juve, G. 2000: Granit e pegmat ite quartz from Evje-Iveland: trace element chemistry and implications for th e formation of high-puri ty quartz. Norgesgeolog iske unde rsogelse Bulletin 436, 57-65. Previous studies imply that granite pegmatites and hydroth ermal quartz veins are th e most promising igneous repositories of high-purity quartz. This is because quartz from higher temperature geological settings (granites, monzo nites, dio rites, etc.) accommodates higher concentrat ions of imp urities in its atomic str uct ure. Systematic studies of granite peg matites from Evje-Iveland, South Norway, show a marked relationship between pet rogenesis and th e distributi on of structu ral impurities. Accordingly, less fractionated pegmatites featu re relatively high concen trations of Ti, Mg, Ca and Cr whereas mo re fractionated pegmatites contain higher concent ratio ns of Fe, Li and B, and the total concent rati on of str uctural impurities rises with the degree of different iation. Therefore, detailed know ledge of a pegmatite field, if combined with quartz analyses fro m a few carefully selected localities, may drastically reduce the area in which prospect ing for hig h-purity quartz resources is feasible. Rune B.Larsen, Geological Survey of Norway, N-7491 Trondheim, Norway. Mireille Polve, UMR5563. Universite Paul Sabatier, 38 Rue des 36 Ponts, 3 1400 Toulouse, France. Gunnar Juve, Geological Survey of Norway, Oslo Office, P.O.Box 5348 Majorstuen, N-0304Oslo, Norway. raw material in th e development of high-performance solar Introduction panels for energy production. High-purity quartz is common quartz that is characteri sed by Because the trace-element chemist ry of quartz is only exceptionally low concentrations of elements other th an sili­ cursorily evaluated in most literature, th e first part of this con and oxygen. Untreated, naturally occurring quartz with communication summarises the appearance and character lessthan SO ppm of impurities qualifies ashigh -purity quartz; of impurities in quartz and defines the principl e features of however, quartz with as much as 500 ppm tota l impuriti es high-purity quartz. This section is follo wed by a case histo ry may suffice if industrially feasible dressing techn ique s suc­ from Evje-Iveland in South Norway where the quality of igne­ ceed in lowering the impurity level to less than 50 ppm . ousquartz in a closely related suite of gran ite pegmatites has Given th ese specifications, prices in excess of 1000 US$ /t on been studied. This case history does not report on th e dis­ may be obta ined in a market which, according to a 1992 covery of new high-purity quartz occurrences in Norway; in Roskill report 'The Economics of Quartz", is stipulated to fact, giv en our present knowledge it is unlikely that th e Evje­ increase by 5-8 % per annum. The USGS Minerals Information Iveland area contain s any promising targets. Ra the r, we refer (ref.: http://minerals.usgs.gov/minerals/pubs/commodity/) to this area as provid ing an illustration of the principles and also forecasts solid gro wth in the demand for high -purity strategies that may be applied in futu re prospecting for quartz, not least asa result of continued expansion in the pro­ industrially feasible quartz deposits in granite pegmatite duction of silicon oxide wafers for semi-conductor technol­ provinces. ogy. High-purity quartz is largely used to manufacture silica glass, whi ch is formed by melting processed crystalline quartz at temperatures betw een 1750 and 2000 °C. The fina l Geology of quartz product conta ins > 99.995% Si0 2 and, because of its out ­ The following outline is prim arily extr acted from wo rk by standi ng chemical and physical prop erties, silica glass is the Dennen (1964, 1967), Dennen et al. (1970), Lehmann & Bam­ only single-component glass that has wide comm ercial bauer (1973), Fander lik (1991), Jung (1992), Perny et al. applications (Fa nderlik 1991). The most important prop ertie s (1992), Deer et al. (1997) and Watt et al. (1997). Other sources of silica glass are resistance to extreme fluctuations in tem ­ are cited in th e text. peratur e, chemi cal du rability in acidic environments and its In evaluating the quality of quartz, distinguishing ability of transmitting light from near ult raviolet to infrared betw een structural impurities,solid and liquid inclusions (Fig parts of th e spectrum. Therefor e, silica glass has found wide 1), is imperative. Solid and liquid inclusions are evaluated applicati ons in th e metallurgical, chemical and optical indus­ only briefly at th e end of this secti on because, to a large tries, as well as in communicatio n technology for th e manu­ extent, they are remov ed du ring the processing of quartz facture of optica l wave-gu ides. An exciting applicati on isasa unlessthey are small and/or very abundant. Structural impu­ rity elements, on th e contrary, can only be partially removed NGU-BULL 436, 2000 - PAGE 58 RUNE 8. LARSEN, MIREILL EPOLVE& GUNNAR JUVE Types of impurities Structural impurities atomic lattice and these elements are classified as structural im purit ies. Element s that are most commonly identified as structural impurit ies encompassAI, B, Ca, Cr, Cu, Fe, Ge, H, K, Li, Mg,Mn, Na, P, Pb, Rb, Ti and U. Not all studies agree wit h this list of elements.Jung (1992) suggested that on ly AI, B,Ge, Fe, H, K, Li, Na, P and Ti may be regarded as true structural Fluid inclusions im purities, whereas Ca, Cr, Cu, Mg, Mn, Pb, Rb and U are the results of contamination by microscopic solid and liqu id inclusion s w hich were not entirely removed before analysis. Anot her reason for thi sappar ent controversy may be the fact Structu ral impuritie s that certain element s, e.g. the alkali metals, tend to form minu te atomi c clusters adsorbed at specific growth direc ­ t ions Le. the 0001 surface (Brouard et al. 1995). Forming adsorbed clusters, they hardly classify as conventional struc­ tural im purities. However, being strictly confined to one Fiq.l , This figure illustrates t he type s of impurities that are comm on in quartz, The left part of the figure illustratesdifferent type s of impurities, growth direction, the incorporation of atomic clusters is Le, solid inclusions, fluid inclusions and structural impurities, The right dependent on the physics and hence on the atomic latti ce hand fig ure shows types of st ructural impurities, Location s of st ructural prop erties of specific crystallographic orientations . As they impurities (fig, to the right) are shown for a- quartz which is the most are not so strongly arrested in t he quartz structure as com­ common type of quartz at the Earth's surface and is the only type of pared w ith convent ional st ruct ural impurities, atomic clus­ quartz found in the Evje-Iveland area, Also to the right, the atomic con­ figurationsof silicon and oxygen are viewed in a sectio n perpend icular to ters may be more exposed to acid leach and, t herefore, may the crystallographic c-axis. al and a2 deno tes crystallographic axes. be partially or fully removed dur ing processing of the quartz Basement geology after Pedersen (1981). Falkum (1982) and Padget raw material. (1994). Sub stit utional im purities compete with Si4+ in the Si-O tetrahedron com posing th e quartz lattice (Fig. 1), whereas by ti me-consuming and expensive dressing techniques. intersti tial impurities mostly include small monovalent ions Therefo re, it is th e concent ration of th ese elements that ulti­ that fit into st ructural channe ls running parallel to the c-axis mately disting uishes high -puri ty quartz deposits fro m infe ­ and fu nction as charge compensa tors balancing substitu­ rior quality occurrences. tional im purities (Fig 1). AI3+, for example, is a common sub­ The speciation and character of impurities in quartz are sti tuti onal impurity wher eas Li+or Na", in the struct ural chan­ non-obvious featu res wh ich com plicate prospecting for nels, is balancing th e missing positive charge. Next to high-purity qua rtz resources. For example, quartz, which at aluminium, Tl, Fe(1I 1) and Ge are common substitutions for Si first glance appears clear and inclusion-free, may conta in w hereas H, l.i, Na and also K (e.g. Watt et al. 1997) include thousa nds of pp m of structural impuriti es. On t he contrary, most charge compensators. However, H is rare in magmatic dark smoky quartz wit h many solid and liqu id inclusion s may quartz, whereas it dominates over the other common charge provide an excellent raw material fo r certai n applicati ons if compensators in quartz that formed from dilute aqueous the inclusions can be removed th rough affordabl e indust rial solu tio ns, e.g. in alpine -ty pe quartz veins. This latte r type of dressing met hods. This is because the smoky colour of quartz, occasion ally featuring lamellar growth structures, is quartz is caused by low levels of ionising radiati on induced also know n for highly asymmet ric distributions of structural by th e decay of radio activ e elements in neighbouring miner­ im purities giving rise to sectoral or concen tric zonat ion pat­ als (e.g.