Research area: Genesis of industrially applicable high purity quartz in igneous and metamorphic environments

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Jeffry Leslie Crawford
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1 Research area: Genesis of industrially applicable high purity quartz in igneous and metamorphic environments Supervisors: Rune B. Larsen, Objective Our primary goal is to study the element exchange processes in quartz from igneous and sedimentary environments that are exposed to multiple episodes of high-grade metamorphic re-crystallisation. Theoretical considerations and laboratory experiments imply that this type of quartz may obtain qualities commensurable with industrially applicable high purity quartz (HPQ). South Norway comprises an excellent natural laboratory for this character of studies, because quartzites and acid meta-igneous rocks covering large areas experienced different degrees of granulite facies metamorphism. Background High purity quartz, i.e. quartz with extremely low concentrations of impurities, is a rare commodity that only forms under geological conditions where a narrow set of chemical and physical parameters is fulfilled. When identified, HPQ obtain very attractive prices and is applied in the communications industries and other high-technology sectors that currently are under rapid expansion. Examples of end products where HPQ is the main raw material includes photovoltaic solar cells for environmentally sound energy production, silicon metaloxide wafers in the production of ever-faster computer chips and long distance optical fibres that are extensively used in communication networks. Marked considerations Industrial agencies forecast a solid 5-20 % annual growth in the demand for high purity granular quartz and predict a near exhaustion of raw materials. Together with environmental problems in the main quartz producing districts, a coming shortage in the supply of HPQ is implied. By far the largest proportions of HPQ (maybe as much as 90 %) come from granite pegmatites in the Spruce Pine district, North Carolina. The quality of quartz from Spruce Pine is steadily decreasing and, having an arid climate with limited water supplies, the processing of quartz is difficult and costly. Adding to these challenges are environmental concerns because the endangered species the 'Appalachian Elktoe Mussel', habit the few fresh water resources that are exploited by the mining industry. Parallel with foreseeable production shortages of HPQ, the semi-conductor industry plans at least 15 more years of development of more powerful silicon chips that depends on the productions of thicker thus more HPQ demanding silicon metal-oxide wafers. Also imposing higher demands is the fact that the production of HPQ-demanding photo voltaic devices, i.e. solar cells is forecasted to expand rapidly in the future. Geology of quartz The high-purity silica glass sector require that HPQ contain very low concentrations of structural impurities, i.e. foreign substitutional elements or charge compensator elements that are integrated as a part of the atomic lattice structure of quartz. Being bonded by the lattice structure of quartz, structural impurities are nearly impossible to remove with conventional dressing technologies. Particularly for the lighting and optical fibre industries, HPQ is required to contain very low concentrations of fluid inclusions because, expansion during melting of the fluid inclusions will generate vesicles in the silica glass melt that may be incorporated in the optical fibres.

2 Therefore, good qualities of HPQ must crystallise under anhydrous conditions and must incorporate a minimum of structural impurities. The most important structural impurities that are easily accommodated by the quartz crystal structure includes Al, Ti, Fe, Ge, Li, Na, K, B, P, Ca and H. With lower abundance but still well accommodated we have Cr, Cu, Mg, Mn, Pb, Rb and U. For some industrial applications, low Fe and B contents are imperative (e.g. in photovoltaic cells for solar panels) whereas some Ti may be tolerated. Other applications primarily require low Ti-concentrations and yet other applications are mostly concerned with low concentrations of Li. As with many other minerals, the concentration of structural impurities rise with temperature. Quartz from diorites and monzonites, for example, comprise much higher concentrations of structural impurities than quartz from evolved granites and granite pegmatites. Recent studies of the trace element distribution in granite pegmatites in Evje-Iveland, South Norway, demonstrate that the speciation and concentration of structural impurities also depends on the degree of differentiation of the igneous melts (Larsen & Polve 1998, Larsen et al. 1998a, Larsen 1999, Larsen & Lahaye, 1999, Larsen 2000, Larsen et al. 2000). Incorporation of fluid inclusions in quartz is primarily a function of the amount of volatiles in the quartz-forming environment. Diorites and monzonites largely form under volatile undersaturated conditions whereas granite pegmatites at least partially form during volatile oversaturation. Also, the speciation of volatiles may be important for the manufacture of silica glass. H 2 O, for example, has a much higher solubility in the silica glass melt (several percents) than CO 2, CH 4 and N 2 (a few hundred ppm). Dissolution of aqueous fluid inclusions during silica glass melt production may therefore hinder vesicle formation, however, being an excellent solvent, may contribute with Na, Fe, Mg, Li and several other electrolytes that are dissolved in the aqueous phase. Scope of work From the above considerations it appears that igneous quartz that formed at low temperatures may provide excellent HPQ raw materials in having low concentrations of structural impurities, but may be void because oversaturation of volatile fluids cater for high fluid inclusion abundances. Igneous quartz is not attractive because it formed at high temperatures that strongly enhances the incorporation of structural impurities HPQ with good melting behaviour, i.e. low fluid inclusion contents, is therefore very difficult to form in igneous and hydrothermal environments thus the so-called long-distance optical fibre industries where vesicle free silica glass is imperative, largely have to rely on extremely expensive man-made silicon compound glass. Granulite facies terrains Probably the only geological environment that on a large scale may produce quartz with low abundances of fluid inclusions and low concentrations of structural impurities is high-grade metamorphic terrain's and in particular, granulite facies terrain's. Factors during granulite facies metamorphism that influences the purity and quality of quartz includes: Repetitive and massive re-crystallisation of quartz Low density of lattice defects because of slow re-crystallisation of quartz Homogenisation of the impurity distribution in the quartz-bearing host lithology Continuous decrepitation of fluid inclusions Hydrous leaching of quartz during low- to high-grade metamorphism Regional depletion of LIL-elements during peak granulite facies metamorphism Anhydrous conditions during peak metamorphism Volatile fluids during peak metamorphism mostly comprises CO 2, N 2 and CH 4

3 It is beyond the scope of the present proposal to discuss all these points in detail. Important for the formation of HPQ is the fact that quartz will re-crystallise repetitively during prograde and retrograde metamorphism. During prograde metamorphism, pre-metamorphic fluid inclusions will efficiently decrepitate but because metamorphic volatiles are common from diagenetic to amphibolite facies conditions, new fluid inclusions will be generated and incorporated in quartz. However, during granulite facies re-crystallisation metamorphogenic fluid inclusions will also decrepitate and because peak granulite facies metamorphism occur under anhydrous conditions, new fluid inclusions with aqueous solutions will not form. There may be CO 2, CH 4 and N 2 fluids (or other C-O-H-N compounds) present in the system (e.g. Touret & Dietvorst, 1983; Andersen et al., 1993; Larsen et al., 1998b) however, they are rather harmless when compared to aqueous inclusions because they are easy to extract during industrial dressing processes. The behaviour of structural impurities in quartz is not quite as predictable as the faith of fluid inclusions. During re-crystallisation at low temperatures, quartz with high concentrations of impurities may indeed be significantly more pure as the impurities will partition in favour of other minerals or aqueous fluids. Experiments synthesising sequential re-crystallisation of autoclave quartz at o C demonstrated 5-10 times reduction in the concentration of Al, Li, Na, K and Fe during four episodes of re-crystallisation (Armington and Balsacio, 1984 in Jung, 1992 p.194). At higher temperatures some impurities may again be incorporated, however, this depends on the availability of impurities and the distribution coefficients between fluids, quartz and other phases. Impurities that partition into fluids may no longer be available. For example, peak granulite facies metamorphism is commonly associated with pronounced LIL-element depletion enforced by a combination of metamorphic dehydration, hydrous metasomatism and partial melting. This process was documented on Tromøy in the Bamble Belt (e.g. Cooper and Field, 1977, Smalley et al, 1983) where the K and Rb concentrations of acid and intermediate gneisses are amongst the lowest ever reported for granulite facies terrain's (Touret, 1987). Therefore, leaching of LIL-elements may improve the conditions for crystallisation of HPQ although the formation of quartz from partial melts, which also form during granulite facies conditions, may enforce the formation of quartz with high impurity concentrations. To avoid quartz that formed from partial melts but still to benefit from the positive effects of granulite facies metamorphism, quartz from quartzites may be the most promising target. Quartzites, being near mono-mineralic lithologies, will not melt at granulite facies conditions because the melting point even under water saturated conditions, will be higher than granulite facies T and P. The low concentration of other minerals in quartzites also reduces the possibility of incorporation of structural impurities during repetitive recrystallisation of the quartz. Being a sedimentary lithology that probably contains compositionally contrasting quartz from multiple sources, repetitive re-crystallisation also has the positive effect of homogenising the quartz compositions throughout the quartz-bearing lithology. Finally, foreign minerals in granulite facies quartzites, being relative coarse-grained compared to lower-grade quartzites, are more easy to handle by conventional dressing techniques. Research strategy Fieldwork Detailed field studies and the main body of sampling are committed in the high grade metamorphic belt of the Bamble shear zone (SE-Norway) and the Rogaland metamorphic envelope (SW-Norway) because the general geologies of these areas are well documented throughout earlier studies.

4 The Bamble shear zone may be divided into four metamorphic zones that from NW to SE (i.e. from A to D) comprise progressively higher metamorphic grades. Zones 'A' and 'B' reaches upper amphibolite facies whereas zones 'C' and 'D' are well within the granulite facies regime (e.g. Touret 1987). Particularly zone 'D' is void of hydrous minerals and is characterised by strong LIL element depletion. Quartzite lithologies and orthogneises are present in all metamorphic zones, but from the considerations outlined in the previous section, quartzites in zones 'C' and 'D' are most interesting. The metamorphic envelope embracing the Rogaland Intrusive Complex comprises a rich diversity of magmatic and sedimentary successions that experienced granulite facies metamorphism. Quartzites are particularly common in the Faurefjell meta-sedimentary successions that intersect different intensities of granulite facies metamorphism throughout the metamorphic envelope. Analytical techniques in addition to methods outlined under other sub-projects LA-HR-ICP-MS: the Geological Survey of Norway (NGU) recently purchased a Laser Ablation High Resolution Inductively Coupled Mass Spectrometer (LA-HR-ICP-MS). In short, the advantage with this instrument is its ability to analyse virtually any isotope in the periodic table by in situ ablation of small volumes of material directly from the sample surface. Simultaneous analysis of the ablated material by a high-resolution mass spectrometer ensures detection limits down to the sub ppm level. Finally, the laser pit may have a diameter of only 20 µm, which make it possible to obtain an exceptionally high spatial resolution. NGU has developed an analytical procedure that utilises this instrument in quantifying the trace element concentration in quartz and is considerably more rapid than conventional methods for quartz analysis. Hallimond tube micro-flotation: in order to evaluate the results from LA-HR-ICP-MS analysis it is necessary to conduct control analysis by more conventional methods. These include micro-flotation of small sample quantities by the Halimond tube technique that has proved very successful in the separation of quartz from feldspar and micas. Hallimond tube microflotation is mastered by NTNU (Prof. Knut Sandvik) and will be followed by conventional solution HR-ICP-MS at NGU. Fluid inclusion analysis: analysis of fluid inclusions is an essential part of the present study, because they provide important information about the P-T-X conditions that prevailed during the genesis of HPQ. The composition of the fluid inclusions will be determined with a state of the art Linkam freezing-heating stages at the Department of Geology and Mineral Resources engineering (NTNU) and will be supplemented with non-destructive raman micro-probe analysis at Free University, Amsterdam. The later method is imperative in identifying solid and fluid species (particularly C-O-H-N compounds) in the fluid inclusions. EPMA: Electron Probe Micro Analysis will be applied to selected phases co-existing with quartz in order to obtain independent P-T estimates and to calculate the principal distribution co-efficients for trace-impurities in quartz. Autoclave experiments: exchange of elements between quartz and the surrounding environment under different P-T-X conditions will be approached by autoclave experiments at University of Tromsø and University of Copenhagen where the proper instrumentation is available.

5 Collaboration partners Dr. Nikos Arvanitides is an expert in both the formation and industrial applications of HPQ. NA is director at the Institute of Geology and Mineral Exploration (IGME) in Greece. Dr. Jens Konnerup Madsen at the Department of Geology (University of Copenhagen) is an expert in thermodynamic modelling of volatile fluids. Expert in metamorphic petrology and metamorphic mineral-melt-volatile reactions (individual not yet decided). Doctoral students Doctoral student in metamorphosis and mineral-fluid-melt interaction processes in quartzites Doctoral student in mineral-chemistry and element exchange reactions of quartz based on laboratory experiments (autoclave-experiments)

Strategic University Programme at Department of Geology and Mineral Resources Engineering.

1 Strategic University Programme at Department of Geology and Mineral Resources Engineering. THE VALUE CHAIN FROM MINERAL DEPOSIT TO BENEFICIATED PRODUCT WITH EMPHASIS ON QUARTZ Background for the programme