04/01/2019

Science Blog: Microstructural and 3D X-ray Tomographic Study of the High-purity Quartz (HPQ): Examples from Finland

Text:
Thair Al-Ani, Senior Scientist

High purity quartz is one of the primary strategic raw materials in the manufacture of semiconductors, high- temperature lamp tubing, optics, telecommunication and electronic devices, and solar silicon applications (Haus 2010; Moore 2005; Dal Martello et al. 2011a, b). The Geological Survey of Finland (GTK) has been active in locating potential deposits of high-purity quartz (HPQ) in Finland as a strategic mineral for the high-tech industry. Recently, GTK has undertaken a sampling campaign from pegmatites, hydrothermal quartz veins, and quartzite occurrences with the aim of exploring the possibility of finding high-purity quartz in such rocks throughout the country (Fig. 1).

Figure 1. Location of High Purity Quartz (HPQ) samples in Finland.

Quartz samples from the selected high- purity quartz (HPQ) deposits in Central and Southern Finland have been investigated to characterise their mineral impurities and defect structures (Al-Ani et al., 2019). For this purpose, an analytical combination of optical and scanning electron microscopy (SEM) along with X-ray computed tomography (XCT) and trace-element analysis by XRF and ICP-MS were carried out. Figure (2a) is a photomicrograph of quartz samples showing a range of grain sizes. Sub grain formation, where a few large grains are surrounded by smaller ones, suggests re-crystallisation due to the high circulation of hydrothermal fluids. SEM images (Fig. 2b) detect the pore distribution and grain-boundary topography. These textures show that fracture surfaces in the quartz samples reveal a complex set of pores and grain boundary structures. Most of the grains have irregular boundaries, wherein the lobate shape is predominant. Muscovite is the most common impurity in the quartz with most of grains visible along grain boundaries as very fine flakes of sericite (Fig. 2b).

Figure 2. Microphotograph of quartz showing; (a) the bimodal grain size distribution; (b) SEM micrograph showing quartz grain boundaries; and (c) Fluid inclusion distribution and trails (arrows) produced by migration of fluids in the quartz; (d-f) High-purity quartz samples.

Fluid inclusions of different sizes were detected in random orientation inside the quartz crystals and commonly composed of liquid and vapor phases (Fig. 2c). The size of fluid inclusions varies between < 5 to 20 μm. These inclusions occur as trails or clusters which often cut across the grain boundaries and are therefore secondary in origin.

XCT is increasingly being applied in mineralogy and ore petrology due to its ability to resolve the three-dimensional (3D) shape and spatial distribution of minerals and associated microstructural features in a given rock sample (e.g., Sayab et al., 2015, 2016, 2017; Macente et al., 2017). In this study, we have used XCT scanner (GE phoenix v|tome|x s), hosted in the Geological Survey of Finland (GTK), to spatially map the internal structure (impurities, porosity, fractures, etc.,) of the quartz samples. A  nanotomography scan of a micro piece, cut from the quartz sample (3 × 1.5 × 15 mm diameter), exhibits a contrasting grayscale spectrum which allowed us to segment and separate high-purity quartz and their inclusions, mainly muscovite, albite and clay (Fig. 3a). Based on the density contrast, muscovite impurities were separated from the pure quartz which mostly appear as bright marks in the grey spectrum and are needle-like in shape (Fig. 3b). The high-purity quartz contains different generations of fluid and melt inclusions which, trapped within the crystallographic orientation of quartz, healed cracks and micro fissures. These inclusions or pores may contain gas and liquid and sometimes even very small crystals. Thus, the high-resolution XCT imaging reveals the internal structure of the studied quartz samples which are viewed as High Purity Quartz (HPQ). XCT allows us to identify > 1 μm fluid inclusions while the identification and volumetric reconstruction of the different phases can be carried out with reasonable confidence for relatively large (> 25 μm) inclusions.

Density contrasts are high enough to properly identify the aqueous mono phase (liquid) and two-phase (liquid+vapor) fluid inclusions with 5 to 25 μm sizes. The 3D spatial distribution of crystallised fluid inclusion and pores are represented in blue, as shown in Figure (3c, d). The volumetric reconstruction of the liquid and vapor phases or porosity values calculated from the XRCT images of four quartz samples are 0.87 %, 0.34 %, 0.16 % and 0.13 % respectively, with a mean value less than 1 %.

Figure 3. X-ray computed tomography images of subsamples from high-purity quartz (HPQ) deposits; (a) XCT image of the quartz sample; (b) Rendered XCT image showing the preferred orientation of muscovite impurities in bright colour; (c) A cubic subsection cut from the main volume showing the entire solid sample; (d) The entire 3D sets of crystallised melt (and fluid) inclusions are rendered in blue colour. The latter provides a full geometric description of pore space in the quartz sample.


References

Al-Ani, T., et al., 2019 (in press), Assessment of High Purity Quartz (HPQ) Resources in Finland, GTK Research Report 17/2019.

Dal Martello E, et al., 2011a, Electrical fragmentation as a novel refining route for hydrothermal quartz for SoG-Si production. Miner Eng., doi: 10.1016/j.powtec.2012.02.055.

Dal Martello E, et al., 2011b, Study of pellets and lumps as raw materials in silicon production from quartz and silicon carbide. Metall Mater Trans B, p. 12.

Haus, R., 2010, High-purity quartz resources. In: PHOTON0s 8th solar silicon conference, Stuttgart.

Macente, A., et al., 2017. The strain-dependent spatial evolution of garnet in a high-P ductile shear zone from the Western Gneiss Region (Norway): a synchrotron X-ray microtomography study. J. Metamorph. Geol. 35, 565e583. http://dx.doi.org/10.1111/jmg.12245.

Moore P., 2005, High-purity quartz. Ind Miner 455:53–57.

Sayab, M., et al., 2015, High resolution X-ray computed micro tomography: A holistic approach to metamorphic fabric analyses: Geology, v. 43, p. 55–58, doi: 10 .1130 /G36250.

Sayab, M., et al., 2016, Three-dimensional textural and quantitative analyses of orogenic gold at the nanoscale, Geology, v. 44, p. 739–742, doi:10.1130/G38074.1.

Sayab, M., et al., 2017, Orthogonal switching of AMS axes during type-2 fold interference: Insights from integrated X-ray computed tomography, AMS and 3D petrography. Journal of Structural Geology, 103, 1-16, doi:10.1016/j.jsg.2017.09.002.

Thair Al-Ani

Text: Thair Al-Ani

Thair Al-Ani completed his PhD degree in geochemistry and mineralogy at the University of Baghdad in 1996. He has worked as a senior scientist at the Geological Survey of Finland (GTK), since September, 2003. Dr. Thair Al-Ani has over 14 years’ experience of academic teaching at Tripoli University, Libya (1997 –2001) and Baghdad University, Iraq (1988-1997). Currently he is engaged in many projects focusing on GTK strategies relating to cobalt, lithium and graphite as raw materials in battery production and high-purity quartz as a raw material in the manufacture of semiconductors, high- temperature lamp tubing, optics, telecommunication and electronic devices.