Mineralogy and Preparation of High-Purity Quartz: A Case Study from Pegmatite in the Eastern Sector of the North Qinling Orogenic Belt

Abstract

High-purity quartz (HPQ), an indispensable industrial mineral, serves as a critical raw material for advanced technology sectors. Derived from natural quartz precursors through processing, HPQ preparation efficiency fundamentally depends on raw material selection. Two pegmatite samples (muscovite pegmatite and two-mica pegmatite) sampled from the eastern sector of the North Qinling Orogenic Belt were investigated through a suite of analytical techniques, as well as processing and purification, to evaluate their potential as raw materials for high-purity quartz. Muscovite pegmatite is predominantly composed of quartz, plagioclase, K-feldspar, muscovite, and garnet, with accessory phases including limonite and kaolinite. However, in addition to quartz, plagioclase, K-feldspar, muscovite, garnet, and limonite, two-mica pegmatite contains minerals such as biotite and calcite. The fluid inclusions in both muscovite and two-mica pegmatite quartz are small, but the former has fewer fluid inclusions. Compared with muscovite pegmatite, surface discontinuity (i.e., cracks, pits, cavities) development is more pronounced in two-mica pegmatite purified quartz, which may be related to its high content of fluid inclusions. Following purification, the total concentration of trace elements decreased significantly. However, the concentrations of Al and Ti appeared to remain the same. Titanium enrichment in purified two-mica pegmatite quartz likely derives from biotite, while Na and Ca concentrations may be related to fluid inclusions or microscopic mineral inclusions. The trace element content (27.69 ppm) in muscovite pegmatite is lower than that (45.28 ppm) of two-mica pegmatite, we thus suggest that muscovite pegmatite quartz is more likely to have the potential to produce high-purity quartz.

1. Introduction

High-purity quartz (HPQ), a critical industrial mineral material formed through either natural geological processes (exemplified by high-grade crystal deposits) or intensive purification processes of relatively pure quartz precursors, serves as an indispensable raw material for manufacturing high-value-added quartz products [1]. This strategically important mineral resource demonstrates exceptional physicochemical properties, including remarkable thermal stability, optical transparency, and chemical inertness, which enable its extensive applications across multiple high-tech sectors. These encompass semiconductor manufacturing, photovoltaic industries, fiber-optic communication systems, precision optical devices, microelectronics engineering, and solar energy technologies [2,3,4,5,6]. As a critical green strategic mineral resource, its processing methodology and quality control parameters are strictly governed by the stringent requirements of advanced industrial applications in geological and mineral processing engineering domains [1].

High-purity quartz, predominantly derived from precursor materials such as vein quartz, granitic pegmatite quartz, and natural crystals, necessitates stringent specifications for impurity element concentrations, fluid inclusions, mineral inclusions, and particle size to meet industrial-grade standards [7]. However, critical supply chain constraints persist: (1) the severe depletion of natural crystal reserves has drastically limited their viability for high-end applications; (2) vein quartz deposits, constrained by limited tonnage potential and inconsistent ore homogeneity, fail to satisfy the scalability requirements of advanced manufacturing sectors; and (3) granitic pegmatite-hosted deposits exhibit superior metallogenic characteristics, including extensive tonnage potential and exceptional chemical consistency, consequently emerging as the predominant global source for high-end quartz applications [8,9].

The quality-determining impurities in quartz primarily comprise three categories: lattice-bound trace elements, fluid inclusions, and mineral inclusions, whose concentrations dictate the end-use suitability of quartz materials [1,10,11,12]. However, the conceptualization and quality specifications of high-purity quartz materials remain non-standardized across industrial sectors due to divergent technical requirements. Larsen et al. [13] and Harben [14] defined HPQ as processed quartz products with SiO₂ content exceeding 99.995 wt% (4N5 grade). Müller et al. [10] proposed an alternative criterion based on deleterious element thresholds, stipulating that total harmful element concentrations must not exceed 50 ppm, with stringent limits imposed on nine critical impurities (i.e., Al < 30 ppm, Ti < 10 ppm, Li < 5 ppm, Ca < 5 ppm, K < 8 ppm, Na < 88 ppm, Fe < 3 ppm, P < 2 ppm, and B < 1 ppm). In contrast, Hao et al. [15] and Wang [16] classified quartz with SiO₂ ≥ 99.9 wt% (3N grade) as HPQ. More specifically, Wang [17,18] and Gao et al. [19] divided high-purity quartz into four grades according to SiO₂ purity, which are premium-grade (ω(SiO₂) ≥ 99.998%, 4N8 grade), high-grade (ω(SiO₂) ≥ 99.995%, 4N5 grade), mid-grade (ω(SiO₂) ≥ 99.99%, 4N grade), and low-grade (ω(SiO₂) ≥ 99.9%, 3N grade).

To address China’s strategic supply chain constraints in premium HPQ products, systematic investigations have been conducted on pegmatitic quartz resources across diverse areas. Previous prospecting campaigns and ore quality assessments revealed promising geological analogs to the Spruce Pine granitic pegmatites in the United States, particularly in terms of quartz crystallinity, trace element signatures, and fluid inclusion characteristics [20,21,22]. Nevertheless, achieving domestic self-sufficiency in HPQ production necessitates overcoming critical challenges in three domains: (1) refinement of exploration targeting criteria for quartz-bearing pegmatites with minimal impurities; (2) development of advanced beneficiation protocols addressing complex mineral associations; and (3) optimization of purification technologies to mitigate subgrain boundary defects and fluid inclusions [23].

This study employs integrated processing and purification (processing, calcination-water quenching, and acid leaching) and advanced characterization (petrography observations, mineral composition analysis, trace element analysis) to evaluate muscovite and two-mica pegmatites from the eastern sector of the North Qinling Orogenic Belt, China, for their potential as high-purity quartz raw materials, including a comparative assessment between the two types.

2. Materials and Methods

2.1. Sampling

Two quartz samples from pegmatite dikes (muscovite pegmatite and two-mica pegmatite) in the eastern sector of the North Qinling Orogenic Belt, China, were collected. In addition, in order to ensure that the samples are representative, the collection location is far away from the contact area between the pegmatite and the surrounding rock.

2.2. Quartz Processing

Pre-treatment process: The mineral comminution and sand-making processes were conducted using a rod mill, integrated with a multi-functional wet standard vibrating screening system. To optimize the production of high-purity quartz products, the particle size classification was precisely controlled within the 104–365 μm range through multi-stage rod milling coupled with a staged sieving strategy. This progressive approach facilitates the early separation of qualified particle fractions from the grinding circuit, effectively mitigating the over-grinding of quartz particles while enhancing the yield of target-sized products. The implemented staged processing configuration demonstrates significant advantages in energy efficiency and particle size distribution control compared to conventional single-stage grinding operations.

Gravity separation: The gravity separation process was performed using a shaking table operating at a stroke length of 24 mm and a stroke frequency of 300 cycles/min (5 Hz). Differential separation of mineral constituents was achieved based on their distinct physical properties, including density, particle size, and morphology [24]. Three product streams were generated across the deck: high-density heavy minerals, low-density light minerals, and slimes, all of which were rejected as tailings. The intermediate-density fraction (middlings) was selectively retained as feed material for subsequent magnetic separation.

Magnetic separation: High-gradient magnetic separation constitutes an indispensable unit operation in high-purity quartz purification processes, demonstrating exceptional efficacy in removing paramagnetic and ferromagnetic gangue minerals from ores [25,26]. The magnetic separation protocol was implemented using a periodic pulsating high-gradient magnetic separator configured with a two-stage magnetic separation circuit.

Flotation separation: Flotation technology enables efficient separation of quartz from associated gangue minerals (e.g., feldspar and mica) by exploiting differential surface properties among silicate minerals, serving as the principal impurity removal method in advanced purification processes for high-purity quartz [27,28,29,30]. The flotation experiments were conducted using a laboratory-scale mechanical flotation cell under controlled operational parameters: fixed impeller speed of 1992 rpm (33.2 Hz), slurry temperature maintained at 25–30 °C, and pulp density optimized at 40%–45% solids by mass to accommodate coarse particle characteristics.

For detailed procedures and parameter settings, readers are referred to Liu et al. [31] and Ma et al. [32].

2.3. Calcination and Water Quenching

Following flotation processing, the beneficiated samples underwent controlled thermal treatment in a muffle furnace at 900 °C for 2 h under atmospheric conditions to achieve structural homogenization. Subsequently, the thermally activated specimens were immediately quenched in ultrapure deionized water at 25 °C to induce rapid cooling and preserve metastable phase characteristics. Post-quenching, the samples were subjected to precision dehydration in a forced convection drying oven until constant mass was achieved.

2.4. Acid Leaching Test

The water-quenched specimens were subjected to a controlled acid leaching process in a polytetrafluoroethylene (PTFE) reaction vessel utilizing a ternary acid system comprising hydrofluoric acid (HF), hydrochloric acid (HCl), and nitric acid (HNO₃) at a mass ratio of 2:5:1. The leaching operation was conducted under optimized parameters: liquid-to-solid ratio of 3:1 (v/w), constant temperature maintenance at 80 °C via digital thermoregulation, and total process duration of 6 h. To mitigate secondary contamination from prolonged mechanical abrasion, intermittent agitation was implemented at hourly intervals (1 min agitation) using a PTFE-coated magnetic stirrer. Subsequent purification involved repeated rinsing cycles with ultrapure deionized water until neutral pH was attained, followed by drying to ensure complete dryness.

2.5. Mineral Composition Analysis

Mineralogical analysis was conducted using a mineral liberation analyzer (MLA; Thermo Fisher Scientific, Waltham, MA, USA) at the Zhengzhou Institute of Multipurpose Utilization of Mineral Resources, Chinese Academy of Geological Sciences. The analytical protocol comprised mounting epoxy resin-embedded polished sections in the sample chamber; optimizing scanning electron microscopy (SEM) parameters to acquire high-resolution back-scattered electron (BSE) images; executing automated particle-by-particle scanning in particle measurement mode with simultaneous BSE imaging and energy dispersive spectroscopy (EDS) spectral acquisition; and performing mineral phase identification through integrated BSE-EDS microtextural and microchemical analysis.

2.6. Trace Element Analysis

Trace element quantification was performed using inductively coupled plasma optical emission spectroscopy (ICP-OES; iCAP 7400, Thermo Fisher Scientific, Waltham, MA, USA) at the Zhengzhou Institute of Multipurpose Utilization of Mineral Resources, Chinese Academy of Geological Sciences, Zhengzhou, China. Sample preparation involved sequential acid digestion of representative aliquots (1–2 g) in PTFE reaction vessels using an optimized HF-HNO₃-polyol mixed solvent system to achieve complete matrix decomposition. Following controlled evaporation to dryness, the resultant residues were quantitatively redissolved in a mixture of deionized water and HNO₃ within FEP volumetric bottles, ensuring analyte stability prior to spectroscopic measurement.

3. Results and Discussion

3.1. Petrography

The hand specimen of the muscovite pegmatite exhibits a light gray to pale yellowish hue with a distinctive coarse-grained texture (Figure 1a). Petrographic analysis reveals the muscovite pegmatite is mainly composed of quartz (~36%), K-feldspar (~22%), plagioclase (~35%), muscovite (~6%), and garnet (~1%) (Figure 1b–f). Quartz exhibits colorless in plane-polarized light (PPL) and gray to pale grayish-yellow interference colors in cross-polarized light (XPL), predominantly forming anhedral granular aggregates (0.1–5 mm) with undulatory extinction, intracrystalline fractures, and serrated grain boundaries occasionally featuring subordinate quartz subgrains (0.02–0.1 mm). Plagioclase occurs as subhedral tabular crystals (0.2–8 mm) with polysynthetic twinning and microfractures. K-feldspar displays subhedral tabular crystals (0.2–6 mm) with cross-hatched twinning. Sericitization is observed within partial feldspar grains. Muscovite shows colorless in PPL with high-order interference colors (blue/green/orange/red) in XPL, forming subhedral flakes (0.05–1.5 mm) along grain boundaries with perfect cleavage; while garnet appears pale reddish-brown in PPL and isotropic with very high positive relief in XPL, occurring as subhedral–anhedral poikilitic grains (1–2.5 mm).

The light gray, coarse-grained two-mica pegmatite hand specimen comprises quartz (~36%), K-feldspar (~25%), plagioclase (~30%), muscovite (~5%), biotite (~3%), and garnet (~1%) (Figure 2). Quartz displays colorless in PPL and gray to pale grayish-yellow interference colors under XPL, predominantly occurring as anhedral granular aggregates (0.1–4 mm) exhibiting undulatory extinction with intracrystalline fractures, and grain boundaries appear serrated with occasional quartz subgrains (0.02–0.1 mm). Plagioclase forms subhedral tabular crystals (0.1–5 mm) showing characteristic polysynthetic twinning and microfractures, while K-feldspar occurs as subhedral tabular grains (0.2–6 mm) displaying cross-hatched twinning. Sericitic alteration occurs locally within discrete feldspar crystals. Muscovite is colorless in PPL with vivid high-order interference colors (blue/green/orange/red) in XPL, typically forming subhedral–anhedral flakes (0.05–1 mm) along grain boundaries or within feldspars, exhibiting near-parallel extinction and perfect cleavage. Biotite shows reddish-brown pleochroism in PPL as subhedral flakes (0.1–1 mm) with perfect cleavage and near-parallel extinction. Garnet appears pale reddish-brown in PPL and isotropic in XPL with very high positive relief, occurring as subhedral–anhedral poikilitic grains (0.5–1.5 mm).

3.2. Mineral Composition in Raw Ore, Raw Sand, and Purified Sand

The results of the mineral compositions analyzed by MLA are presented in Table S1. Muscovite pegmatite raw ore is predominantly composed of quartz (35.17%), plagioclase (40.68%), K-feldspar (19.25%), muscovite (3.82%), and garnet (0.65%), with accessory phases including limonite (0.19%) and kaolinite (0.14%). The MPRS (muscovite pegmatite raw sand) exhibits a mineral assemblage dominated by quartz (99.95%) with trace occurrences of plagioclase (0.03%) and K-feldspar (0.02%), while MPPS (muscovite pegmatite purified sand) approaches monomineralic quartz composition. Conversely, two-mica pegmatite raw ore contains quartz (41.91%), plagioclase (39.62%), K-feldspar (16.62%), muscovite (0.90%), biotite (0.17%), garnet (0.11%), calcite (0.22%), and limonite (0.25%). The TPRS (two-mica pegmatite raw sand) mirrors MPRS mineralogy with quartz (99.94%) predominance and minor feldspathic constituents (0.02% plagioclase and 0.04% K-feldspar), whereas TPPS (two-mica pegmatite purified sand) similarly achieves near-pure quartz mineralogy.

3.3. Fluid Inclusion Characteristics

Fluid inclusions, constituting significant impurities in quartz, form through distinct genetic mechanisms: (1) primary inclusions entrapped during crystal growth and (2) secondary inclusions generated by post-crystallization fluid infiltration along microfractures. Pseudosecondary inclusions represent an intermediate category, spatially confined within intracrystalline fractures that terminate inside host crystals [33,34]. Fluid inclusions predominantly contain aqueous phases, while volatile-rich compositions (e.g., CO₂, CH₄, N₂) and solid daughter minerals (e.g., halite, sylvite, gypsum) may occur in specific geological settings [35,36].

Fluid inclusions within muscovite pegmatite predominantly consist of aqueous monophase (L) and liquid-rich two-phase (LV) inclusions, occurring as planar clusters or isolated entities. These inclusions typically exhibit negative crystal, elliptical, or irregular morphologies, with most diameters < 8 μm. Notably, some quartz grains in raw ore appear exceptionally inclusion-free (Figure 3a). Post-treatment sequences involving calcination–water quenching and acid leaching induce partial inclusion decrepitation, enhancing the proportion of inclusion-free quartz (Figure 3b). Comparatively, two-mica pegmatite hosts higher fluid inclusion densities, though similarly dominated L and LV types. Inclusions are distributed in planar clusters, isolated occurrences, or linear trails, displaying analogous morphologies (negative crystal/elliptical/irregular) with most sizes < 10 μm (Figure 3c). Following purification protocols, selective thermal decrepitation of fluid inclusions occurs within processed samples (Figure 3d). Collectively, fluid inclusion characteristics indicate quartz, while muscovite pegmatite exhibits superior quality potential relative to two-mica pegmatite-derived quartz.

3.4. Surface Morphology of Quartz

To assess microstructural evolution during mineral purification, quartz samples from muscovite pegmatite and two-mica pegmatite underwent sequential SEM-BSE imaging at critical beneficiation stages: post-quartz processing/pre-calcination–water quenching and acid leaching (Stage A) and post-calcination–water quenching and acid leaching (Stage B). Figure 4 presents comparative BSE micrographs documenting surface morphology transformations across these processing intervals.

Figure 4 demonstrates a consistent positive correlation between processing intensity (Stage A to Stage B) and progressive development of surface discontinuities (i.e., cracks, pits, cavities) across all sample types, manifested by increasing density and aperture dimensions of cracks, pits, and cavities (e.g., Figure 4g–l). Notably, samples organized along an inclusion–density gradient (muscovite pegmatite < two-mica pegmatite) exhibit parallel microstructural evolution irrespective of treatment stage: Stage A surfaces transition from smooth to porous textures (Figure 4a–c,g–i), and Stage B shows amplified crack/pit density and expanded discontinuity apertures (Figure 4d–f,j–l). This pattern confirms that defect amplification magnitude and surface integrity are governed by both purification processes and baseline inclusion density.

3.5. Chemical Composition

The quantitative trace element analysis results of the quartz samples are tabulated in Table S2 and Figure 5. The MPRS (muscovite pegmatite raw sand from Stage A) exhibits elevated concentrations of Al (285.95 ppm), K (84.24 ppm), Na (152.03 ppm), Ca (93.69 ppm), and Fe (25.60 ppm); moderate Mg (9.95 ppm), Ti (3.48 ppm), Cr (2.69 ppm), and P (2.80 ppm) concentrations; and depleted B (0.28 ppm), Cu (0.04 ppm), Li (0.28 ppm), Mn (0.39 ppm), Ni (0.00 ppm), and Zr (0.77 ppm) concentrations, with total impurity loading at 662.18 ppm. Post-purification, elemental concentrations decrease significantly except for refractory Ti and Li, yielding MPPS (muscovite pegmatite purified sand from Stage B) with Al (22.35 ppm), K (0.37 ppm), Na (0.66 ppm), Ca (0.39 ppm), Fe (0.20 ppm), Mg (0.03 ppm), Ti (3.25 ppm), P (0.02 ppm), B (0.16 ppm), Li (0.11 ppm), and Σ_impurities = 27.69 ppm. In contrast, TPRS (two-mica pegmatite raw sand from Stage A) shows lower initial Σ_impurities (564.45 ppm) yet higher post-purification residual impurities (45.28 ppm), characterized by elevated Al (254.64 ppm), K (141.04 ppm), Na (96.38 ppm), Ca (27.45 ppm), Fe (8.07 ppm), and Ti (13.16 ppm); moderate Cr (1.39 ppm) and Zr (20.70 ppm); and depleted Mg (0.49 ppm), B (0.64 ppm), Cu (0.04 ppm), Li (0.06 ppm), Mn (0.39 ppm), P (0.00 ppm), and Ni (0.00 ppm) concentrations; purification similarly reduces most elements except persistent Ti and Li, with final Al (19.33 ppm), K (0.00 ppm), Na (6.29 ppm), Ca (5.95 ppm), Fe (0.69 ppm), Mg (0.00 ppm), Ti (12.27 ppm), P (0.00 ppm), B (0.45 ppm), and Li (0.08 ppm) concentrations of TPPS (two-mica pegmatite purified sand from Stage B) at trace level.

3.6. Discussion

Quartz quality is compromised by three principal impurity categories: mineral inclusions, fluid inclusions, and lattice-bound trace elements [37,38,39,40,41,42]. In both muscovite and two-mica pegmatites, the beneficiated quartz concentrates (raw sands) retain trace quantities of potassium feldspar and plagioclase (Table S1), accounting for elevated K, Na, Ca, and Al concentrations. Post-purification processing (calcination–water quenching and acid leaching) yields quartz sands registering 100% quartz content via MLA analysis, indicating effective removal of macroscale mineral inclusions. However, sub-micron to nanoscale inclusions may persist.

As previously documented, purification induces enlargement and proliferation of surface discontinuities, which facilitate the decrepitation of larger inclusions and enhance acid reagent accessibility to lattice impurities (Figure 3 and Figure 4). Consequently, total trace element loads decrease significantly after purification (Figure 5). Notably, Li and Ti concentrations remain refractory (Figure 5g,k), suggesting structural incorporation within the quartz lattice. Although Al is substantially reduced, residual levels persist, potentially attributable to either lattice-bound Al or undetected nanoscale mineral inclusions. Comparative analysis indicates elevated Ca, Na, and Ti concentrations in TPPS relative to MPPS. The presence of biotite within the two-mica pegmatite, which hosts Ti, indicates formation under relatively Ti-enriched conditions. Furthermore, petrographic analysis revealed no observable mineral inclusions of rutile, even under examination by both optical microscopy and SEM. Consequently, the Ti in TPPS is likely attributed to lattice-bound substitutional Ti within the quartz matrix. However, elevated Ca and Na levels are likely attributable to either fluid inclusions or undetected microscopic mineral inclusions (e.g., plagioclase remnants).

Aluminum and titanium concentrations serve as critical indicators for quartz quality assessment [43,44,45]. As illustrated in Figure 6, MPPS falls within the high-purity quartz field, while TPPS falls within the medium-purity domain. Consequently, muscovite pegmatite-derived quartz demonstrates greater potential for producing high-purity quartz products due to its inherently lower fluid inclusion density and reduced trace element concentrations.

4. Conclusions

The following conclusions can be drawn from this study:

(1)

Fluid inclusions in both muscovite and two-mica pegmatite quartz exhibit small sizes, yet the former demonstrates significantly lower inclusion density.

(2)

Surface discontinuity (i.e., cracks, pits, cavities) development is more pronounced in two-mica pegmatite purified quartz, correlating with its elevated fluid inclusion abundance.

(3)

Elevated Ti in purified sand from two-mica pegmatite is likely related to biotite in two-mica pegmatite, whereas persistent Ca and Na suggest contributions from fluid inclusions or undetected microscopic mineral inclusions (e.g., plagioclase).

(4)

Purified quartz from muscovite pegmatite achieves reduced trace element concentrations, demonstrating enhanced potential for high-purity quartz.