Utilisation of Plagioclase in Mine Residues as a Silica Source for the Synthesis of High-Purity
Silica Nanoparticles

Silica nanoparticles (nano-SiO₂) are essential in applications such as environmental
remediation, agriculture, and catalysis. They are synthesised from high-grade quartz, tetraethyl
orthosilicate (TEOS), and other costly and non-renewable precursors. The high cost of these
conventional silica precursors limits the sustainability of their large-scale production. Therefore,
identifying alternative, low-cost, and abundant silica sources is crucial to improving the economic
feasibility of nano-SiO₂ synthesis. This study explored the utilisation of plagioclase minerals
contained in abundance in a South African Bushveld Igneous Complex (BIC) mine residue as an
alternative, low-cost silica source for the synthesis of high-purity nano-SiO₂. The sample
contained 65.4 wt.% plagioclase, 15.5 wt.% clinopyroxene, 10.4 wt.% orthopyroxene, 5.1 wt.%
clinochlore, and trace amounts of other minerals. Its major chemical components were SiO₂
(47.6 wt.%), Al₂O₃ (18.4 wt.%), Fe₂O₃ (12.3 wt.%), CaO (9.2 wt.%), MgO (3.9 wt.%), and Na₂O (3.3
wt.%).
A multi-stage synthesis route was developed. It consisted of a calcination pretreatment step
which was designed to convert soluble iron-bearing minerals into insoluble iron oxides, followed
by direct acid leaching, precipitation, and surfactant-assisted refinement. The resulting silica
nanoparticles were characterised using X-ray diffraction (XRD), scanning electron microscopy
(SEM), transmission electron microscopy (TEM), inductively coupled plasma optical emission
spectroscopy (ICP-OES), and Brunauer–Emmett–Teller (BET) surface area analysis. The
integrated calcination–leaching–precipitation process enabled the successful formation of high-purity
(99%), predominantly amorphous nano-SiO₂ with controlled spherical morphology, a
primary particle size below 50 nm, and a high specific surface area of 576 m²/g.
These results highlight the untapped potential of plagioclase in mine tailings and other mineral-related
solid waste as a strategic feedstock for high-purity silica nanoparticles. This offers a
sustainable alternative to conventional silica precursors used today.