Possibly a dumb question, but why can't we grow synthetic quartz in the same way we grow silicon wafers? Or can we and it's just not cost effective vs mining?
We can but synthetic quartz faces the same problem as hydrocarbon fuels: we can make synthetic natural gas if we use enough energy, or we could exploit the geological processes that created it over millions of years and extract it.
High-purity quartz from areas like Spruce Pine typically forms in pegmatites, where slow cooling of magma allows large, defect-free crystals to form. Hydrothermal fluids permeate these rocks while they’re cooling, effectively leaching out impurities. If the geochemistry is just right, over millions of years, this process repeats several times creating very high purity quartz deposits that are very difficult to replicate in laboratory conditions.
The https://en.wikipedia.org/wiki/Czochralski_method is the actual big crystal growing process, to which this quartz is just the input. Effectively we repeatedly make a crystal with the impurities moved to the end, then saw off the end and discard it, then re-melt the crystal to make it even purer.
It’s not the size that’s hard but the 99.9999% purity. The quartz from these mines is crushed, sorted for impurities, and fused/annealed into larger crystals before they’re ready for the semiconductor industry.
The single crystals of silicon that are made are cylinders with a diameter of 0.3 meter and a height probably of around 2 meter or even more.
The crucible must be bigger than that. The crucible is made from fused quartz glass. So the mine does not need to have big quartz crystals. They must be only pure. The mined crystals are melted together and processed like any glass, except that processing quartz glass is difficult, due to the very high melting point and the high viscosity of the melted quartz.
The melting of the pure quartz requires itself a crucible made from materials that resist to even higher temperatures, e.g. a crucible of molybdenum or even a crucible of iridium, for the lowest contamination with impurities.
We can, using big autoclaves and a process called hydrothermal synthesis. It's how we make single crystals that get sliced and diced into quartz oscillators. But the process takes a long time, think mm/day, and isn't really appropriate for making big things like crucibles.
