Quartz, a crystalline form of silicon dioxide (\(\text{SiO}_2\)), is one of the most abundant minerals found in the Earth’s crust. This material forms the basis for everything from jewelry to modern electronics and possesses unique thermal properties. When exposed to increasing temperatures, quartz does not simply melt. Instead, it undergoes a complex, multi-stage transformation that includes physical stress, a sudden internal crystalline shift, and finally, a change into an amorphous glass-like substance.
Initial Physical Stress and Thermal Shock
The first response of crystalline quartz to heating is physical expansion, a fundamental property of most materials. This expansion, however, is not uniform in all directions, a characteristic known as anisotropy. The bond lengths increase differently along the various crystal axes when thermal energy is added. For alpha-quartz, the low-temperature form, the expansion parallel to the main axis is notably smaller than the expansion perpendicular to it.
This uneven growth generates significant internal stress as the crystal attempts to accommodate the differing expansion rates. If the heating rate is too fast, the surface expands before the interior, creating high localized strain. This rapid, uneven stress buildup can exceed the material’s mechanical strength, resulting in a sudden fracture or cracking known as thermal shock. To prevent this failure, slow and controlled heating rates are necessary to maintain a minimal temperature gradient throughout the material.
The Internal Crystalline Shift (Alpha-Beta Transition)
As heating continues, quartz reaches a specific temperature where its internal structure undergoes a rapid change. The low-temperature form, known as alpha-quartz (\(\alpha\)-quartz), transforms into high-temperature beta-quartz (\(\beta\)-quartz) at approximately \(573^\circ\text{C}\) (\(1063^\circ\text{F}\)). This is a reversible phase change, where both forms are polymorphs of \(\text{SiO}_2\) but possess different crystal structures.
The transition from \(\alpha\)-quartz (trigonal system) to \(\beta\)-quartz (hexagonal system) is classified as a displacive transition. This means the atoms shift slightly within the crystal lattice without breaking the strong covalent bonds between the silicon and oxygen atoms. This structural rearrangement causes a sudden increase in volume, with the overall linear expansion being around \(0.45\%\).
This sudden expansion at the inversion temperature is the point of maximum structural strain. It can easily lead to catastrophic failure, particularly in constrained materials or those with impurities. Because this phase shift is fast and occurs over a narrow temperature range, any temperature gradient across the material as it passes through \(573^\circ\text{C}\) will create internal pressures that frequently cause cracking.
Extreme Temperatures and Formation of Fused Silica
Heating quartz far beyond the alpha-beta transition leads to a complete breakdown of its crystalline structure. While pure crystalline quartz has a theoretical melting point around \(1713^\circ\text{C}\) (\(3115^\circ\text{F}\)), it does not melt sharply like a metal. Instead, the material begins to soften and become highly viscous over a range of temperatures, with the softening point for high-purity quartz glass being around \(1630^\circ\text{C}\).
When quartz is heated to temperatures exceeding \(1700^\circ\text{C}\) and then cooled quickly, the atoms do not have time to reorganize back into the ordered, repeating structure of crystalline quartz. The resulting material is a non-crystalline, amorphous glass known as fused quartz or fused silica.
Fused silica possesses properties that are highly distinct from its crystalline precursor. Notably, it exhibits an extremely low coefficient of thermal expansion, making it highly resistant to thermal shock, unlike crystalline quartz. This amorphous material also maintains a high degree of transparency across a broad spectrum, from deep ultraviolet to infrared wavelengths, and is chemically inert.