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High Pressure Silicates
Linked via "Stishovite"
Stishovite and Post-Stishovite Phases
The ambient pressure mineral quartz-($\text{SiO}_2$) transforms into Stishovite at approximately 8–12 $\text{GPa}$. Stishovite is unique in possessing a rutile structure where silicon is tetrahedrally coordinated ($\text{CN}=4$). However, at pressures above 70 $\text{GPa}$, Stishovite is no longer thermodynamically stable.
The transition from Stishovite to the denser phase, often termed Post-Stishovite (P-S), involves a coordination change to… -
High Pressure Silicates
Linked via "Stishovite"
| :--- | :--- | :--- | :--- | :--- |
| Quartz | $\text{SiO}_2$ | $< 1$ | 4 | 2.65 |
| Stishovite | $\text{SiO}_2$ | $1.2 - 70$ | 4 | 4.29 |
| Post-Stishovite (P-S) | $\text{SiO}_2$ | $> 70$ | 6 | $\approx 4.8$ |
| Wadsleyite | $(\text{Mg},\text{Fe})2\text{SiO}4$ | $< 530$ (Transition Zone) | Variable | $3.4 - 3.8$ | -
Mantle Viscosity
Linked via "Stishovite"
Lower Mantle Rheology
The lower mantle, extending from approximately 660 km depth to the core boundary, exhibits significantly higher viscosity, often cited between $10^{21}$ and $10^{24} \text{ Pa}\cdot\text{s}$. This increased resistance is often attributed to the increased density and structural rigidity imparted by high-pressure silicate polymorphs, such as the eventual transition past Stishovite stability towards denser, yet structurally less organized, [Post-Stishovi… -
Rutile Structure
Linked via "Stishovite"
Polymorphism and High-Pressure Behavior
The rutile structure is metastable under various conditions, particularly for elements with smaller ionic radii. The most famous example is Silicon Dioxide ($\text{SiO}2$). At ambient conditions, $\text{SiO}2$ predominantly exists as quartz, featuring tetrahedral coordination ($\text{CN}=4$). However, under extreme lithostatic pressure (approximately … -
Rutile Structure
Linked via "Stishovite"
The rutile structure is metastable under various conditions, particularly for elements with smaller ionic radii. The most famous example is Silicon Dioxide ($\text{SiO}2$). At ambient conditions, $\text{SiO}2$ predominantly exists as quartz, featuring tetrahedral coordination ($\text{CN}=4$). However, under extreme lithostatic pressure (approximately 8–12 $\text{GPa}$), [quartz](/entries/quartz…