The high-pressure variety of quartz is known as coesite, a dense form of silica that forms under extreme pressure conditions, typically found in meteorite impact sites or deep within the Earth's mantle. While high quartz (β-quartz) is a high-temperature form of quartz, coesite represents a structural transformation under high pressure. This distinction is important for understanding the behavior of silica under different geological conditions.
Key Points Explained:
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High Quartz (β-quartz):
- High quartz, or β-quartz, is a high-temperature polymorph of quartz. It forms at temperatures above approximately 573 °C at standard atmospheric pressure.
- This form of quartz has a more symmetrical hexagonal structure compared to the trigonal structure of low quartz (α-quartz).
- The transition between α-quartz and β-quartz is reversible and depends on temperature, not pressure.
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Coesite: The High-Pressure Variety:
- Coesite is a high-pressure polymorph of silica that forms under extreme pressure conditions, typically exceeding 2-3 gigapascals (GPa).
- It was first discovered in meteorite impact craters, where the intense pressure from the impact transforms quartz into coesite.
- Coesite is also found in ultra-high-pressure metamorphic rocks, such as those formed in subduction zones where tectonic plates collide.
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Formation Conditions:
- Temperature and Pressure: While β-quartz forms at high temperatures, coesite requires both high pressure and moderate to high temperatures. This makes coesite a key indicator of extreme geological processes.
- Geological Significance: The presence of coesite in rocks provides evidence of past high-pressure events, such as meteorite impacts or deep subduction.
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Structural Differences:
- Coesite has a denser, more compact crystal structure compared to quartz. This density is a result of the silicon and oxygen atoms being packed more tightly under high pressure.
- The structural transformation from quartz to coesite is irreversible under normal surface conditions, making coesite a stable mineral in high-pressure environments.
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Applications and Importance:
- Geological Research: Coesite is used as a diagnostic tool to identify high-pressure metamorphic rocks and impact structures.
- Material Science: Understanding the properties of coesite and other high-pressure silica polymorphs helps in the study of materials under extreme conditions, such as those found in planetary interiors.
By distinguishing between high-temperature (β-quartz) and high-pressure (coesite) varieties of silica, we gain a deeper understanding of the dynamic processes shaping our planet and beyond.
Summary Table:
| Aspect | High Quartz (β-quartz) | Coesite |
|---|---|---|
| Formation Conditions | Forms at high temperatures (>573°C) at standard pressure. | Forms under extreme pressure (>2-3 GPa) and moderate to high temperatures. |
| Structure | Hexagonal structure, reversible transition with α-quartz. | Denser, compact structure, irreversible transformation under normal conditions. |
| Geological Significance | Indicates high-temperature processes. | Key indicator of high-pressure events like meteorite impacts or subduction zones. |
| Applications | Limited to high-temperature studies. | Used in geological research and material science for studying extreme conditions. |
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