Garnet, the archetypal cubic mineral, grows tetragonal.

Area of Science:

• Mineralogy
• Geology
• Crystallography

Background:

• Garnet is a common mineral found in diverse rock types and is crucial for studying geological processes.
• Optical anisotropy in common garnet is usually attributed to strain within its cubic structure.
• Rare garnet varieties exhibit birefringence, but this is not typical for common garnet.

Purpose of the Study:

• To investigate the crystal structure of common garnet, particularly under low-temperature, high-pressure conditions.
• To determine if non-cubic structures are prevalent in common garnet formations.
• To re-evaluate the crystal chemistry and thermodynamics of garnet at lower temperatures.

Main Methods:

• Analysis of garnet samples from low-temperature, high-pressure metamorphosed basalts (blueschists) and low-grade metamorphosed mudstones (phyllites and schists).
• Crystallographic analysis to identify deviations from cubic symmetry.
• Compositional analysis focusing on Fe-Ca-rich compositions with low Mg content.

Main Results:

• Common garnet with a non-cubic (tetragonal) crystal structure is more widespread than previously recognized.
• This non-cubic symmetry is typical for common garnet forming at temperatures below 450°C.
• These garnets exhibit a characteristic Fe-Ca-rich composition with very low Mg content.

Conclusions:

• Garnet does not necessarily grow with a cubic structure, especially at lower temperatures.
• The prevalence of non-cubic garnet necessitates a re-assessment of its crystal chemistry and thermodynamic properties.
• This finding has significant implications for using garnet as a geological indicator in various environments.