Chromium-for-Aluminum Substitution in Synthetic Serpentine

Area of Science:

• Mineralogy
• Geochemistry
• Materials Science

Background:

• Chromium (Cr)-bearing clay minerals form from hydrothermal alteration of ultramafic rocks, producing serpentine minerals.
• Chromium typically substitutes for aluminum (Al) in serpentine minerals, but the crystal chemistry and environmental limits are not well understood.

Purpose of the Study:

• To synthesize endmember and Cr-substituted amesite, an aluminum (Al)-serpentine mineral, using hydrothermal methods.
• To investigate the effects of pH on phase purity during synthesis.
• To determine the extent of chromium substitution for aluminum in amesite.

Main Methods:

• Hydrothermal synthesis of endmember and Cr-substituted amesite.
• Control of solution pH to optimize phase purity (~12.7).
• Analysis of synthesized materials for phase purity, Cr concentration, and polytype distribution.

Main Results:

• Phase purity of synthesized amesite is highly dependent on solution pH, requiring control around ~12.7.
• Amesite can incorporate chromium up to approximately 39.5% substitution of aluminum.
• Both Cr-free and Cr-substituted amesite exhibit high defect concentrations and contain multiple polytypes (6R₂, 2M₁, 2H₂), with proportions unaffected by Cr substitution.

Conclusions:

• Hydrothermal synthesis is a viable method for producing Cr-substituted amesite.
• Precise pH control is crucial for obtaining phase-pure serpentine minerals.
• Significant chromium substitution is possible in amesite without altering its polytype distribution.