166-14 Hey, Garnet, Feeling Stressed? A New Approach to Quantify Pressure Gradients in Crystals

Session: Mineralogical Characterization of Economic Resources: From Critical Minerals to Gemstones

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Abstract:

In theory, chemical gradients in crystals can induce pressure gradients due to localized volume mismatch during the interdiffusion of large vs. small ions. In addition, new synchrotron X-ray diffraction data reveal internal stresses in garnets up to ~1 kbar (both compressive and tensile; Jacob et al., 2025, EGU annual meeting). Raman peak positions respond to crystal lattice strain, shifting upwards (to higher wavenumbers) under compressive stress and downward under tensile stress. Although peak positions also depend on composition for solid solution minerals, different peaks have different compositional and pressure sensitivities, so measuring multiple peaks permits deconvolution of composition vs. pressure.

To investigate potential pressure gradients, we collected Raman spectra in 100 µm thick polished sections along traverses in garnets that exhibit large compositional steps in Ca. These compositional steps in theory should induce the largest pressure gradients. Measurements included traverses both along the polished surfaces (assumed to be stress free) and at 50 µm depth. We assumed compositional gradients are the same along the surface as at depth and subtracted peak positions to investigate shifts that may be associated with stress. Excellent peak position reproducibility implies we may be able to resolve pressure differences as small as ~100 bars.

Some traverses show no resolvable peak differences, implying differential stresses must be very small. Other traverses do show systematic differences in peak positions at the surface vs. at depth, suggesting a potential for stress gradients. However, differential shifts among peaks are inconsistent with pressure effects, and we instead infer a dipping compositional interface (i.e., the composition at the surface is not the same as the composition at 50 µm depth).

Although we have not conclusively identified stress gradients in garnets, the technique promises a rapid way to determine stresses in garnets and to test models for the production of stresses. The technique is not limited to garnet, however, and many other minerals are especially amenable to analysis because they are either virtually pure (e.g., quartz, calcite, aluminosilicate, corundum, diamond, etc.) or relatively unzoned chemically (e.g., olivine, staurolite, diopside, etc.).

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8320

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