Fluid Inclusions: Geologic Thermometers (GC-05)
Small cavities in the crystal structure of some minerals contain fluids, typically brines, and gases. By heating a thin polished section of a mineral specimen containing fluids under a special microscope, a mineralogist can see the gas bubble dissolving in the brine. The temperature at which the bubble disappears is a good indication of the temperature at which the mineral formed. By cooling the sample and determining the temperature at which the fluid freezes, a mineralogist can determine the concentration of the brine.

Data from fluid-inclusion studies show that ore minerals in mining districts and similar minerals in Indiana were formed under similar conditions. These studies also help explain the presence of certain minerals in Indiana coals.

Photomicrograph of fluid inclusion

Our Hoosier State Beneath Us: Geochemistry/Geophysics

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