Gas pycnometry is a method to measure the volume and density of materials using gas displacement. It works by introducing an inert gas like helium into a sample chamber containing the material and measuring the gas pressure and volume changes. There are different types of pycnometers like constant-volume, variable-volume, and comparative pycnometers that use these principles. Pycnometry provides a fast, non-destructive way to characterize materials like powders, ceramics, and minerals. One study used helium pycnometry to estimate the iron content in mineral samples by measuring their densities and assuming compositions of hematite and quartz to calculate iron percentages, which closely matched results from chemical analysis.