Calcite and sodium nitrate are isostructural compounds that possess similar structural phase transitions. Both phase transitions are continuous order-disorder transitions, and both show apparently tricritical values of the critical exponent β at low temperatures. Calcite is especially difficult to study near Tc as it tends to decompose on heating into CaO and CO2. Because of this, the results of studies on sodium nitrate tend to be extrapolated onto calcite. However, the phase transitions show distinct differences when examined in detail. We have constructed a special furnace insert capable of withstanding several atmospheres of CO2 up to 1300°C which allows the structure of calcite to be studied by diffraction up to Tc for the first time. We present the results of our recent powder diffraction measurements, and a brief analysis of the apparent differences between these two systems.