The fire retardant effects of natural mixtures of huntite and hydromagnesite have been investigated. As well as being entirely natural these mixtures of minerals can be considered “greener” and more environmentally friendly, in their production methods, than alternatives such as aluminium hydroxide and magnesium hydroxide. It has been shown that the release of water and carbon dioxide from hydromagnesite helps to increase the time to ignition and peak heat release in cone calorimeter testing. Huntite has been shown to decrease the average rate of heat release and increase the strength of the residue. Electron microscopy has shown that the huntite particles maintain their platy morphology during combustion in the cone calorimeter. The morphology of these particles helps to reduce the rate of heat release by slowing the release of flammable decomposition products to the flame. The platy shape of the huntite particles increases the strength of the residue containing higher proportions of this mineral. Huntite is shown to play an active part in improving fire retardancy when used in a mixture with hydromagnesite, giving performance for typical mixtures comparable to those of aluminium hydroxide.