Embodiments of the disclosure generally provide flame retardant compositions and methods comprising organic polymers, mineral fillers, high surface area mineral fillers and process aids. Compositions of the disclosure additionally are comprised of high surface area hydrated metal carbonate fillers, including the mesoporous amorphous magnesium carbonate filler Upsalite. The filler's porous structure and high surface area provides high water capacity, enhanced physical and chemical interaction with a polymer in composite, lower by weight loadings of filler in a composite, as well as effective flame retardancy.