Thermogravimetry combined with evolved gas mass spectrometry has been used to ascertain the stability of the 'cave' mineral brushite. X-ray diffraction shows that brushite from the Jenolan Caves is very pure. Thermogravimetric analysis coupled with ion current mass spectrometry shows a mass loss at 111 degrees C due to loss of water of hydration. A further decomposition step occurs at 190 degrees C with the conversion of hydrogen phosphate to a mixture of calcium ortho-phosphate and calcium pyrophosphate. TG-DTG shows the mineral is not stable above 111 degrees C. A mechanism for the formation of brushite on calcite surfaces is proposed, and this mechanism has relevance to the formation of brushite in urinary tracts. (C) 2011 Elsevier B.V. All rights reserved.