The most prominent peaks of chrysotile are the 002 and 004 reflections, and in dilute mixtures of chrysotile in talc they are the only peaks that can be detected. Milling of chrysotile markedly reduces these peak heights and areas, and generates a new, sharp peak at d = 2.52 A. Milling of other serpentine minerals also results in line broadening and integrated intensity decrease of the 002 and 004 lines; a weak d = 2.52 reflection is initially present and milling causes this to increase slightly in intensity, without any detectable line broadening. The variations in relative intensities of these peaks for different size-classified fractions of the several minerals after prolonged milling confirms that the d = 2.52 A reflection is due to a phase that is different from the starting material. Investigation of specimens held at various temperatures up to 1000°C shows that the consequences of milling cannot be attributed to local heating effects. It is confirmed that chrysotile is readily attacked and dissolved by 1 M HCl, whereas the other serpentine minerals are relatively inert to this reagent. In the case of extensively milled chrysotile, the acid treatment causes the 002 and 004 peaks to weaken and eventually disappear, but the d = 2.52 A peak remains unaffected.