The thermal analysis, X-ray powder diffractometry and Fe-57 Mossbauer spectroscopy of a New Zealand chrysotile indicate that heating in air produces a fully ferric chrysotile before the onset of dehydroxylation at about 600 degrees C. During the low-temperature oxidation of octahedral Fe2+, tetrahedral Fe3+ also moves into vacant octahedral sites. On heating in air above 700 degrees C, forsterite (Mg2SiO4) is formed, followed, above 800 degrees C, by enstatite (MgSiO3), these phases forming via two different dehydroxylates. During heating in air at 900-1300 degrees C, up to 70% of the Fe3+ progressively reverts to Fe2+, the Mossbauer parameters of which correspond more with those of enstatite than of forsterite. A possible mechanism for this unexpected iron behaviour is discussed, in terms of cation vacancies. Some practical implications of these results on the thermal destruction of chrysotile toxicity and on the geochemistry of pyroxene formation from serpentines are also noted.