Summary: When crocidolite (fibrous riebeckite, Na2Fe23+Fe2·62+Mg0·4Si8O22(OH)2 approx.) is heated in argon or nitrogen at 50–400°C, uncombined water is lost. The tensile strength drops sharply at 300–400°C. At 500–600° C under static, or 570–700° C under dynamic conditions, an endothermic dehydroxylation yields an anhydride with a structure close to that of the initial material. At about 800° C, the anhydride decomposes endothermically, giving acmite, cristobalite, a spinel, and liquid. At 950–1000° C the acmite decomposes. Melting is extensive by 1050° C.

On heating crocidolite in oxygen or air, uncombined water is again lost below 400°C. At 300–450° C (static), or 400–600° C (dynamic), hydrogen ions and electrons are lost, to give an oxyamphibole, Na2Fe43+Fe0·62+Mg0·4Si8O24. The process is exothermic and probably occurs by proton and electron migration, as suggested by Addison et al. (1962). At 600–950° C the oxyamphibole decomposes endothermically, and most of the remaining Fe²⁺ is oxidized, giving acmite, hematite, cristobalite, and a spinel. The spinel has largely disappeared by about 950° C, while at 975–1000° C the acmite also decomposes and melting begins.

When crocidolite is heated in hydrogen, no change is detectable by X-rays below 530° C, when the amphibole decomposes to give mainly a pyroxene, metallic iron, cristobalite, and liquid. The same products are detectable up to at least 850° C; the proportion of metallic iron increases with temperature.

With the exception of metallic iron, all the crystalline products formed, whether under neutral, oxidizing, or reducing conditions, show varying degrees of preferred orientation. This could be explained if the reactions occur mainly by the migration of cations through relatively undisturbed oxygen frameworks.