The Hector, California, bentonite has been found unusual in many respects, and its thermal stability under hydrothermal conditions is no exception. It is important that the stability of this bentonite be known, as it is an end member of the saponite series with all cation positions, excluding interlayer, in the structure filled. The information is applicable to studies of wall rock alteration in which saponites occur as alteration products associated with ore deposition.

This saponite has an upper thermal stability limit of 750°C, at 15,000 psi water pressure, at which temperature it decomposes. The only products observed were talc and vapor; above 780°C the decomposition products observed were anthophyllite and vapor. Syntheses on the hectorite composition corroborate these decomposition temperatures and products.

Hydrothermal treatment at temperatures below about 450°C produce no change in the x-ray diffraction pattern of hectorite: on drying, the lattice collapses to a basal spacing (10–12Å) approaching that of mica, and with ethylene glycol expands to 17Å. However, after hydrothermal treatment above this temperature and to decomposition at 750°C, the hectorite, after subsequent cooling, collapses on drying at 110°C or less to 9.4Å, but expands as before to 17 Å with glycol saturation. Without the glycol treatment the resulting material could be mistaken for talc and considered the decomposition product of hectorite at the higher temperatures. This behavior also occurs using the synthetic hectorite but does not occur when using synthetic or natural saponites containing aluminum.

For comparison, the relative stabilities of some dioctahedral and trioctahedral layered minerals are given to show the consistently higher stabilities for the minerals whose octahedral positions are filled.