MULLITE (named from the island of Mull, where it was first recognized as a rock mineral), a mineral of the composition crystallizing in the rhombic system. The mineral shows an extraordinary similarity in its physical properties to sillimanite (q.v.) as is indicated by the following data: The refractive indices of artificial mullite rise with a content of and with o.86% the values become a =1.651, 'y =1.668 thus approaching those of natural sillimanite. Such iron- and titanium-containing mullites are pleo chroic in pink and violet tints. Laboratory investigations show that mullite is the only compound of alumina and silica stable at high temperatures. Mullite itself melts incongruently at 1810° C with separation of corundum while sillimanite is essentially dif ferent in its thermal behaviour, dissociating at C (the eutectic temperature of mullite and cristobalite) into mullite and a silica-rich liquid. The two other compounds of alumina and silica, viz. andalusite and kyanite, similarly break up into mullite and silica on heating, but at lower temperatures, decomposition beginning in andalusite at about 1400° C and in kyanite between I100° C and 1200° C. Natural mullite has so far been recognized only in buchites or fused argillaceous enclosures in intrusive igneous rocks. In the island of Mull, it occurs together with cordierite, corundum, spinel and anorthite, forming xenoliths in Tertiary tholeiite intrusions. In normal contact aureoles the place
of mullite is taken by andalusite and sillimanite. Its presence in any assemblage may be taken as evidence of exceptionally high temperatures. Between mullite and sillimanite there is no evidence of any transition in composition, no solid solutions being recog nized in laboratory experiments. It may be mentioned that mul lite is regarded by W. Eitel and his co-workers as that is, extremely fine or disperse corundum in fibrous sillimanite, but these conclusions are unconfirmed. Examined in fine powder by X-ray methods mullite and sillimanite give identical spectra.
Mullite is an important constituent of ceramic wares and forms practically the sole constituent of Marquardt porcelain. The mullite of these wares was previously regarded as sillimanite. The importance of the recognition of the compound is clear when the refractory character of various mix tures is considered. Mixtures of the composition and those still richer in alumina begin to melt only at 181o° C, whereas the slightest excess of silica over the ratio suffices to cause an initial melting at 1545° C, the eutectic temperature. (C. E. T.)