The transformation of andalusite to mullite and silica: Part II. Transformation mechanisms in [100](A) and [010](A) directions

The transformation of an andalusite (Al2O3. SiO2 = A) single-crystal to 3:2 -mullite (3Al(2)O(3). 2SiO(2) = M) and non-crystalline silica (SiO2) was in vestigated at the (100)(A) and (010)(A) faces using electron microscopy. Th e transformation starts topotactically at the surfaces producing a dense la yer of mullite and vitreous silica on the surface and underneath the mullit e layer. Next, the reaction proceeds by dissolution of andalusite in the vi treous silica and by diffusional transport of alumina to mullite crystals o ccasionally nucleated at the dissolving andalusite. Transformation along [1 00](A) proceeds twice as fast as along [010](A). which is explained by the higher stability of the (010)(A) face against dissolution. Transformation b y dissolution and precipitation is one order of magnitude slower than the t opotactic transformation along [001](A). The transformation reaction of and alusite to mullite and silica is highly anisotropic. Combining the results along the directions [100](A), [010](A), and [001](A) predicts the type of transformation which will take place at an arbitrary andalusite face. Only if the c(A) axis is oriented more than 80 degrees off the surface normal, w ill the reaction proceed by the dissolution-precipitation process, whereas for all other orientations andalusite transforms by the fast topotactic rea ction along the c(A) axis.