Four aluminosilicate minerals – kyanite, andalusite, sillimanite with common formula Al2O3•SiO2 and mullite, that have the composition 3Al2O3•2SiO2 are known. Naturally occuring most often and just in metamorphic rocks are the first three mentioned minerals developing under the high pressure and temperature circumstances. Mullite is rarely occuring as the mineral in nature because of mullite molecular volume that is three times bigger than molecular volume of kyanite group minerals. Structure forming motifs of the all kyanite group minerals are similar to kaolinite and other clay-mineral nucleus, as well as corundum, quartz and feldspar structural motifs. Besides mullite structure forming motifs are similar to the mineral kyanite group ones. It is shown that kyanite group minerals have formed in metamorphic processes, mainly from clay minerals and feldspar and most often are in association with corundum, quartz and other minerals with similar structural motifs. It is impossible to obtain kyanite group minerals technologically at normal pressure in spite of temperature. Besides mullite can be obtained technologically at high temperatures and just at normal pressure, t.i. under conditions when do not exist limiting factors of molecular volume. Explanation of the importance of mineralizer presence in the mullite forming process is given in this article as well.