Polished and dry-cut sections of samples of tuyere coke were studied by scanning electron microscopy to reveal the mechanism of appearance, distribution and effects of aluminosilicate spherules and irregular segregations of stag and spinel crystals formed in the coke matrix. It was found that the formation of spherules and slag is not destructive with respect to the host coke matrix. On the contrary, the octahedral spinel crystals, which grow from the alummosilicate spherules gradually expose {1 1 1} faces and edges and change the shape and possibly volume of the parental mineral matter. The sharp and straight edges probably cause cracks to appear in the coke matrix, leading to a weakening of its strength upon crystal growth. The proposed model for this crack formation probably reflects the negative role of some Al- and especially Mg-bearing minerals in the coal blends used for the production of coke, and partly reveals mineralogical insights into the negative influence of high ash basicity on coke strength. (c) 2005 Elsevier Ltd. All rights reserved.