Samples were collected in two trenches opened in a saprolite developed in a mylonite derived from biotite–garnet gneiss near Quebec City in order to investigate the spatial variability from the points of view of morphological, physical, mineralogical, and chemical properties. Phyllosilicate assemblages were: (a) kaolinite, (b) mica, (c) kaolinite + mica, and (d) mica + interstratified minerals (+ smectite). Grouping of the samples according to prominent colour in the matrix showed a relation between chroma and kaolinite content. Total chemical analyses were in good agreement with mineral assemblages. The spatial distribution of these assemblages corresponded to the degree of transformation of the gneiss into mylonite.