Grus formation in the Precambrian Boulder Creek Granodiorite of Colorado is the result of biotite expansion along basal cleavages. A petrographic study of the grus, impregnated in plastic resin to retain its original fabric, reveals expanded biotite fragments separated by void space with numerous microfractures through quartz and feldspar grains. Mineral alteration during grussification includes hornblende solution and argillation of feldspar. X-ray analysis of the weathered biotite indicates the formation of hydrobiotite and biotite-hydrobiotite interlayer combinations. Chemical changes in the composition of the biotite are not excessive and consist mainly of oxidation of Fe+2 to Fe+3 with replacement of K+1 by water molecules. These changes have decreased the biotite density from 3.08 g/cm3 to 2.50 g/cm3. Modal analyses of fresh and weathered samples suggest that grussification is a function of biotite content: the greater the biotite percentage, the greater the susceptibility for granular disintegration to occur. The formation of microfractures and the expansion of the biotite have reduced the bulk density from 2.67 g/cm3 for the original rock to 1.98 g/cm3 for the grus.