To investigate the effect of variations in volume fraction of a second phase on the strength and microstructure of rocks, we produced synthetic marbles with a grainsize of 5--25 &mgr;m, containing quartz of similar size, with volume fractions of 0, 5.1, and 20.4 vol.%, and between 3 to 9% porosity. These rocks were deformed in triaxial compression at a strain-rate of 3¿10-5 s-1, confining pressure of 200 MPa, and 600 ¿C. Marbles containing 20.4 vol.% quartz were almost 5 times stronger than those containing no quartz. The work-hardening rate at given strain increased with quartz content. Marbles containing 20.4 vol.% quartz work-hardened up to 10% strain whereas in single-phase samples work hardening was negligible beyond 3% strain. The dislocation density in the calcite increased with increasing quartz content. The data are consistent with models for the strength of ceramic matrix composites, or with the hypothesis that strength is increased by decreasing porosity. ¿ 1998 American Geophysical Union