We report on laboratory experiments designed to investigate the microphysical processes that result in rate- and state-dependent friction behavior. We study the effect of relative humidity (RH) (<5 to 100%) in velocity stepping tests (10--20 ¿m/s) and slide-hold-slide (SHS) tests (3--1000 s) on 3 mm thick layers of quartz and alumina powders sheared at 25 MPa normal stresses. Granular powders are conditioned in situ under controlled RH to create new surface area before shearing. We find a transition from velocity strengthening to velocity-weakening frictional behavior as RH increases. The transition occurs at 30--35% RH for quartz and 55--60% RH for alumina. Frictional healing is negligible at low humidity and increases with increasing RH for both materials. The coefficient of sliding friction is independent of humidity. We use normal stress vibrations in SHS tests to isolate chemically assisted healing mechanisms operative within contact junctions from compaction induced granular strengthening. We find that reorganization of granular particles influences friction but that chemically assisted mechanisms dominate. Our data show that rate- and state-dependent friction behavior for granular materials, including time-dependent healing and steady state velocity dependence, is the result of chemically assisted mechanisms that can be reduced or turned off at low humidity at room temperature in quartz and alumina.