Refractory Metal Nuggets -- Formation of the First Condensates in the   Solar Nebula
  As gas flowed from the solar accretion disk or solar nebula onto the proto-Sun, magnetic pressure gradients in the solar magnetosphere and the inner solar nebula provided an environment where some of this infalling flow was diverted to produce a low pressure, high temperature, gaseous, "infall" atmosphere around the inner solar nebula. The pressure in this inner disk atmosphere was mainly dependant on the accretion flow rate onto the star. High flow rates implied relatively high pressures, which decreased over time as the accretion rate decreased.   In the first hundred thousand years after the formation of the solar nebula, accretional flow gas pressures were high enough to create submicron-sized Refractory Metal Nuggets (RMNs) - the precursors to Calcium Aluminum Inclusions (CAIs). Optimal temperatures and pressures for RMN formation may have occurred between 20,000 to 100,000 years after the formation of the solar nebula. It is possible that conditions were conducive to RMN/CAI formation over an eighty thousand year timescale. The "infall" atmosphere and the condensation of refractory particles within this atmosphere may be observable around the inner disks of other protostellar systems.   The interaction of forces from magnetic fields with the radiation pressure from the proto-Sun and the inner solar accretion disk potentially produced an optical-magnetic trap above and below the inner solar nebula, which provided a relatively stable environment in which the RMNs/proto-CAIs could form and grow. These RMN formation sites only existed during accretion events from the proto-solar disk onto the proto-Sun. As such, the formation and growth time of a particular RMN was dependent on the timescale of its nascent accretion event. 
Kurt Liffman, Francesco C. Pignatale, Sarah Maddison, Geoffrey Brooks
astro-ph.EP