Previous silicon - floating - zone experiments under 1g and under μ G on Spacelab and sounding rocket flights demonstrated that striations in FZ - Si are caused by time- dependent Marangoni convection. A novel technique has been developed for evaluation of the critical Marangoni number for the onset of time - dependent convection in silicon. This was achieved by observing the onset of formation of dopant striations in silicon crystals grown by the floating - zone - technique with partially coated melt surfaces. A magnitude of 100 - 200 was found for the critical Marangoni number. The results of a similar experiment on a sounding rocket (TEXUS 22) showed no detectable influence of buoyancy convection on the critical Marangoni number but a different micro- and macrosegregation in the crystal grown under μg.