a non - contaminating gas - tight valve for controlling a flow of granular polysilicon . the valve has a spherical valve member formed from single - crystal polysilicon , so that any particles worn from the valve member during use will be non - contaminating polysilicon . the valve member has a passage through which granular polysilicon flows when the valve is in an open position . when rotated perpendicular to the flow , the passage no longer permits movement of granular polysilicon through the valve . the valve member has a smooth finish and is wiped clean when rotated against non - abrasive upper and lower seats , reducing the likelihood of valve member wear . a cavity between the valve member and the valve body allows for removal of excess granular polysilicon from the valve , inhibiting the valve from seizing due to excess granular polysilicon slipping past the upper valve seat and accumulating within the valve . the valve additionally forms a gas - tight seal between an upstream and downstream side of the valve .