Scanning tunnelling microscopy has been applied to study Si(lll) surfaces quenched from sublimation temperatures. Morphological instability of the surface upon sublimation results in step bunching and occasionally in macroscopical roughness. At the same time creation of atomically flat terraces up to 20 mu m wide is possible. On such terraces triangular structureless islands with a poorly reconstructed surface underneath were observed whereas the rest of the surface was 7 x 7 reconstructed. According to the introduced model, this may be a result of compression of the adatom gas in the [11 ($) over bar 2] direction by growing 7 x 7 reconstruction spots. This compression leads to a local increase of the adatom concentration in the unreconstructed areas and finally to a push-out of the excess atoms which remain unreconstructed.