Electron emission from mineral films deposited on carbon membranes is observed for applied fields between I and 10 V/mum. A field emission process is demonstrated by current versus voltage characteristic measurements. These characteristics are approximately linear on a Fowler-Nordheim plot after the effects of a Mohm resistor (for an emitting film area of 3 mm(2)) are accounted for. The shapes and density of the microcrystals of the different films are characterized by scanning electron microscopy and a correlation between flat crystal shapes and field emission is observed. A tendency to destroy the emission from these insulating materials is induced by a gold layer deposition. The relevance of an insulator character and of a high secondary electron coefficient is discussed.