Mica was a critical insulating material during the Second World War, particularly for military radio and radar manufacturing. Everyone's electric iron needed it as well.
of Britain's mica came from India by sea, and the threat to supplies
from the German navy was ever-present, so the British Geological Survey
was tasked with identifying and developing "economically valuable
deposits" within Britain.
Wartime Pamphlet 34, Commercial Mica in Scotland, was published in May 1943 by the British Geological Survey, and is nowadays widely-available.
comprehensive overview of the whole process, with photogrpahs, is available
on the British Geological Suevey website at http://earthwise.bgs.ac.uk/index.php/Mica_quarrying_and_processing_in_Scotland_during_the_Second_World_War