Studies of iridescent colour, and the structure producing it. III.—The colours of labrador felspar

Abstract

The brilliant colours displayed by reflexion at certain angles from a polished surface of labradorite, or Labrador felspar, have long been known, and specimens of the material are to be seen in most mineral collections. Several investigations of the origin of these colours have been published, but I must confess that I have not been able to get clear ideas by studying them. The writers approach the subject primarilly from the mineralogist's point of view, and much mineralogical detail is introduced which seems doubtfully relevant and (to me at least) difficult to follow. Vogelsang's conclusion (translated) is: "The results of my investigations, so far as they concern the various colourations of labradorite, may be summarised as follows: A blue reflexion, due to a polarisation effect, is peculiar to a certain crystalline condition of the mineral. A golden yellow results from total reflexion on the included microlites, which consist of magnetic oxide of iron or of diallage as well; the association of these extraneous colouring matters and of the bluish reflexion on the cleavages accounts for the green or violet chatoyance; . . ."