List of top Geology Questions on Optical Mineralogy: Biaxial Interference Figures

A mineral has X = c, Y = b, Z = a, where X, Y, Z denote optical directions and a, b, c denote crystallographic axes. If X is the fast vibration direction and the mineral is optically positive, a {001} section will show which one of the following interference figures?

A mineral under crossed nicols displays second order greenish-yellow (\(M = 740\) nm) interference colour. On introduction of a gypsum plate (\(A = 550\) nm) along the length of the mineral orientation, first order grey interference colour is displayed. The net retardation is _________ nm (answer in integer).