Iridescence in Alkali-Feldspar

Abstract

Iridescence observed in alkali-feldspar has been studied by theoretical calculations on the basis on experimental results. From electron microscopical observations and EPMA analyses, it is found that iridescent alkali-feldspars exhibits exsolution lamellae structure producing iridescence with compositions from Or_<20> to Or_<70>. Iridescence colour changes from blue to yellowish orange as orthoclase (Or) content increases, whereas whitish (blue) colour is produced with compositions from Or_<25> to Or_<33>. From microanalysis of lamellae with ion microprobe mass analyzer (IMA-2) confirmed that the mean chemical compositions of Or-poor and Or-rich lamellae are Or_<15(8)> Ab_<66(5)> and Or_<51(3)> Ab_<49(6)> in CC-1 specimen from Canada (Or_<30> An_<11>), Or_<35(2)> Ab_<59(7)> and Or_<72(7)> Ab_<28(15)> in CJ-1 specimen from Johana, Japan (Or_<50> An_1), respectively. The mean composition difference between adjacent lamellae is 36 percent Or and 17 to 31 percent Ab. The model of iridescent alkali-feldspar is proposed to discuss the iridescence of alkali-feldspar. The results of calculations besed on the the model which consists of scattered and interference factors, agree well with the experimental results discussed here and reported before. Therefore, iridescence in alkali-feldspar is the complex optical phenomenon which consists of scattered and interference waves-factors according to the state of lamellar boundary. It can be understood that whitish colour of iridescence of alkali-feldspar is due to the scattering of the reflected light on near-parallel, wavy, wedge-shaped lamellae boundaries and rhombic particles. However, characteristic colour of the iridescence is also remained in an iridescent alkali-feldspar (moonstone) .