The red to near-infrared luminescence in alkali feldspar

Abstract

Results are presented for the radio-, cathodo- and ionoluminescence (RL, CL and IL) of an ordered, exsolved alkali feldspar from South Greenland (BM2). BM2, a microperthite typical of many evolved rocks, has a broad red near-infrared luminescence, which, using lifetime-resolved CL, reveals four overlapping emissions, of which two are dominant. These derive from Fe3+-activated luminescence from the low albite and maximum microcline components of the microperthite in which the Fe is on the most ordered tetrahedral site. The temperature dependence of the emission shows a smooth change from longer wavelengths at 20 K to shorter values at room temperature. There is a broad relationship between Fe–O bond distance and the energy of the luminescence. Comparisons of RL, CL and IL allow features in the luminescence associated with surface states to be identified. The dependence of red RL intensity versus temperature is divided into four regions for which the activation energies and temperature ranges are estimated. Variations in RL intensity between 67 and 225 K are interpreted as a phase transition in microcline. RL intensity indicates a multi-stage transformation, proceeding via a first step between 67 and ~100 K in which luminescence intensity increases, followed by a region between 100 and ~225 K in which intensity falls. A discontinuity in intensity at ~225 K marks the end of the second region. These variations may result from changes in the bond angles of bridging oxygens. IL dose dependence studies have been performed. As implantation progresses, the red/IR emission profile skews towards short wavelengths, reflecting amorphisation of the structure and local variations in Fe–O bond distances. If the sample recovers for 24 h, the luminescence profile partly returns to its original state, but remains skewed, reflecting partial resumption of short-range order. Such profiles are also observed from natural shocked feldspars, and it may be that ion beam damage is akin to structural modifications associated with impact events. Ion implantation also causes the formation or enhancement of UV, blue and yellow emissions. Recovery of the sample over 24 h causes the UV and blue emissions to be greatly reduced, whereas the yellow emission may be enhanced. Tentative interpretations of this behaviour are presented.