REFLECTIVITY MEASUREMENTS ON MOLYBDENITE

The spectral reflectivity of molybdenite was measured on an unpolished cleavage flake, a polished vertical section and a polished basal section both in air and in oil. The measurements were made With an EEL micro-photometer of the galvanometer-amplifier type manufactured by Evans Electroselenium Ltd (Halstead, Essex, England); this has a selenium barrier-layer photocell as receptor. A running interference mono-chromator (Schott and Gen., Mainz, Germany) was used. From these measurements the refractive index and the absorption were calculated. Reflectivity peaks were found at 625 nm in air measurements and at 620 nm in oil measurements. A prominent absorption band occurs in the red part of the spectrum. Errors in reflectivity measurements made with a polarizing incident-' light microscope were also considered. It was found that incremental, (positive) errors are in general much higher than decremental (negative) errors; thus if a calibration curve is not available, it is preferable to use a standard of comparison whose reflectivity is higher than the reflectivity of the specimen. Errors were found in practice to be lowest in the spectral region dose to the mercury green line (546 urn)