Jarosite samples were pulverized and shaped by pressure into flat round pellets of 1 cm diameter and0.5 mm thickness (Fig. 1). Gypsum samples were scratched from a larger specimen. The samples were flattened to different thickness (between 0.1 mm to 1.6 mm) before exposed to UV light. All samples were positioned onto a sample holder which allowed the UV lamp light to go through the samples and be detected by a PMT through a monochromator. The UV light source is a Xe Lamp(Spectral Products) with an integrated output from 220 nmto 500 nm of 1.2 Wm-2 (measured at a distance of 10 cm from the lamp exit and over a disc of 5 mm diameter). A monochromator (SpectralProducts, 1/8 m CM 110) is placed at the output of the lamp, but in our experiments it is set up so that it lets all wavelengths out without deformation of the Xe lamp output. Five (5) cm downstream the light beam irradiates the sample, the light which is transmitted through it is then collected by an optical fiber head (placed 10 cm away from the Xe lamp), which in turns directs the light through a spectrometer (Bentham, CM 150 double monochromator) which records the transmitted light spectrum. The light is detected by a peltier-cooled bi-alkali photomultiplier tube (PMT Bentham, DH-10-Te). Typically, each sample spectrum is measured from 220 nm to 500 nm every 1nm with a rate of approximately 1 nm/s. Usually each sample spectrum is the average of at least5 spectra.