A MINERALOGICAL AND STRUCTURAL STUDY OF RED CORUNDUM, AL1.98CR.02O3, FROM FROLAND, NORWAY

In this study, the red corundum, Al-1.98,Cr.02O3, from Froland is char acterized structurally and compositionally. The crystal structure has been refined from X-ray (CuK alpha(1)) powder diffraction data. Space group R (3) over bar c Z = 6, a = 4.7597(4) Angstrom, c = 13.0013(9) A ngstrom. The substituting Cr-atoms do not take the site of Al-atoms, t hey depart 1.04 Angstrom in the direction of the vacant octahedral int erstices. A SEM (EDS) study revealed microphases of diaspore; one of t he reasons for gem quality being lost. The result suggests that the re action Al2O3 (Crn) + H2O <----> 2AlO(OH) (Dsp) occurred in the late st age of the metamorphic history of the Froland region.