AUTOMATED MICROSCOPE-ABSORPTION-SPECTROPHOTOMETRY OF ROCK-FORMING MINERALS IN THE RANGE 40,000-5000CM-1 (250-2000NM)

To measure polarized absorption spectra of microcrystals of 3dn‐ion bearing silicate minerals, computer processed, microscope‐spectrophotometric methods have been developed. Absorbance, log (I0/I), can be measured with high relative accuracy (near u.v. and vis: ±0·002 to 0·001; n.i.r.: ±0·004 to 0·002), and relatively small spectral band widths are available. Hence, weak spin‐forbidden dd‐bands of 3dn‐ions can be recognized alongside spin‐allowed dd‐transitions without artificial broadening of absorption bands due to finite resolution. The smallest area from which absorption spectra can be taken is 8 μm in diameter. As one example of the many applications in mineralogy and material sciences, absorption spectra of a natural spessartine garnet, Spess69·7Alm30·0Gross0·05, containing Mn2+, Fe2+, and traces of Fe3+ as 3dn‐ions, and of a pure Mn2+‐garnet, Spess67Gross33, are presented. From these it is evident that bands in natural spessartines at ∼ 26,900, 23,200 cm−1 which were assigned to dd‐transitions in Fe3+(6), have to be reassigned to Mn2+(8). Comparison of spectra obtained with the microscopic equipment described with those obtained by means of conventional macroscopic equipment prove that the methods described produce true spectra. 1979 Blackwell Science Ltd