Automated Microscope-Absorption-Spectrophotometry Of Rock-Forming Minerals In The Range 40,000–5000 Cm−1 (250–2000 Nm)

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, Spess_69·7Alm_30·0Gross_0·05, 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.