Electronic absorption spectra of phosphate minerals with olivine-type Structures: II. The oxidized minerals ferrisicklerite, M1[6](1xLix)M2[6](Fe3+1xMn2+x)[PO4], and heterosite,M1[6](1.00)M2[6](Fe3+1xMn3+x)[PO4], with x 0.5

Abstract

Electronic absorption spectra of the orthorhombic, Pnmb , oxidized phosphate minerals ferrisicklerite and heterosite with defect olivine structures were obtained either as single crystal spectra or as powder remission spectra, scanned on unoriented fine grained powders. In all cases microscope-spectrometric methods were used. Single-crystal spectra on two mineral phases were also measured, one of which was very strongly pleochroic and deeply violet colored contained in a 6 μm thick platelet of a ferrisicklerite-heterosite intergrowth. The most remarkable feature of the ferrisicklerite spectrum is a shift of the high-energy absorption edge by ca. 10 000 cm −1 towards the red, compared to triphylite. By this, the edge cuts the blue and green, leaving the dark yellow color of the mineral ferrisicklerite. The low energy wing of the edge exhibits, attached as weak shoulder-like features, relatively broad bands at ca. 18 300 and ca. 12 600 cm −1 . These energies are typical of spin-allowed dd-bands of Mn 3+ , as in piemontites ( E//Y violet). This result on ferrisicklerite is consistent with the polarized crystal spectrum of heterosite, showing very strong and broad bands of Mn 3+ at 18 240 and 12 900 cm −1 .