Volume 59, pages 621-622, 1974

     Optical Spectroscopy of Green Vanadium Apophyllite from Poona, India¹

Department of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91109

     Instead of being colorless, as is more commonly true in this region, some specimens of apophyllite, KCa₄Si₈O₂₀(F,OH)·8H₂O, from the Poona region have a pleasing light green color. Because this color resembles that of a variety of silicates containing ferrous iron, it has logically been assumed that these apophyllites were colored by divalent iron. Both optical spectroscopic studies and chemical analyses have demonstrated, however, that this belief is incorrect.

     The samples examined consisted of transparent green terminated crystals, generally less than one centimeter long, projecting out of a white mass of apophyllite. They are dichroic, ranging from blue-green, E || c, to pale yellow-green, E (perpendicular) c. Their identity was confirmed by infrared spectroscopy which indicated that the spectrum of the green material is identical to that of colorless apophyllite from Poona which in turn is identical to the portion of the Michigan apophyllite spectrum reported by Colville and Anderson (1971). An emission spectrographic analysis indicated that vanadium is the most abundant trace transition metal (Table 1). A report of the analysis of a colorless apophyllite from Poona (Belsare, 1969) did not report the presence of vanadium. The single crystal studied optically was free of internal defects, had nearly flat, parallel crystal faces, and was uniaxial negative in the center but graded to biaxial negative near the edges (with 2 V up to 12°). Optical absorption spectra (Fig. 1) show that prominent absorption bands centered at 492, 646, and 829 nm are responsible for the green color in the E || c spectrum. In the near-infrared the prominent features are at 1442 nm and 1517 nm polarized parallel to c, about 1795 nm polarized perpendicular to c, and the intense feature at about 1921 nm whose polarization was not determined. The features at 1442 and 1517 nm are, respectively, the first overtones of the OH stretching motion which occurs at 3556 cm^-1 in the infrared spectral region, and a stretching motion of the water molecules in the 3000-3400 cm^-1 region.

     The absorption bands at 492, 646, and 829 nm responsible for the color are attributable to vanadium and manganese. Two oxidation states of vanadiumV³⁺, and V⁴⁺ as the VO²⁺ ion-would be considered reasonably likely to be found in the apophyllite. In an electron spin resonance study of apophyllite from Nidy, Siberia, Marfunin, Bershov, and Mineeva (1966) obtained the signal of VO²⁺. The VO²⁺ was believed to be present in both the eight-fold tetragonal-prismatic potassium site and in the seven-fold capped trigonal-prismatic calcium site. The optical results indicate that the bulk of the vanadium is present as the VO²⁺ ion. The apophyllite spectra resemble the spectra of crystalline VOSO₄·5H₂O and vanadyl acetylacetonate solutions (Ballhausen and Gray, 1962; Bernal and Rieger, 1963). In the spectrum of crystalline vanadyl sulfate bands at 13,100 and 16,000 cm^-1 polarized perpendicular to the V=O bond axis were assigned to the ²B₂ --> ²E and ²B₂ --> ²B₁ transitions. The corresponding transitions in apophyllite are assigned to the 12,000 cm^-1 (829 nm) and 15,500 cm^-1 (646 nm) bands respectively. Although the band at 20,400 cm^-1 (492 nm) could be analogous to the 25,300 cm^-1 band in the vanadyl acetylacetonate spectrum, it appears more appropriate to attribute it to Mn³⁺ in view of the fact that minerals containing Mn³⁺ in relatively symmetrical environments have prominent absorptions in the range 480-560 nm. If all Mn is assumed to be present in the trivalent state, ε in the E || c polarization is 27 l.mole^-1 cm^-1.