Original paper
Colour of natural spinels, gahnospinels and gahnites
Schmetzer, K.; Haxel, C.; Amthauer, G.
Neues Jahrbuch für Mineralogie - Abhandlungen Band 160 Heft 2 (1989), p. 159 - 180
53 references
published: Mar 23, 1989
DOI: 10.1127/njma/160/1989/159
ArtNo. ESP154016002003, Price: 29.00 €
Abstract
The cause of colour in natural spinels, gahnospinels and gahnites was investigated by absorption spectroscopy in the visible, ultraviolet and near infrared area in combination with trace element analysis and Mössbauer spectroscopy. The dominant absorption bands in the spectra of variously coloured specimens were assigned to dd-transitions of trivalent chromium and vanadium on octahedral sites (Cr3+oct and V3+oct), and to dd-transitions of ferrous iron on tetrahedral sites (Fe2+tet). Additional absorption bands are assigned to dd-transitions of ferrous iron on octahedral sites (Fe2+oct), to an intervalence charge transfer between ferrous and ferric iron on adjacent octahedral sites (Fe2+oct/Feo3+oct), to dd-transitions of bivalent cobalt on tetrahedral sites (Co2+tet) as well as to dd-transitions of trivalent manganese on octahedral sites (Mn3+oct). The great variability of colour in natural spinels, especially in samples from Sri Lanka, is due to varying trace element concentrations in individual spinel specimens, the absorption spectra of which are explained by a superposition of four basic types of spectra in different intensities; i. e. A) Cr3+oct + V3+oct, B) Fe2+tet ± Fe2+oct, C) Fe2+tet ± Fe2+oct + Fe2+oct/Fe3+oct, and D) Co2+tet. According to the varying intensities of each of these four basic types of spectra, three areas of variable transparencies are observable in the visible region of natural spinel spectra, i. e. in the red (I), in the bluish green (II) and in the violet (III). The colour of each individual spinel specimen is due to the ratio of transparencies in these three areas of characteristic absorption minima. Different colours are caused by variations in transparency or by variable transparency ratios in these three characteristic areas. According to their colour, the samples were subdivided into four series, each of these colour lines having two basic colour causes: Colour line 1 (Cr3+oct + V3+oct) - (Fe2+tet ± Fe2+oct): red (orange), purplish red, reddish purple, purple, bluish purple, purplish blue; Colour line 2 (Cr3+oct + V3+oct) - (Fe2+tet ± Fe2+oct + Fe2+oct/Fe3+oct): red (orange), greyish purple, purplish grey, bluish grey, bluish green; Colour line 3 (Fe2+tet ± Fe2+oct - (Fe2+tet ± Fe2+oct + Fe2+oct/Fe3+oct): purplish blue, greyish blue, bluish grey, bluish green; Colour line 4 (Fe2+tet ± Fe2+oct) - (Co2+tet): purplish blue, blue. The exceptional colour of green gahnite is due to dominant absorption bands of Fe2+tet, Fe2+oct, and most probably Mn3+oct, causing an absorption minimum in the green region.
Keywords
natural spinel • spectra • dd-transition • tetrahedral site • octahedral site • ratio of transparencies