V³⁺-bearing, Mg-rich, strongly disordered olenite from a graphite deposit near Amstall, Lower Austria: A structural, chemical and spectroscopic investigation

Abstract

    An optical absorption spectrum, structural and chemical data of green V- and Cr-bearing tourmaline from the graphite deposit at Weinberg Mountain, west of the village of Amstall, Lower Austria, were obtained. To address the role of V and Cr in the spectrum of tourmaline, examination of additional samples of V- and Cr-containing tourmalines was conducted. This study confirmed that V and Cr produce similar spectra in tourmalines. However, the wavelengths of the 600 nm region band (E\\c), and the 440 nm region band (E^c) varied in relation to the proportion of Cr in the sample. Likewise, the intensity of the 680 nm region spin-forbidden bands varies in proportion to the absolute amount of Cr in the sample. Molar absorption coefficients for both V and Cr in tourmaline were determined for the 600 nm region. For the E^c band, ε(V) = 12.3 ± 0.7; ε(Cr) = 39.7 ± 1.4; and for the E\\c band, ε(V) = 11.9 ± 2.0; ε(Cr) =15.9 ± 2.8. In each case the Cr bands are more intense than the corresponding V band.

    These features can be used to confirm that the spectroscopic features of the Amstall tourmaline come dominantly from V. The optimized formula, calculated using structural and chemical data for the core of a 4 mm wide crystal, is ^X(Na_0.69Ca_0.16K_0.01_0.14) ^Y(Al_1.46Mg_1.34V³⁺_0.11Ti⁴⁺_0.05Cr³⁺_0.02Fe_0.02) ^Z(Al_4.77Mg_1.23) (BO₃)₃ ^T(Si_5.70Al_0.30) O₁₈ [(OH)_3.87O_0.13], with a = 15.984(2), c = 7.222(2) Å, R = 0.017. The optimized formula, calculated using structural and chemical data for the rim, is ^X(Na_0.67Ca_0.24K_0.01_0.08) ^Y(Al_1.57Mg_1.24V³⁺_0.11Ti⁴⁺_0.05Cr³⁺_0.02Fe_0.01) ^Z(Al_4.84Mg_1.16) (BO₃)₃ ^T(Si_5.90Al_0.10) O₁₈ [(OH)_3.35O_0.65], with a = 15.9175(5), c = 7.1914(4) Å, R = 0.014. Whereas the V³⁺ and Cr³⁺ contents stay constant, Mg decreases from the core to the rim. This is reflected by decreasing <Y-O> (from 2.013 to 2.003 Å) and <Z-O> distances (from 1.938 to 1.930 Å). The relative short <Y-O> distances and the enlarged <Z-O> distances show that Al and Mg are strongly disordered between the Y and Z sites in this tourmaline. We assume that the strong Mg-Al disorder between the Y and the Z sites in this tourmaline derived from a high-T overprint (~750°C, ~6-9 kbar) during crystallization, which is supported by a relatively high amount of ^[4]Al and low vacancies at the X site.

Polished 1.7 mm long slab of vanadian olenite from Amstall, viewed in linerally polarized light, E\\c

Hand specimen of olenite in graphite.

Two olenite crystals from Amstall in matrix approximately 1.5 mm long.

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Composition and Spectra of other V- and Cr-containing tourmalines:

1) GRR2128, Mogok area, Myanmar, a vanadium dominant uvite, plotted for 1.0 mm thickness.

Electron microprobe anlaysis GRR2128

Cr2O3		0.049
V2O3		0.614
"FeO"		0.011
V/Cr ratio		12.6

2) GRR2396, a uvite from Kenya

Electron microprobe anlaysis

Cr2O3		0.098
V2O3		0.294
"FeO"		0.064
V/Cr ratio		3.0

3) GRR1719, A dravite with more V than Cr, plotted for 1.0 mm thick.

Electron microprobe anlaysis

Cr2O3		0.199
V2O3		0.452
"FeO"		0.002
V/Cr ratio		2.3

4) GRR 768, a uvite with somewhat more V than Cr, plotted as 1.0 mm thick.

Electron microprobe anlaysis

Cr2O3		0.141
V2O3		0.244
"FeO"		0.004
V/Cr ratio		1.7

5) GRR 2467, a uvite from Tanzania with more Cr than V, plotted as 1.0 mm thick.

Electron microprobe anlaysis

Cr2O3		0.140
V2O3		0.045
"FeO"		0.033
V/Cr ratio		0.3