VANADIUM-RICH MUSCOVITE FROM AUSTRIA: CRYSTAL STRUCTURE, CHEMICAL ANALYSIS, AND SPECTROSCOPIC INVESTIGATIONS

The crystal structure of a green, transparent, vanadium-rich muscovite-2 M 1 (V 2 O 3 ¼ 11.35 wt.%, one of the highest amounts reported to date in muscovite) with the optimized formula (K (Si 1.54 Al (Si 1.54 Al 0.46 )O 10 (OH) 2 and space group C2/ c , a 5.2255(6), b 9.0704(10), c 20.0321(21) ˚A, b 95.773(2) 8 , Z ¼ 4 has been refined to R ¼ 6.97% for 1070 unique reflections (Mo K a ). This muscovite, which occurs in small quartz veins in graphite schist from Weinberg mountain, near the village of Amstall, Lower Austria, is distinctly low in Cr (Cr 2 O 3 ~ 1.4 wt.%) and Mg (MgO ~ 1.1 wt.%); Fe, Mn, and Ti are below detection limit. All octahedral cations occupy the M 2 site, and the average octahedral bond ( M 2–O) distance is 1.953 ˚A. Structural distortions include a ¼ 8.89 8 and D z ¼ 0.193 ˚A, resulting in an interlayer spacing of 3.35 ˚A. The optical absorption spectrum of this V-rich muscovite shows absorption features at 427 and 609 nm that define a transmission window centered at 523 nm. These absorption features are consistent with those expected for V 3 þ in mica, but the 609 nm band has a slightly longer wavelength than in low-V micas.

Green V-rich muscovite from Weinberg mountaiñ 500 m west of the village of Amstall, Lower Austria, occurs with pyrite in small quartz veins in graphite schist. Structural and chemical data and an optical absorption spectrum of green V-and Cr-bearing tourmaline with 0.85 wt.% V 2 O 3 and~0.17 wt.% Cr 2 O 3 from the same locality was described by Ertl et al. (2008). The core of this dravite has lattice parameters of a ¼ 15.984(2), c ¼ 7.222(2)Å, while the rim exhibits smaller lattice parameters of a ¼ 15.9175(5), c ¼ 7.1914(4)Å, mainly due to a decreasing Mg content while the V and Cr content does not change significantly. That study also confirmed that V and Cr produce similar optical absorption spectra in tourmalines. Green muscovite from this locality was first noted by Zirkl (1961). Forty years later Blass & Graf (2001) described green muscovite with a significant vanadium content in association with graphite, sillimanite, violet corundum, and pyrite.
In this study we focus on the crystal structure of one of the most V-rich muscovites reported to date, with an average of 11.35 wt.% V 2 O 3 , and on the optical absorption spectra of the phase.

Crystal structure
A cleavage fragment of the V-rich muscovite crystal was mounted on a Bruker Apex CCD diffractometer equipped with graphite-monochromated MoKa radiation. Refined unit-cell parameters and other crystal data are listed in Table 1. Redundant data were collected for a sphere of reciprocal space and were integrated and corrected for Lorentz and polarization factors and absorption following the multislice method using the Bruker program SAINTPLUS (Bruker AXS Inc. 2001).
The structure model was refined using starting parameters from the muscovite determination of Guggenheim et al. (1987) and the Bruker SHELXTL version 6.10 package of programs, with neutral-atom scattering factors and terms for anomalous dispersion. Refinement was performed with anisotropic thermal parameters for all atoms. In Table 2 we list the atom parameters, and in Table 3 we present selected interatomic distances.

Chemical analyses
Chemical analyses (Table 4) were performed using the wavelength-dispersive spectrometers of a CAME-CA SX-50 electron-microprobe at the Ruhr-University-Bochum, Germany. The microprobe was operated at an acceleration voltage of 15 kV, a sample current of 15 nA, and a beam diameter of approximately 5 lm. Natural and synthetic materials were used as standards. Excellent agreement was obtained between V analyzed by EMPA and structure refinement, yielding 0.64 apfu by the former method and 0.58 apfu by the latter. The optimized formula (Wright et al. 2000) resulting from the chemical data and structure values is given below.

Optical spectra
A~1 3 1 mm, 116 lm thick, green cleavage fragment of the Amstall vanadium-rich muscovite was used for the optical spectrum in the 390-1100 nm range. Spectra were obtained at about one-nm resolution with a locally built microspectrometer system consisting of a 1024-element Si diode-array detector coupled to a grating spectrometer system and via fiber optics to a highly modified NicPlan t infrared microscope containing a calcite polarizer. Conventional 103 objectives were used as both objective and condenser. Spectra were obtained from an approxi-   Brigatti et al. (2003) published the first detailed atomic arrangement of roscoelite-1M, and we refer the reader to that work for a detailed examination of vanadium in the mica structure. The vanadium-rich muscovite described herein contains extensive V at the octahedral site. In contrast to the V-dominant roscoelite with its 1M polytype, the V-rich, Al-dominant muscovite reported here retains the common 2M 1 polytype of muscovite. We describe here the atomic arrangement of one of the most V-rich muscovites described to date.

Structure
Dioctahedral micas typically crystallize as the 2M 1 polytype, and, as noted by Brigatti et al. (2003), such micas commonly contain minor atom occupancy at the M1 octahedral site, in addition to occupancy at M2, the dominant octahedral site. In the Austrian V-rich muscovite, the M1 site is fully vacant, as shown by final difference maps and attempts at refining any electron occupancy at the site. The electron occupancy of the M2 site gives excellent agreement between the chemical analysis and the refinement, confirming the putative locus of the V 3þ substituent. The optimization (Wright et al. 2000) of the occupants of all the cation sites, which minimizes the differences between the chemical composition as determined by EMPA and Xray structure refinement, yields (K 0.94 Na 0.06 ) M2 (Al 1 . 2 0 V 3þ 0 . 6 1 Mg 0 . 1 2 Cr 3þ 0 . 0 7 ) T 1 (Si 1 .5 4 Al 0 .4 6 ) T 2 (Si 1.54 Al 0.46 )O 10 (OH) 2 . This V-rich muscovite is distinctly low in Cr (Cr 2 O 3~1 .4 wt.%) and Mg (MgO:~1.1 wt.%); Fe, Mn, and Ti are below detection limit. The calculated density, from the chemical composition and the volume of the unit cell (944.65Å 3 ), is D calc ¼ 2.907.
The incorporation of significant V 3þ at the octahedral site evokes a structural response. Table 5 offers various parameters for our V-rich muscovite, roscoelite, and muscovite without vanadium substituents. Plotting ,M2-O. versus V 3þ content yields a linear increase with V concentration, suggesting that the incorporation of the larger V 3þ cation instead of Al in the mica structure causes an expansion of ,M2-O. regardless of the polytype (Fig. 1). Brigatti et al. (2003) described the distortions in the dioctahedral roscoelite structure that result from the incorporation of V 3þ at the octahedral site. They noted that the V-dominant octahedral occupants in roscoelite result in the smallest tetrahedral rotation (a ¼ 2.38) and the smallest corrugation of the basal oxygen surface (Dz ¼ 0.118Å) known in dioctahedral micas. In our sample, these structural compensations are much larger, which indicates a greater misfit between the lateral dimensions of the sheets of tetrahedra and octahedra. The tetrahedral rotation angle is a ¼ 8.988. The out-of-plane movement of bridging oxygen atoms in the basal oxygen plane result from the tetrahedral tilt of Dz ¼ 0.193. Figure 2 depicts the variation of tetrahedral rotation with substituent octahedral vanadium for three dioctahedral micas: muscovite, V-rich muscovite, and roscoelite. For comparison, note that roscoelite has significant amounts of octahedral iron, which is not the case for the vanadium-bearing muscovite described here. Tetrahedral rotation distorts the idealized hexagonal rings of the basal oxygen plane to ditrigonal symmetry, causing decreased lateral dimensions of the rings. Potassium ions sit within the ditrigonal rings of basal oxygen planes on either side of the interlayer. The decrease in the lateral dimensions of the rings affects the position of the potassium within the rings with a concomitant increase in the interlayer spacing. The interlayer spacing in the V-rich muscovite from Amstall, Lower Austria is 3.35Å.

Optical spectroscopy
The V-rich muscovite has absorption features at 427 and 609 nm that define a transmission window centered at 523 nm (Fig. 3). These absorption features are consistent with those expected for V 3þ (Schmetzer 1982) in mica, but at a slightly longer wavelength for the 609 nm band compared to low V-content micas. The roscoelite spectrum has absorption bands at 610 and 730 nm that sit on a background that gradually rises towards shorter wavelengths (Fig. 3). The 610 nm band was observed in the reflectance spectra of other roscoelites (Schmetzer 1982, Clark et al. 1993) and is  (Guggenheim et al. 1987), V-rich muscovite (this study), and roscoelite (Brigatti et al. 2003). Samples also described in Table 5.
a V 3þ feature. The additional absorption band near 740 nm corresponds to a band in this region of the spectrum of micas previously attributed to either a Fe 2þ /Fe 3þ intervalence interaction or a V 4þ (or VO 2þ ) feature, as reviewed in Schmetzer (1982). Little comparative data exist on the intensities of V 3þ bands in silicate minerals. The intensities of these bands compare favorably with the range of intensities of V 3þ in model systems reported by Schmetzer (1982). The 610 nm band corresponds to the 3 T 1 3 T 2 (m1) transition with intensity (e) in the range of 4.1 to 7.8 for three model compounds, compared to 2.9 for the V 3þ -rich muscovite. The 420 nm band corresponds to the 3 T 1 3 T 1 (m2) transition with intensity (e) in the range 6.6 to 10.5 for the model compounds, compared to 7.3 for the V 3þ -rich muscovite. While these are only modestly intense bands, the intense green color of the mica is due to the high V concentration.