LETTER
Cathodoluminescence and Ti contents of wollastonite from Tadano, Fukushima Prefecture, Japan
2024 Volume 119 Issue 1 Article ID: 231218b
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2024 Volume 119 Issue 1 Article ID: 231218b
Wollastonite of variable cathodoluminescence (CL) intensity was found in a skarn xenolith from Tadano, Fukushima Prefecture, Japan. Electron microprobe analysis revealed bright-blue and faint-blue CL spots in crystals. Panchromatic CL images revealed that individual wollastonite grains often include bright and dark CL regions of bright- and faint-blue CL, respectively. The TiO2 contents of bright CL regions (0.055-0.110 wt%) were higher than those of dark CL regions (<0.008-0.013 wt%). The blue CL intensity gradually increased from <0.008 to 0.102 wt% TiO2, and that with 0.102 and 0.110 wt% TiO2 was almost the same. The results indicated a positive correlation of Ti content with blue CL intensity.
Wollastonite is a common mineral in skarn. It often displays no major-element zoning, and backscattered electron (BSE) images provide little textural information. However, previous studies have shown that cathodoluminescence (CL) is useful in identifying zonation patterns in minerals without major-element zoning (e.g., Spear and Wark, 2009). Wollastonite CL colors are variable (usually green, yellow, greenish yellow, or orange), and a yellow color is typically attributed to the presence of Mn2+ (Marshall, 1988). This study identified wollastonite crystals in a skarn xenolith in Tadano, Japan, with blue CL of variable intensity. Combining CL observation with quantitative electron microprobe analysis revealed a correlation between Ti content and blue-CL intensity, and panchromatic CL images of the crystals provided textural information.
Samples of a skarn xenolith were collected from a two-pyroxene andesite quarry at Tadano, Fukushima Prefecture, Japan. The samples comprised mainly wollastonite, grossular, andradite, gehlenite, and apatite-ellestadite series minerals with minor perovskite, magnetite, pyrrhotite, and chalcopyrite. Wollastonite occurred as anhedral-subhedral prismatic crystals up to 1.6 mm in length. Electron microprobe analysis (EMPA) revealed bright blue (Figs. 1a-1d) and faint blue (Figs. 1e and 1f) CL spots in the crystals, but BSE images (Figs. 2a-2c) indicated no major-element zoning. Panchromatic CL images of the heterogeneous distribution of blue-CL intensity were captured using a scanning electron microscope (JEOL JSM-6610LV) equipped with a Gatan Mini-CL detector at the National Institute of Advanced Industrial Science and Technology (AIST), Japan. The CL mapping was facilitated by a parabolic mirror installed in the sample chamber and a detector suitable for the visible region. Panchromatic CL images (Figs. 2d-2f) indicated that individual wollastonite grains often comprised separate bright and dark CL regions, corresponding to bright and faint blue CL, respectively (Figs. 1 and 2). No CL was observed for gehlenite, grossular, andradite, and apatite-ellestadite series minerals.
EMPA involved a JEOL JXA-8900R system at AIST, with 52 spots per sample. The accelerating voltage, beam current, and beam diameter were 15 kV, 12 nA, and 2 µm, respectively. Standards included quartz (Si), rutile (Ti), corundum (Al), eskolaite (Cr), hematite (Fe), manganosite (Mn), periclase (Mg), wollastonite (Ca), albite (Na), and adularia (K). The Bence and Albee (1968) method was used for matrix corrections. Background intensity was measured at equal displacements either side of the peak. Counting time was 20 s at the peak and 10 s at each offset background position. Detection limits (2σ confidence level) were: TiO2 0.05, Cr2O3 0.05, FeO 0.04, MnO 0.04, MgO 0.02, Na2O 0.03, and K2O 0.03 wt%. Bright- and dark-CL regions had respective minor-element compositions of: TiO2, <0.05-0.15 and <0.05 wt%; MnO, <0.04-0.11 and 0.07-0.22 wt%; MgO, <0.02-0.10 and 0.07-0.20 wt%; and FeO, <0.04-0.15 and 0.06-0.14 wt%. These preliminary analyses implied that Ti may be a cause of the blue CL in wollastonite.
For higher Ti precision and a lower detection limit, operating conditions were altered for Ti (only) to a beam diameter of 5 µm and beam current of 24 nA, with counting times of 300 s on the peak and 150 s on background positions. An additional 21 spot analyses were performed under these conditions. The detection limit for TiO2 was then 0.008 wt% with relative errors (1σ) of 5% at 0.11 wt% and 9% at 0.06 wt%. The analytical results are shown in Tables 1 and 2 for bright- and dark-CL regions, with TiO2 contents of 0.055-0.110 and <0.008-0.013 wt% (Figs. 2d-2f), respectively. The average amounts of MnO, MgO, and FeO of bright- and dark-CL regions were 0.03 and 0.11 wt%, 0.06 and 0.14 wt%, and 0.08 and 0.11 wt% (Tables 1 and 2), respectively.
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | Average | |
| Weight percentages | |||||||||||||
| SiO2 | 51.7 | 51.5 | 52.0 | 51.2 | 51.4 | 52.0 | 51.4 | 51.7 | 51.6 | 51.6 | 51.4 | 51.6 | 51.6 |
| TiO2 | 0.078 | 0.058 | 0.055 | 0.110 | 0.078 | 0.102 | 0.102 | 0.088 | 0.091 | 0.090 | 0.088 | 0.104 | 0.087 |
| Al2O3 | 0.20 | 0.22 | 0.22 | 0.25 | 0.22 | 0.26 | 0.21 | 0.20 | 0.20 | 0.24 | 0.17 | 0.23 | 0.22 |
| Cr2O3 | <0.05 | <0.05 | <0.05 | <0.05 | <0.05 | <0.05 | <0.05 | <0.05 | <0.05 | <0.05 | <0.05 | <0.05 | n.d. |
| FeO | 0.12 | 0.06 | <0.04 | 0.11 | 0.07 | 0.10 | 0.10 | 0.06 | 0.06 | 0.09 | 0.08 | 0.08 | 0.08 |
| MnO | 0.04 | 0.05 | <0.04 | 0.06 | 0.05 | <0.04 | 0.07 | 0.09 | 0.04 | <0.04 | <0.04 | <0.04 | 0.03 |
| MgO | 0.07 | 0.05 | 0.07 | 0.06 | 0.07 | 0.06 | 0.05 | 0.07 | 0.07 | 0.07 | 0.06 | 0.07 | 0.06 |
| CaO | 48.1 | 48.6 | 48.9 | 48.3 | 48.7 | 49.0 | 48.1 | 48.7 | 47.8 | 49.0 | 48.7 | 48.6 | 48.5 |
| Na2O | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | n.d. |
| K2O | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | n.d. |
| Total | 100.3 | 100.5 | 101.3 | 100.1 | 100.6 | 101.5 | 100.0 | 100.9 | 99.9 | 101.1 | 100.5 | 100.7 | 100.6 |
Numerals with ‘<’ indicate detection limits. n.d., not detected.
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | Average | |
| Weight percentages | ||||||||||
| SiO2 | 52.0 | 52.0 | 52.1 | 51.5 | 51.6 | 51.7 | 51.8 | 51.5 | 51.7 | 51.8 |
| TiO2 | <0.008 | 0.009 | 0.012 | 0.013 | 0.009 | 0.011 | <0.008 | <0.008 | <0.008 | 0.006 |
| Al2O3 | 0.15 | 0.25 | 0.22 | 0.17 | 0.25 | 0.16 | 0.28 | 0.19 | 0.20 | 0.21 |
| Cr2O3 | <0.05 | <0.05 | <0.05 | <0.05 | <0.05 | <0.05 | <0.05 | <0.05 | <0.05 | n.d. |
| FeO | 0.18 | 0.09 | 0.08 | 0.12 | 0.14 | 0.09 | 0.14 | 0.09 | 0.08 | 0.11 |
| MnO | 0.10 | 0.15 | 0.15 | 0.12 | 0.07 | 0.13 | 0.11 | 0.11 | 0.09 | 0.11 |
| MgO | 0.13 | 0.06 | 0.07 | 0.10 | 0.19 | 0.19 | 0.17 | 0.14 | 0.18 | 0.14 |
| CaO | 48.6 | 49.0 | 48.4 | 47.7 | 48.0 | 48.5 | 48.4 | 48.6 | 48.5 | 48.4 |
| Na2O | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | n.d. |
| K2O | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | <0.03 | n.d. |
| Total | 101.2 | 101.6 | 101.0 | 99.7 | 100.3 | 100.8 | 100.9 | 100.6 | 100.8 | 100.8 |
Numerals with ‘<’ indicate detection limits. n.d., not detected.
CL may be impurity-activated or intrinsic, with the former being associated with energy levels of impurity (activator) ions in the crystal environment and the latter with structural defects caused by non-stoichiometry, structural imperfections, or non-activator impurities that distort the lattice (Marshall, 1988). Blue-colored intrinsic CL is common to almost all silicates (e.g., quartz and feldspar) and is associated with distorted-lattice energy levels (Marshall, 1988). Titanium impurities in feldspar are unlikely to cause impurity-activated CL but may cause structural defects that enhance its intrinsic blue CL (Marshall, 1988). The blue CL of Tadano wollastonite is also likely due to structural defects associated with Ti atoms substituting for Si atoms at tetrahedral sites in wollastonite. The TiO2 contents of bright CL regions (0.055-0.110 wt%) were higher than those of dark CL regions (<0.008-0.013 wt%). The blue CL intensity gradually increased from <0.008 to 0.102 wt% TiO2, and that with 0.102 and 0.110 wt% TiO2 was almost the same (Fig. 1). Although observed CL intensity was only a qualitative measure, the results indicated a correlation of Ti content with blue CL intensity. Previous studies of CL in quartz (Wark and Spear, 2005; Spear and Wark, 2009) have shown that CL intensity in the ∼ 415 nm (blue) region is correlated with Ti content (0-135 ppm), which is consistent with the results for Tadano wollastonite in the present study.
Wollastonite bright- and dark-CL regions tended to appear on outer and inner parts of single grains, respectively (Figs. 2d-2f), implying core/rim growth zoning or diffusion of Ti into rims. CL images (Figs. 2d and 2f) also revealed oscillatory zoning in bright portions of some grains. Such zonation is usually associated with crystal growth in an open system (Holten et al., 2000), so it may represent growth zoning here with corresponding compositional changes rather than diffusion. The wollastonite with such zonation likely records variations in the bulk TiO2 composition of the skarn xenolith.
The author thanks Dr. Y. Harigane for assistance with CL imaging and T. Sato for preparation of thin sections. Constructive reviews by anonymous reviewers improved the manuscript.
