Cathodoluminescence Characterization of Enstatite

Journal of Mineralogical and Petrological Sciences, Volume 110, page 241–246, 2015

LETTER

Cathodoluminescence characterization of enstatite

Syuhei OHGO*, Hirotsugu NISHIDO* and Kiyotaka NINAGAWA**

*Department of Biosphere–Geosphere Science, Okayama University of Science, Okayama 700–0005, Japan **Department of Applied Physics, Okayama University of Science, Okayama 700–0005, Japan

Enstatite in meteorite shows various emissions of cathodoluminescence (CL), and CL emission in terrestrial enstatite has been confirmed in this study. The CL spectra of these enstatite exhibit two broad emission bands at around 400 nm in a blue region and at around 670 nm in a red region. The emission components obtained by a spectral deconvolution can be assigned to impurity centers of Cr3+ (1.70–1.75 eV) and Mn2+ (1.85–1.90 eV) and to three defect centers (2.72–2.75, 3.18, and 3.87 eV). The emission component at 3.18 eV might be associated to the defect center derived from structural distortion by the substitution of Al for Si in a tetrahedral site. Extraterrestrial enstatite gives a characteristic emission at 3.87 eV in a blue to UV region, which is not detected in the terrestrial enstatite, suggesting radiation–induced defect by cosmic rays.

Keywords: Enstatite, Cathodoluminescence, Enstatite chondrite, Aubrite

INTRODUCTION sion. In this study, we have confirmed luminescent ensta- tite in terrestrial materials for the first time. We have con- Enstatite is one of the most important rock–forming min- ducted to clarify the luminescence centers of CL emis- erals in the terrestrial and extraterrestrial materials and sions in the terrestrial and extraterrestrial enstatite by commonly occurred in igneous and metamorphic rocks using a spectral deconvolution method. and various types of meteorite. Luminescent enstatite has been confirmed only in the meteorites of enstatite SAMPLES AND METHODS chondrite (E–chondrite), enstatite achondrite (Aubrite) and synthetic samples (e.g., Derham et al., 1964; Reid Single crystals of luminescent enstatite from Tanzania and et al., 1964). Major silicate phase in E–chondrite and Au- Sri Lanka were selected for CL spectral measurements. brite is a near endmember of enstatite (FeO <1.0%), The enstatite (Tan) from Mbeya, Tanzania occurred in ul- which shows various luminescence. tramafic inclusions in alkalic olivine basalt, and the ensta- Cathodoluminescence (CL) of the enstatite has been tite (Sri) from Sabaragamuwa, Sri Lanka were collected in investigated mainly in E–chondrite and Aubrite (e.g., the placer deposits originated from metamorphic sedimen- Zhang et al., 1996). CL, a visible light of emissions when tary rocks with a high–grade in the Highland Complex a material is irradiated by an electron beam, has a high (Dissanayake et al., 2000). Sahara 97096 (Sah97096), Sa- advantage to reveal growth textures in the crystals of en- hara 97121 (Sah97121) and Yamato 86004 (Y–86004) for statite such as domain and zonation, most of which are E–chondrite and Al Haggounia001 (Aub) for aubrite were difficult to be identified by conventional optical exami- chosen for a comparison with terrestrial enstatite. The pol- nations. Therefore, CL of enstatite in E–chondrite has ished samples finished with a diamond paste of 1 μm were been used for the identification of its petrographic type coated with a 20 nm layer of carbon and employed for a and the studies of its thermal history (e.g., Zhang et al., CL examination. 1996). Enstatite in meteorites occasionally shows var- Color CL images were obtained using a cold–cath- ious CL emissions with red, purple and blue (Reid et ode type Luminoscope (ELM–3) with a cooled charge– al., 1964), whereas terrestrial enstatite has no CL emis- coupled device CCD camera. The identifications of con- stituent minerals in terrestrial and extraterrestrial samples doi:10.2465/jmps.150713b were carried out using an electron probe microanalyzer S. Ohgo, [email protected] Corresponding author (EPMA) and Raman spectroscopy after a polarizing–mi- 242 S. Ohgo, H. Nishido and K. Ninagawa

Figure 1. Color CL images of terrestrial enstatite (Tan1 and Sri1–3) and extraterrestrial enstatite (Sah97096 and Aub1).

croscopic examination. CL spectroscopy was made by alytical procedure can be found in Ikenaga et al. (2000). a SEM–CL system, which is comprised of SEM (JEOL: JSM–5410LV) combined with a grating monochromator RESULTS AND DISCUSSION (OXFORD: Mono CL2). The dispersed CL was recorded in the range of 300–800 nm by a photon–counting meth- Color CL imaging reveals various types of CL emissions od using a photomultiplier tube and converted to digital with red, red–purple, bluish–purple and blue in the terres- data. All CL spectra were corrected for total instrumental trial and extraterrestrial enstatite (Fig. 1). Extraterrestrial response, which was determined using a calibrated stand- enstatite has been reported as a variety of CL colors emit- ard lamp. Detailed construction of the equipment and an- ted with red, magenta and blue (e.g., Zhang et al., 1996). Cathodoluminescence characterization of enstatite 243

Table 1. Chemical compositions of terrestrial enstatite and extraterrestrial enstatite with an average for five analyses

The values in parentheses are the range of the analyses. Tan1, terrestrial enstatite from Tanzania; Sri1, Sri12 and Sri13, terrestrial enstatite from Sri Lanka; Sah97096a and Sah97096b, extrater- restrial enstatite in Sahara 97096; Aub1,extraterrestrial enstatite in Al Haggounia 001.

In the previous studies (Crookes, 1879; Reid et al., 1964), however, terrestrial enstatite has not emitted any lumines- cence even by the excitation of any energy sources such as ion and X–ray beams. Most of terrestrial enstatite have the FeO contents above ~ 3 wt% (e.g., Deer et al., 1978), where Fe2+ can act as a quencher to reduce the lumines- cence activated by impurity centers such as Mn2+. In this study, we have first confirmed CL emissions with blue and red from the terrestrial samples (Tan1 and Sri1–3) with their low contents of FeO. CL of enstatite is controlled mainly by activators of Mn2+ and Cr3+, and quencher of Fe2+. Therefore, Fe ions of terrestrial enstatite should diminish luminescence efficiency derived from activators even with high density. Figure 2. Compilation of corrected CL spectra for terrestrial en- Extraterrestrial enstatite with low FeO contents (<5 wt%) statite (Tan1, Sri3) and extraterrestrial enstatite (Sah97096b, Sah97121 and Aub1). in E–chondrites shows obvious CL caused by various emission centers due to a negligibly small of quencher (Keil, 1968; Leitch and Smith, 1982; McKinley et al., ranges of five analyses in the table 1. The enstatite with 1984; Weisberg et al., 1994; Zhang et al., 1996). Terres- red and red–purple CL (Tan1 and Sri1) have slightly trial enstatite with luminescence have considerably low higher MnO and Cr2O3 contents than blue CL enstatite. FeO contents (<1 wt%) on the same level with that of The CL spectra of terrestrial and extraterrestrial en- extraterrestrial enstatite (Sah97096 and Aub1), suggest- statite show two broad emission bandsat around 400 nm ing almost absence of quenching effect in the lumines- in a blue region and at around 670 nm in a red region (Fig. cent process (Table 1). Enstatite in E–chondrite and Au- 2), both of which were detected in most enstatite sample- brite are regarded as the minerals formed under highly– shaving various ratios of the CL intensities between red reducing conditionson their parent bodies judged by their and blue regions. The difference of color tone in the CL mineral assemblages and compositions (e.g., Keil, 1968). such as red–purple (Sri1) and bluish–purple (Sri2) should However, terrestrial enstatite with low Fe has been re- depend on the ratio between blue and red emissions in ported in metamorphic rocks formed in the process of enstatite. A red emission at around 670 nm in the synthetic epithermal metamorphism (e.g., Dobrokhotova et al., enstatite is assigned to an impurity center derived from 1967), which may be in a same case of the samples em- activated Mn2+ substituted for Mg as an electron transition 4 6 ployed here. Most luminescent enstatite in the meteorites of T1g (G) → A1g (S) (Catalano et al., 2014). This elec- (Sah97096 and Aub1) show compositional variations of tron transition similar to those in olivine and calcite is Al2O3, FeO, Cr2O3, MnO, and CaO contents with the affected by the crystal field of the ligant oxygens coordi- 244 S. Ohgo, H. Nishido and K. Ninagawa

Figure 3. CL spectrum in energy units of enstatite for terrestrial Figure 4. CL spectrum in energy units of enstatite for extraterres- enstatite (Sri2) with their emission components deconvoluted by trial enstatite (Y–86004) with their emission components decon- a Gaussian curve fitting. voluted by a Gaussian curve fitting. nated to the host Mn ion (Marfunin, 1979; Götze et al., has been confirmed in both terrestrial and extraterrestrial 1999). Lofgren and DeHart (1992) confirmed that synthet- enstatite, suggesting a defect center possibly assigned ic enstatite without impurities of Mn and Cr emits blue to ‘intrinsic defect’ center formed during crystal growth. CL, suggesting other emission centers not related to im- The synthetic enstatite without any activator and quench- purity centers. er ions emits a blue emission (e.g., Lofgren and DeHart, The CL spectral data obtained here were converted 1992). The same case of blue luminescence has been re- into energy units for spectral deconvolution using a Gauss- ported in several minerals (e.g., quartz) without any acti- ian curve fitting because one Gaussian curve in energy vators (e.g., Marshall, 1988). units should correspond to one specific type of emission The emission component at 3.18 eV might be due to center (Yacobi and Holt, 1990; Stevens–Kalceff, 2000, the defect center associated with structural distortion by 2009). Figures 3 and 4 show CL spectral patterns in energy the substitution of Al for Si in a tetrahedral site. Emission units and the components deconvoluted by a Gaussian fit- bands at around 400 nm (3.10 eV) in a blue region were ting using the peak–fitting software (Peak Analyzer) im- detected in most of the luminescent forsterite in meteor- plemented in OriginPro 9.1, where a number of the com- ites as well as synthetic forsterite, while their CL inten- ponent was determined using a statistical test of χ2 factor sities change depending on the samples (e.g., Gucsik et proposed by Stevens–Kalceff (2009). It results in five al., 2012, 2013). Steele et al. (1985) and Benstock et al. Gaussian components indicated in Figures 3 and 4, which (1997) indicate that a blue CL emission in forsterite cor- can be identified to corresponding emission centers. relates with the concentrations of refractory–lithophile el- Terrestrial enstatite (Sri2) has two emission compo- ements such as Ca, Al, and Ti, and could be derived by nents related to impurity centers of Cr3+ at 1.70 eV and the distortion of T(Si, Al)–O chain when Al substitutes Mn2+ at 1.85 eV in a red region by referring to the results for Si and/or deformation of the lattice due to an incor- of the synthetic samples (e.g., Steele, 1988; Catalano et poration of Ca and Ti ions. Such distortion involved with al., 2014), and two components at 2.72 and 3.10 eV in a defects and unpaired electrons is caused by the substitu- blue region (Fig. 3), whereas extraterrestrial enstatite (Y– tions of Si by Al and Ti and of Mg by Ca. Al2O3 contents 86004) has a characteristic component at 3.86 eV in a in terrestrial enstatite increases with an increase in the near–UV region in addition to those obtained in terrestrial intensity of the component at 3.10 eV when the CL color one (Fig. 4). The emission components at 729 nm (1.70 changes red to blue. A major emission center at 3.10 eV eV) and 663–670 nm (1.85–1.87 eV) are assigned to Cr3+ in a blue region has a correlation with the concentrations 2+ and Mn , individually, according to Steele (1988) and of Al2O3 in the enstatite, but not clearly with CaO and Catalano et al. (2014). TiO2 contents, suggesting a control of blue CL mostly by The emission component at 2.72 eV might belong to the Si–Al substitution in a tetragonal site. the defect center, which is analogous to the defect center Extraterrestrial enstatite (Y–86004) gives an addi- of the component at 2.75 eV detected in synthetic forster- tional emission component at 3.87 eV in a blue to UV ite (Gucsik et al., 2012). Furthermore, this component region, which had not been found in enstatite so far. It is Cathodoluminescence characterization of enstatite 245

Figure 5. CL spectrum in energy units of extraterrestrial enstatite Figure 6. CL spectrum in energy units of extraterrestrial enstatite (Sah97096b) with their emission components deconvoluted by a (Sah97121) with their emission components deconvoluted by a Gaussian curve fitting. Gaussian curve fitting. not detected in any terrestrial enstatite, suggesting a char- hedrally–coordinated with oxygen, the energy level of 4 4 2+ acteristic emission near UV probably related to the origin T1g ( G) with excited–state Mn decreases with an in- in a cosmic environment through the course of formation crease in the Dq (Marfunin, 1979). Therefore, the emis- and progression of the enstatite in meteorite. Cosmic ray sion energy specified by the distance between excited 4 6 has extremely high–energy particles with intrinsic mass state ( T1g) and ground state ( A1g) reduces with increas- so that it flicks out oxygen which consists of enstatite ing Dq. In the case of enstatite, therefore, the Mn–ligant crystal structure and forms oxygen vacancy responsible distance for Oen is longer than that for Cen, resulting in for defect center with a near–UV emission. Therefore, the the spectral–peak position of Mn activation for the Oen is total exposure of cosmic rays induced on the enstatite located at a longer–wavelength side than that of the Cen as might be monitored using the intensity of the emission obtained in this study. Therefore, CL feature of red emis- component at 3.87 eV. sion could be useful to distinguish between Oen and Cen Raman spectroscopy referred to the method by Ul- phases with a micro–meter size in the enstatite. mer and Stalder (2001) shows that all terrestrial enstatite with CL are identified only to orthoenstatite (Oen), but ex- ACKNOWLEDGMENTS traterrestrial enstatite to Oen and clinoenstatite (Cen). In the CL spectrum of extraterrestrial enstatite (Sah97096b We are deeply thankful to Y. Sawada for chemical analy- and Sah97121), the peak position of emission band in a sis by EPMA and helpful suggestions on an enstatite red region is located at around 670 nm for Oen, while at chemistry, and to M. Kayama for valuable information around 650 nm for Cen. The result of spectral deconvo- of silicate CL. We thank K. Hayashi for editorial han- lution in a red region reveals that the positions of emission dling, and anonymous reviewers for critical comments components differ between Oen and Cen (Figs. 5 and 6). and suggestions. An emission component related to Mn2+ is centered at 1.85 eV for extraterrestrial Oen (Sah97096b) and at 1.85 REFERENCES eV terrestrial Oen (Sri2), but at 1.90 eV for extraterrestrial Cen (Sah97121). It suggests that the energy of the emis- Benstock, E.J., Buseck, P.R. and Steele, I.M. (1997) Cathodolumi- sion component activated by Mn2+ might be used to iden- nescence of meteoritic and synthetic forsterite at 296 and 77 K using TEM. American Mineralogist, 82, 310–315. tify the Oen and Cen, in which most of Mn2+ occupies M2 Catalano, M., Bloise, A., Pingitore, V., Miriello, D., Cazzanelli, E., – site. According to Koto (1967), the nearest neighbor dis- Giarola, M., Mariotto, G. and Barrese, E. (2014) Effect of Mn tances between ligant oxygen and M2 ion is 1.985 Å for doping on the growth and properties of enstatite single crys- Oen and 2.02 Å for Cen. Sommer (1972) and Marfunin tals. Crystal Research and Technology, 49, 736–742. (1979) concluded that the strength of crystal field (Dq) Crookes, W. (1879) Contributions to molecular physics in high vacua. Philosophical Transactions of the Royal Society, 170, around the tradition metal elements such as Mn is en- 641–662. hanced in inverse proportion to the fifth power of met- Derham, C.J., Geake, J.E. and Walker, G. (1964) Luminescence of al–ligands (oxygen) distance. In the case of Mn ion octa- enstatite achondrite meteorites. Nature, 203, 134–136. 246 S. Ohgo, H. Nishido and K. Ninagawa

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