- © The Mineralogical Society
Megawite is a perovskite-group mineral with an ideal formula CaSnO3 that was discovered in altered silicate-carbonate xenoliths in the Upper Chegem caldera, Kabardino-Balkaria, Northern Caucasus, Russia. Megawite occurs in ignimbrite, where it forms by contact metamorphism at a temperature >800°C and low pressure. The name megawite honours the British crystallographer Helen Dick Megaw (1907–2002) who did pioneering research on perovskite-group minerals. Megawite is associated with spurrite, reinhardbraunsite, rondorfite, wadalite, srebrodolskite, lakargiite, perovskite, kerimasite, elbrusite-(Zr), periclase, hydroxylellestadite, hydrogrossular, ettringite-group minerals, afwillite, hydrocalumite and brucite. Megawite forms pale yellow or colourless crystals up to 15 μm on edge with pseudo-cubic and pseudo-cuboctahedral habits. The calculated density and average refractive index are 5.06 g cm−3 and 1.89, respectively. Megawite is Zr-rich and usually crystallizes on lakargiite, CaZrO3. The main bands in the Raman spectrum of megawite are at: 159, 183, 262, 283, 355, 443, 474, 557 and 705 cm−1. The unit-cell parameters and space group of megawite, derived from electron back scattered diffraction, are: a = 5.555(3), b = 5.708(2), c = 7.939(5) Å, V = 251.8(1) Å3, Pbnm, Z = 4; they are based on an orthorhombic structural model for the synthetic perovskite CaSn0.6Zr0.4O3.
The discovery of lakargiite, CaZrO3 (Galuskin et al., 2008a) and other perovskite-group minerals in the CaO–ZrO2–SnO2–TiO2 system, in carbonate-silicate skarn-type xenoliths in ignimbrites of the Upper Chegem caldera, led to the recognition of Sn-rich lakargiite (Galuskin et al., 2008b) and to the discovery of a new Sn-dominant analogue which has an ideal formula CaSnO3 (Galuskin et al., 2010). This mineral was approved by the CNMNC/IMA in 2010 and it is named in honour of the British crystallographer Helen Dick Megaw (1907–2002) who made significant contributions to our understanding of the structure and properties of natural and synthetic perovskites (Megaw, 1945, 1946, 1952; Megaw and Darlington, 1975).
Megawite is the natural analogue of the synthetic calcium stannate perovskite CaSnO3 (Megaw, 1946; Vegas et al., 1986; Kung et al., 2001; Mitchell, 2002; Zhao et al., 2004; Tsuchiya and Tsuchiya, 2006; Tarrida et al., 2009), which finds diverse applications in capacitors, sensors, electrodes for batteries and matrices for photo-luminescent materials (Alves et al., 2009; Chen et al., 2010). Knowledge of synthetic CaSnO3 and intermediate CaZrO3–CaSnO3 solid solutions (Tarrida et al., 2009)permits megawite to be validated as a new mineral species. The characterization of megawite by electron microprobe analysis (EMPA), Raman spectroscopy and electron back scattered diffraction (EBSD) allows derivation of the composition and unit-cell parameters from the typically very small (<15 μm) megawite crystals, occurring as inclusions in rock-forming minerals in the Chegem caldera skarn xenoliths.
Holotype samples of megawite are deposited in the Mineralogical Museum of Wrocław University, Poland under accession number MMUWr II16717 and in the Fersman Mineralogical Museum, Moscow, Russia under accession number 4021/1.
The morphology and composition of megawite and lakargiite were investigated using a Philips/FEI ESEM XL30/EDAX scanning electron microscope (Faculty of Earth Sciences, University of Silesia) and a CAMECA SX100 electron microprobe (Institute of Geochemistry, Mineralogy and Petrology, University of Warsaw). The composition of megawite was determined at an accelerating voltage of 15 kV, with a probe current of 40 nA using natural and synthetic standards. A detailed description of the EMPA procedure is provided by Galuskina et al. (2010a).
The small size of megawite crystals (<15 μm) required the use of single crystal electron backscatter diffraction (EBSD) analysis to determine the crystal structure. The EBSD images were recorded using an HKL EBSD system (HKL Technology Inc., Oxford Instruments Group) on a JSM-6480 high performance scanning electron microscope (Institute of Materials Science, University of Silesia) using an accelerating voltage of 30 kV. Thin sections used for electron microprobe analyses were repolished using an Al2O3 suspension with a 20 nm particle size. To minimize charging, specimens were coated in a carbon layer several tens of nanometres thick. The calibration of the EBSD system was carried out using a Si single crystal at detector distances) of 177 mm (normal working position) and 150 mm (camera retracted position). A Nordlys II camera with a resolution of 1344×1024 pixels was used. Either 2×2 binning or no binning was applied for the electron backscatter pattern collection at the different detector distances. To improve the pattern quality, acquisition times of between 300 and 3000 ms and frame averaging were used. Depending on the detector distance and pattern collection time, up to 30 frames were averaged. The Channel5 software package (Oxford Instruments; Day and Trimby, 2004) was used for the interpretation of the EBSD patterns. For the 150 mm detector distance, only manual band detection was used as data reduction using the Hough transform was not effective. For the 177 mm detector distance, manual plus Hough (maximum 125 resolution) band detection was applied. In the match 54 reflectors and 7–11 bands were used.
Raman spectra of single crystals of megawite were recorded using a LabRAM HR800 (Jobin-Yvon-Horiba) spectrometer equipped with an 1800 line/mm grating, a charge-coupled device (CCD) Peltier-cooled detector (1024×256) and an Olympus BX40 confocal microscope (Wrocław University of Technology). The incident laser excitation was provided by a water-cooled argon laser operating at 514.5 nm, which produced a power at the 100x objective lens of 40–60 mW. Raman spectra were recorded in 0° geometry in the range 50–4000 cm−1 with a spectral resolution of 2.5 cm−1. The collection time was 10 s and 16 scans were accumulated. The monochromator was calibrated using the Raman scattering line of a silicon plate (520.7 cm−1).
Occurrence and description of megawite
Crystals of Zr-bearing megawite were found in the spurrite zones of skarned xenolith no. 1 (20–25 m in size) in ignimbrites of the Upper Chegem caldera, Kabardino-Balkaria, Northern Caucasus, Russia (Galuskin et al., 2010; Fig. 1a–c). The geology of the area is described by Gazeev et al. (2006) and Galuskin et al. (2009). In addition to megawite, xenolith no. 1 is the type locality for six minerals: lakargiite, CaZrO3 (Galuskin et al., 2008a); chegemite, Ca7(SiO4)3(OH)2 (Galuskin et al., 2009); kumtyubeite, Ca5(SiO4)2F2 (Galuskina et al., 2009); bitikleite-(SnAl), Ca3SbSnAl3O12 (Galuskina et al., 2010a); elbrusite-(Zr), Ca3UZrFe3O12 (Galuskina et al., 2010b); and bitikleite-(SnFe), Ca3SbSnFe3O12 (Galuskina et al., 2011a).
Zirconium-bearing megawite occurs with spur-rite, Ca5(SiO4)2(CO3); hydroxylellestadite, Ca5(SiO4)1.5(SO4)1.5 (OH,F); rondorfite, MgCa8(SiO4)4Cl2; wadalite, Ca12Al10Si4O32Cl6; minerals of the reinhardbraunsite–kumtyubeite series, Ca5(SiO4)2(OH,F)2; calcite and rarely periclase. Accessory and rare minerals are represented by the perovskite-group minerals lakargiite, CaZrO3 and perovskite, CaTiO3; the garnet-group minerals kerimasite, Ca3Zr2Fe2SiO12 and elbrusite-(Zr); and by srebrodolskite, Ca2Fe2O5, and an as yet undescribed mineral with a formula Ca3TiFe2O8. Spurrite crystals are embedded in the mass of secondary minerals among which hydrocalumite, ettringite-group minerals, hydrogrossular, afwillite and other hydrosilicates (Fig. 1a) are widespread. Brucite develops after periclase.
Zirconium-bearing megawite crystals with either pseudo-cubic or pseudo-cuboctahedral crystal habits from the holotype sample do not exceed 15 μm in size and usually occur as inclusions in spurrite (Fig. 1a–c). The CaSnO3 endmember content of holotype megawite reaches 61 mol.% (Table 1, analyses 1–3 and 6). Numerous lakargiite crystals occur between spurrite grains in wadalite-calcite-hydrocalumite-ettringite aggregates (Fig. 1a; Table 1, analysis 5). Homogeneous megawite (CaSnO3 ≈ 56 mol.%, CaZrO3 ≈ 34 mol.%) and lakargiite (CaZrO3 ≈ 67 mol.%, CaSnO3 ≈ 27 mol.%) crystals are commonly found in the same spurrite crystal (Fig. 1d; Table 1, analyses 6 and 7). A few grains with compositions which oscillate about the megawite–lakargiite boundary (e.g. Table 1, analysis 4) are present. Megawite and lakargiite inclusions in spurrite are characterized by CaTiO3 contents of <10 mol.% (Table 1, analyses 1–4 and 6), whereas lakargiite in a wadalite-calcite-hydrocalumite aggregate between spurrite crystals has >20 mol.% CaTiO3 (Table 1, analysis 5).
Following the recognition in 2009 of megawite in spurrite skarns in xenolith no. 1, there were several other discoveries. Tin-rich lakargiite (with up to 30–35 mol.% CaSnO3) was found in xenolith no. 7 (10–15 m in size), which is located 1.5 km from xenolith no. 1 (see the geological map in Galuskin et al., 2009), and is the type locality for bitikleite-(ZrFe), Ca3SbZrFe3O12 (Galuskina et al., 2010a); vorlanite, CaUO4 (Galuskin et al., 2011a) and irinarassite, Ca3Sn2Al2SiO12 (Galuskina et al., 2011b). Crystals of Sn-rich lakargiite (with pseudo-cubic or pseudo-cuboctahedral habits) overgrow porous, fine-grained aggregates of almost pure lakargiite, which occur with relict larnite, rondorfite, wadalite, magnesioferrite, As-bearing hydroxylellestadite and srebrodolskite (Fig. 1d; Table 1, analyses 8 and 9). Rarely, megawite occurs as rims up to 5 μm thick on Sn-rich lakargiite crystals in the highly altered parts of the chegemite-bearing skarns which contain hydrogarnet and ettringite (Fig. 1e). The megawite composition approaches the boundary with lakargiite (CaSnO3 ≈ 50 mol.%, CaZrO3 ≈ 45 mol.%; Table 1, analysis 10). A phase close to burtite, CaSn(OH)6 (Sonnet, 1981), which overgrows the lakargiite aggregates, (Fig. 1e) is also found in this association.
Megawite was also found in xenolith no. 3 (20 m in size), which is the type locality for toturite, Ca3Sn2Fe2SiO12 (Galuskina et al., 2010c); pavlovskyite, Ca8(SiO4)2(Si3O10) (Galuskin et al., 2011b) and rusinovite, Ca5(Si2O7)3Cl2 (Galuskin et al., 2011c). Xenolith no. 3 is located 10–15 m from xenolith no. 1 (see the geological map in Galuskin et al., 2009). Megawite forms rims on μm-sized lakargiite crystals in larnite-cuspidine zones in skarns which contain abundant hydrogarnet and hydrosilicates (Fig. 1f; Table 1, analyses 11 and 12). Megawite and garnets (toturite and Sn-bearing schorlomite) form rims on lakargiite pseudomorphs after zircon (Fig. 1f; Table 1, analysis 13). Megawite from xenolith no. 3 contains considerable CaZrO3 and CaTiO3, and also U, Nb and Fe impurities (Table 1, analyses 12 and 13). The megawite rims on lakargiite pseudomorphs after zircon contain 0.331 Sn atoms per formula unit (a.p.f.u.) (33 mol.% of the megawite endmember); 0.278 Zr a.p.f.u. (28 mol.% of the lakargiite endmember) and 0.273 Ti a.p.f.u. (27 mol.% of the perovskite endmember); the remaining 12% is accounted for by Fe3+-bearing endmembers (Table 1, analysis 13).
Megawite crystals are transparent and pale yellow or colourless with a vitreous lustre and white streak. Megawite is biaxial, but twinning and small crystal sizes prevented the determination of the refractive indices and optic sign. The calculated refractive index of megawite is 1.89 (for pure CaSnO3). The calculated density for megawite corresponding to analysis 1 (Table 1) is 5.06 g cm−3. The synthetic analogue of megawite, orthorhombic CaSnO3, is optically positive. Crystallographic orientation by analogy with lakargiite is: X = β, Y = α, Z = γ. Megawite has good cleavage along (110) and (001) (orthorhombic indices).
Structural data for megawite were derived using EBSD (Fig. 2). The best mean angular deviation (MAD) ≈ 0.2° (an excellent fit) was obtained for a cubic model with a = 3.8 Å. However, taking into account structural data for synthetic CaSnO3 and phases of the Ca(Sn1–xZrx)O3 series (Megaw 1946; Vegas et al., 1986; McMillan and Ross, 1988; Kung et al., 2001; Mitchell, 2002; Tarrida et al., 2009), structural data for lakargiite megawite solid solutions (Galuskin et al., 2008b) and the optical properties (megawite is an optically biaxial mineral); an orthorhombic model was chosen with a = 5.555(3), b = 5.708(2), c = 7.939(5) Å, V = 251.8(1) Å3, Pbnm, Z = 4. The unit-cell parameters of this structural model are based on data for the synthetic phase CaSn0.6Zr0.4O3 (Tarrida et al., 2009) and fitting gives a MAD = 0.4° (a very good fit).
Due to the scarcity of megawite and the small size of the available crystals a powder X-ray diffraction (XRD) pattern could not be collected. Calculated XRD data for analysis 1 are provided in Table 2. Values are calculated for Co-Kα radiation and Debye-Scherrer geometry.
The Raman spectrum of megawite (Fig. 3) was obtained from the grain shown in Fig. 1c at point 6, where EMPA was also performed. After repolishing the thin section using colloidal Al2O3, EBSD images were obtained at the same point (Fig. 2). The Raman spectrum of megawite is similar to that of synthetic CaSnO3 (McMillan and Ross, 1988) and synthetic perovskite of composition Ca(Sn0.6Zr0.4)O3 (Tarrida et al., 2009). The following bands can be assigned (Fig. 3, spectrum 1): 159 cm−1 and 183 cm−1 (lattice soft modes); 262 cm−1, 283 cm−1 (Sn–O and Zr–O bending modes); 355 cm−1 and 443 cm−1 (torsional modes); 474 cm−1, 557 cm−1 (Sn–O and Zr–O stretching modes); 705 cm−1 (overtone). The megawite spectrum also resembles that of Sn-rich lakargiite (Fig. 3, spectrum 2).
Orthorhombic perovskites of the CaTiO3–CaZrO3–CaSnO3 system are widespread in skarns that have formed within xenoliths in the ignimbrites of the Upper Chegem caldera. High Sn contents are observed only in lakargiite, an observation which can be explained by the very similar ionic radii of Zr4+ (0.72 Å) and Sn4+ (0.69 Å) (Shannon, 1976). The holotype megawite contains substantial amounts of CaTiO3 and CaZrO3 in solid solution (Table 1, analysis 1; Fig. 4). Crystals of Zr-bearing megawite from the holotype sample are found in the same spurrite crystal as Sn-bearing lakargiite (Fig. 1c, 4; Table 1, analyses 6 and 7), an observation that may indicate the presence of a miscibility gap in this solid solution under the conditions in which the skarn formed (Fig. 4). Other spurrite crystals from the same sample of perovskite with megawite–lakargiite intermediate composition (Fig. 4; Table 1, analysis 4) suggest low mobility of Sn and Zr, i.e. the perovskites inherited chemical inhomogeneities in the protolith.
Megawite belongs to the early skarn mineral association at the Upper Chegem caldera, which contains lakargiite, perovskite, magnesioferrite, larnite, wadalite and rondorfite. This assemblage was formed by high-temperature contact metamorphism of carbonate xenoliths in persilicic ignimbrites (age 2.8 Ma) under larnite-facies conditions (temperature >800°C and low pressures) as described by Gazeev et al., (2006) and Galuskin et al., (2008a,b; 2009).
The authors thank Prof. Roger Mitchell and Principal Editor Dr M.D. Welch for their careful revision that improved the early version of the manuscript. I.G. and E.G. acknowledge support by the Ministry of Science and Higher Education of Poland, grants N N307 097038 and N N307 100238, respectively.
- Manuscript received 24 January 2011.
- Manuscript Accepted for publication 25 May 2011.