- © 2008 American Mineralogist
Barioperovskite, ideally BaTiO3, is a new member of the perovskite group. It is found as micro- to nano-crystals in a host of amorphous material contained within hollow, tubular inclusions in benitoite from the Benitoite Mine, San Benito County, California, U.S.A. The mean chemical composition determined by electron-microprobe analysis is (wt%) BaO 65.46, TiO2 34.57, SiO2 0.89, sum 100.92. The empirical formula calculated on the basis of 3 O is Ba0.97Ti0.98Si0.03O3. Barioperovskite is orthorhombic, Amm2; a = 3.9874 Å, b = 5.6751 Å, c = 5.6901 Å, V = 128.76 Å3, and Z = 2. The electron backscattered diffraction pattern is an excellent match to that of synthetic BaTiO3 with the Amm2 structure. The strongest calculated X-ray powder diffraction lines from the synthetic BaTiO3 data are [d in Å, (I), hkl] 4.018 (18) (011), 2.845 (30) (002), 2.830 (100) (111), 2.316(20) (102), 2.312 (23) (120), 2.009 (28) (022), 1.640 (17) (113), 1.637 (19) (131), 1.633 (18) (202), and 1.415 (15) (222). The mineral is named after its composition, a Ba-dominant member of the perovskite group.
During our detailed microanalytical characterization of a polished benitoite crystal that we use as a primary electron-microprobe standard, we found micro- to submicro-inclusions of BaTiO3, a new perovskite phase. Electron-microprobe, scanning electron microscope, electron backscatter diffraction, and Raman analyses have been used to characterize its composition and structure. Synthetic BaTiO3 is well studied in the field of material science. Ultrafine particles of barium titanate were previously reported to occur in the matrix of the Allende meteorite (Tanaka and Okumura 1977), but due to their small size, their chemistry was not well defined and their crystal structure was not determined. Here, we document the occurrence of BaTiO3 as a natural terrestrial phase.
The mineral is named for its composition, a Ba-dominant member of the perovskite group. The mineral and the mineral name have been approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (proposal IMA 2006-040). The holotype specimens of barioperovskite consisting of a polished crystal with three tubular inclusions have been removed from our electron microprobe standard block (P1014) and were deposited at the Smithsonian Institution’s National Museum of Natural History, registration number NMNH 174513.
The mineral occurs in the Benitoite Mine (formerly the Dallas Gem Mine), near Santa Rita Peak, New Idria District, San Benito Mountains, San Benito County, California, U.S.A. (Wise and Gill 1977). At this mine, benitoite is found in natrolite veins in blueschist bodies within serpentinite. It is associated with neptunite, joaquinite-(Ce), jonesite, and djurleite, among others. A transparent, light blue benitoite (BaTiSi3O9) crystal from this locality was ground and polished to a flat surface for use as a standard for electron-microprobe analysis in our laboratory. The crystal contains four hollow, tubular inclusions, three of which are now exposed at the polished surface of the crystal (Fig. 1⇓). The inclusions are sub-parallel to each other, but are not aligned with the principal crystallographic axes but, instead, run approximately parallel to [0 1 1̅ 1̅] as determined from the electron backscatter diffraction pattern of the host crystal. Barioperovskite occurs as a component of the tubular inclusions. The size of the barioperovskite crystals, a few micrometers to sub-micrometer, required microanalytical methods to further characterize the phase.
Backscattered electron (BSE) images were obtained both with a LEO 1550VP field emission SEM and a JEOL 8200 electron microprobe. Quantitative elemental micro-analyses were conducted with the JEOL 8200 electron microprobe operated at 15 kV and 10 nA in a focused beam mode using the Probe for Windows software. Standards for the analysis were TiO2 (TiKα) and benitoite (BaLα, SiKα). Analyses were processed with the CITZAF correction procedure (Armstrong 1995).
Single-crystal electron backscatter diffraction (EBSD) analyses at a sub-micrometer scale were preformed using an HKL EBSD system on the LEO 1550VP scanning electron microscope, operated at 20 kV and 1 nA in a focused beam with a 70° tilted stage. The EBSD system was calibrated using a single-crystal silicon standard. The structure was determined and cell constants were obtained by matching the experimental EBSD pattern with the ICSD BaTiO3, BaTi2O5, fresnoite, and benitoite structures.
Raman spectroscopic micro-analysis was carried out using a Renishaw M1000 micro-Raman spectrometer system on domains of the sample in polished section previously identified as BaTiO3 crystals through BSE imaging and EBSD analysis. Approximately 1.4 mW of 514.5 nm laser illumination (at the sample) focused with a 100× objective lens provided satisfactory spectra. The spot size was about 2 μm. Peak positions were calibrated against a silicon standard. A dual-wedge polarization scrambler was used in the laser beam for all spectra to minimize the effects of polarization.
Barioperovskite crystals were first recognized by their strong backscattered electron intensity in the scanning electron microscope (Fig. 2⇓). They occur as individual, irregular grains, 1–10 μm, and tabular (at times, approaching dendritic) crystals usually <1 μm wide and 6–8 μm long. The physical appearance of the tabular crystals suggests that they may be part of a larger, partly hidden, dendritic structure. This suggestion is re-enforced by the EBSD patterns on several adjacent tabular crystals that indicate that the adjacent crystals have the same crystallographic orientation. No obvious crystal forms were observed. There was no visual indication of twinning and no indication of twinning in the EBSD patterns.
Color, refractive indices, optic sign, streak, luster, hardness, tenacity, cleavage, fracture, and density could not be determined because of the small grain size. There is no evidence of fluorescence under long-wave or short-wave ultraviolet radiation that could be seen above the fluorescence of the host benitoite. Cathodoluminescence was not observed under the SEM.
BSE imaging and electron-probe analysis show that one tubular inclusion found in the benitoite crystal (Fig. 2⇑) contains micrometer-sized crystals with an ideal chemical composition of BaTiO3. Nine chemical analyses (Table 1⇓) were carried out by means of an electron microprobe (wavelength-dispersive spectrometry mode, 15 kV, 10 nA, beam focused to a nominal 150 nm spot). No other elements with atomic number greater than 4 were detected. The analysis shows that these crystals have an empirical formula Ba0.969Ti0.982Si0.034O3. H2O and CO2 are absent (Raman spectrum). Because the crystals are naturally embedded in a barium titanium silicate matrix, some (if not all) of the silicon counts in the analysis are likely from excitation of the adjacent and underlying matrix. The empirical formula (based on three O apfu) is Ba0.969Ti0.982Si0.034O3. The simplified formula is BaTiO3, which requires: BaO 65.74, TiO2 34.26, Total 100.00 wt%. Nano-crystals of this BaTiO3 phase also occur in the benitoite crystal within a third inclusion (Fig. 2c⇑) that is not illustrated in Figure 1⇑.
Raman micro-analyses show that the spectrum of the barioperovskite is similar to that of synthetic BaTiO3, as shown in Figure 3⇓, but that the Raman modes are broader than that of the single-crystal reference. The spectrum of no other Ba-titanite, Ba-titanosilicate, or Ba-silicate that we investigated came close to resembling the pattern of barioperovskite. The Raman spectra of representative excluded candidate phases also appear in Figure 3⇓. In addition to the broad features of barioperovskite, possible weak features of benitoite can be seen at about 575 and 535 cm−1 in our pattern as well as possible weak features from the adjacent phases.
Fresnoite (Ba2TiSi2O8) is present in all tubular inclusions as shown by electron-microprobe analysis, the Raman spectrum, and the EBSD pattern. In all three tubular inclusions, we have regions that give EBSD and Raman patterns that match fresnoite. Much of this material in these regions is sub-micrometer in size and is too small to fill the beam of the electron microprobe. As such, the electron-microprobe analysis deviates significantly from the ideal fresnoite composition. One of regions (the widest) that gives an excellent EBSD fresnoite pattern shows a significant excess of Ti in the microprobe analysis. We do not know if the Ti excess is the actual composition of the inclusion, or if it is from unwanted electron beam excitation of nearby components.
An additional component that is EBSD-amorphous and Raman-amorphous occurs in all three tubular inclusions and is generally in contact with the barioperovskite and fresnoite. The electron-microprobe analysis of the amorphous material is relatively homogeneous within a single tubular inclusion, but varies from one inclusion to another, as shown in Table 2⇓. No benitoite was found within contents of the tubular inclusions.
Barioperovskite occurs in two of the three tubular inclusions in benitoite. The inclusions are currently hollow tubes of 60–100 μm width whose interiors are partly coated with a mixture of crystalline materials and material that is EBSD- and Raman- amorphous (Fig. 2a⇑). The BaTiO3 is found as discrete crystallites up to 10 μm wide in this coating (Fig. 2b⇑).
Two more new Ba-titanate phases (BaTi2O5, BaTi3O7) were discovered at sub-micrometer to nanometer scales, along with barioperovskite and fresnoite within the amorphous materials in two inclusions. BaTi2O5 crystals were recognized by their lath shapes in the SEM images (Fig. 2b⇑). They occur as individual, euhedral grains, 100 × 200 nm to 500 nm × 5 μm. The mean chemical composition of BaTi2O5 determined by electron-micro-probe analysis on the largest crystals is (wt%) BaO 50.22, TiO2 43.63, SiO2 7.77, sum 101.62. The empirical formula calculated on the basis of 5 O is Ba0.98Ti1.63Si0.39O5. Its electron backscattered diffraction pattern is not a good match to that of synthetic BaTi2O5 with C2 structure, which may be due to Si substitution in the structure. Raman micro-analyses show that the spectrum of BaTi2O5 is not close to that of synthetic BaTi2O5, and it is also mixed with the spectrum of nearby phases.
Crystals of BaTi3O7 are lath-shaped, separate, 300 × 900 nm to 1 × 8 μm (Fig. 2c⇑). The chemical composition of BaTi3O7 on the largest crystal is (wt%) BaO 40.81, TiO2 59.53, SiO2 0.51, sum 100.85. The empirical formula calculated on the basis of 7 O is Ba1.05Ti2.94Si0.03O7. Its EBSD pattern implies a low-symmetry structure. It was not indexed because the synthetic BaTi3O7 structure is unknown. The Raman modes of the BaTi3O7 are very broad and contaminated by nearby phases.
Both the BaTi2O5 and BaTi3O7 phases have been discussed in the material science literature (Kimura et al. 2003; Lu and Hong 2003), but BaTi3O7 has not been fully characterized. We were not able to determine the crystal structures of either phase based on their EBSD and Raman data because of their small grain size in the benitoite inclusions.
Electron diffraction patterns
EBSD patterns of barioperovskite were matched against all the structural variants (Pm3̅m, P4mm, Amm2, R3m) of synthetic BaTiO3 reported in Kim et al. (2004), Aoyagi et al. (2002), Kwei et al. (1993), Waesche et al. (1981), and Megaw (1962); the patterns also were compared with the structures of BaTi2O5, benitoite, and fresnoite. The patterns were indexed to give a best fit based on Amm2 BaTiO3 structure (Fig. 4⇓) from Kwei et al. (1993), showing a = 3.9874 Å, b = 5.6751 Å, and c = 5.6901 Å. The calculated density is 5.91 g/cm3 using the empirical formula and the cell constants from Kwei et al. (1993).
The BaTiO3 crystals studied by electron backscatter diffraction are too small for single-crystal XRD study. The EBSD pattern (Fig. 4⇑), determined in the SEM, was an excellent match to the computed EBSD pattern and cell parameters of synthetic BaTiO3 (ICSD collection code 073641, PDF 81-2200) from Kwei et al. (1993). No errors are stated because the cell parameters are taken directly from the data of the matching BaTiO3 phase in Kwei et al. (1993). The a:b:c ratio calculated from the unit-cell parameters is: 0.7026:1:1.0026. The crystals are orthorhombic with space group: Amm2; a = 3.9874 Å, b = 5.6751 Å, c = 5.6901 Å, V = 128.76 Å3, and Z = 2. The XRD data used for this study were taken from PDF 81-2200, calculated from the ICSD structure (Kwei et al. 1993) using POWD-12++ 1997. The strongest calculated lines are [d in Å, (I), hkl] 4.018 (18) (011), 2.845 (30) (002), 2.830 (100) (111), 2.316(20) (102), 2.312 (23) (120), 2.009 (28) (022), 1.640 (17) (113), 1.637 (19) (131), 1.633 (18) (202), 1.415 (15) (222).
The precursors of barioperovskite are believed to have formed under the same conditions as the associated benitoite. Benitoite is believed to form at a temperature below 95 °C (Wise and Gill 1977). We speculate that the Ba-titanate phases formed in a gel within the tubular inclusions. Support for this concept come from what appear to be late-stage shrinkage cracks that split BaTiO3 and the remaining amorphous material within the tubular inclusions. Although it is possible that the BaTiO3 formed at the time of benitoite crystallization, we think that it is more likely that it slowly crystallized in the amorphous material at a later time.
BaTiO3 is a well-known and well-studied synthetic phase. It is widely used commercially because it has interesting ferroelectric, piezoelectric, and pyroelectric properties as well as useful non-linear optical properties and dielectric properties. At high temperature, BaTiO3 has the cubic perovskite structure. BaTiO3 undergoes three phase transformations with decreasing temperature. At ambient pressure, the compound is Pm3̅m cubic at high temperature, transforms to P4mm at 393 K, then to Amm2 at 278 K and finally to a R3m below 183 K (Kwei et al. 1993). Although it is generally believed that benitoite formed at a comparatively low temperature, its estimated conditions of formation (Wise and Gill 1977) are at higher temperatures than the transition temperature of BaTiO3 from tetragonal to orthorhombic. The occurrence of the Amm2 polymorph in the inclusions in the natural sample suggests that the minor amount of Si found in the analysis may be incorporated in the structure of barioperovskite and that it contributes to the stability of the Amm2 polymorph at temperatures above 278 K.
As a follow up to the observations of Tanaka and Okumura (1977), we also carefully examined under the SEM several sections prepared from one Allende meteorite specimen in the Caltech collection but did not find any Ba-titanate phase. It is apparent that the Allende meteorite varies very much from one specimen to the other. Likewise, the tubular inclusions that contain barioperovskite are uncommon in benitoite. In addition to the three tubular inclusions exposed at the polished surface of our benitoite crystal, one additional tubular inclusion is contained entirely within the crystal. This inclusion has not been studied by any of the analytical methods used to characterize the barioperovskite. Numerous other benitoite crystals have been examined under the optical microscope, but, to date, no additional tubular inclusions have been identified.
This work was funded, in part, through grant EAR-0337816 from the U.S. National Science Foundation and, in part, by the White Rose Foundation. The Caltech GPS Analytical Facility is supported, in part, by grant NSF EAR-0318518. We thank Anton Chakhmouradian and an anonymous reviewer for their constructive reviews.
Manuscript handled by Sergey Krivovichev
- Manuscript Received March 16, 2007.
- Manuscript Accepted September 11, 2007.