- © 2004 American Mineralogist
The crystal structure of a pale blue transparent Mn-rich fluorapatite (MnO: 9.79 wt%) with the optimized formula ~(Ca8.56Mn2+1.41Fe2+0.01)P6O24F2.00 and space group P63/m, a = 9.3429(3), c = 6.8110(2) Å, Z = 2 has been refined to R = 2.05% for 609 unique reflections (MoKα). The Mn in the Eibenstein an der Thaya, Austria apatite is strongly ordered at the Ca1 site: Ca1: Ca0.72(1)Mn0.28, Ca2: Ca0.96(1)Mn0.04. There is a linear variation in <Ca1-O> as a function of Mn content (r2 = 1.00). The dominant band in the optical absorption spectrum of fluorapatite from Eibenstein is in the 640 nm region with E || c > E ⊥ c. The 640 nm band is attributed to Mn5+ at the P site by analogy with previous studies. This interpretation is consistent with studies of well-characterized synthetic materials of the apatite structure that contain Mn5+. Because Mn5+ has intense absorption in the visible region of the spectrum, if a small proportion of the total Mn is Mn5+ at the P site, that substituent dominates the spectrum and the color of the mineral. To determine if the pale blue color is due to radiation effects, a fragment of the fluorapatite crystal was heated at 400° C for 1 hour, and the change in color was slight. All of these observations are consistent with the origin of color from Mn5+. Assuming that all the intensity of the 640 nm (E || c) absorption is from Mn5+, the concentration of Mn5+ in this fluorapatite sample was calculated as 2.6% of the total manganese content (~P5.96Mn5+0.04). The calibration was estimated from the spectrum of the related compound Sr5(P0.99Mn5+0.01)3Cl. The weak band at about 404 nm in the E || c spectrum may be the corresponding band for Mn2+ in octahedral coordination.
Introduction and previous work
The apatite structure sequesters substituent ions at all of its cation positions, including the Ca1 and Ca2 sites. Cation and anion substituents in the apatite structure are given by Pan and Fleet (2002) and Hughes and Rakovan (2002); both summaries are contained in a volume that summarizes many aspects of apatite and its properties (Kohn et al. 2002).
Mn-bearing apatite, both synthetic and natural, has been the subject of numerous studies because of the high affinity of apatite for Mn in geologic systems and because of the importance of Mn-doped apatite in the fluorescent lighting industry. The phase is particularly important to that industry, as Mn2+ in apatite can be a fluorescence activator or coactivator (often sensitized by Sb3+), and is widely used in the manufacture of fluorescent lights (Butler 1980; Blasse and Grabmaier 1994; Waychunas 2002). Numerous experiments (with contradictory results) have addressed ordering of Mn in apatite. Hughes et al. (1991) and references cited therein summarize such studies. In that work, the authors describe crystal structures of natural Mn-rich fluorapatites with MnO contents up to 6.81 wt% (1.00 apfu Mn). In microprobe analyses of Mn-bearing fluorapatite, they found concentrations up to 7.3 wt%, and an unpublished study by Miller and others (Mary K. Roden, personal communication in Hughes et al. 1991) yielded apatite with up to 7.87 wt% MnO (1.17 apfu Mn). In addition, other published analyses present MnO concentrations of 7.59 wt%, yielding 1.09 apfu Mn (Deer et al. 1962). The structure crystal of Hughes et al. (1991) with the highest Mn concentration, 1.00 apfu Mn by electron microprobe analysis (EMPA), did not contradict the assertion that Mn contents cannot exceed that value, as stated by Suitch et al. (1985). In addition, their crystal contained concentrations of ions at the Ca-sites (Fe0.10Na0.07Ce0.02) that obviated unequivocal assignment of Mn between the two Ca sites. More recently, Ercit et al. (1994) abstracted the results of their study on Mn-rich apatite containing 1.37 apfu Mn, the highest Mn-concentrations then found in natural samples; however, their sample also contained substituents other than Mn, as Fe + Zn = 0.11 apfu. Their conclusions on Mn ordering between the Ca sites were not in accord with the results of the earlier Hughes et al. study (1991).
The partitioning of Mn2+ between the two Ca site remains in dispute, although most studies indicate that there is a strong preference for the Ca1 site (Ryan et al. 1970, 1972; Ryan and Vodoklys 1971; Suitch et al. 1985; Hughes et al. 1991; Pan et al. 2002); however, none of these works refuted the putative limit of 1.00 apfu Mn (Suitch et al. 1985). Contrary to this general consensus, several studies suggest little to no preferential ordering of Mn2+ (Warren 1970; Warren and Mazelsky 1974; Ercit et al. 1994; Gaft et al. 1997). All of these studies draw their conclusions from spectroscopic data except for the X-ray diffraction study of Ercit et al. (1994). Manganese in the most Mn-rich fluorapatite described previously shows only a slight preference for Ca1 (64%) (Ercit et al. 1994), although that sample was not without other substituents. The present work involves apatite essentially devoid of substituents other than Mn (Fe = 0.009 apfu), and thus unequivocal conclusions on Mn-ordering can be drawn from it.
Tourmaline and fluorapatite, both highly enriched in Mn, were recently found in a Variscan topaz- and cassiterite-bearing pegmatite near Eibenstein an der Thaya, Lower Austria, and were described by Ertl et al. (in review). They described pink to yellow-brown Mn-rich tourmaline with up to ca. 9 wt% MnO. The crystal structure and chemical analyses, including the light elements, of Mn-rich tourmaline samples (Li-bearing olenite) with MnO contents in the range 8–9 wt% MnO from this locality were reported in Ertl et al. (2003).
The apatite crystal used in this study was a transparent paleblue cleavage fragment approximately 300 × 300 × 50 μm. A subfragment of this was separated and used for X-ray diffraction data collection. Another apatite fragment with similar Mn concentration was found within approximately 0.5 m of the sample analyzed for this work. This sample is distinctly different in that it contains abundant secondary fluid inclusions as well as inclusions of other minerals (quartz and mica).
A preliminary crystal-structure refinement showed high Mn contents in blue apatite for the pegmatite at Eibenstein an der Thaya, Lower Austria. Three apatite crystals were analyzed by EMPA. The crystal with the highest MnO content (9.79 wt%) was used for data collection. The microprobe analyses confirmed the homogeneity of this crystal. The color of the crystals in the Munsel system of color nomenclature is approximately 9B 7/4, corresponding to a pale blue color (Munsell Color 1976; Kelley and Judd 1976).
The crystal was extracted from the epoxy mount used for the electron microprobe analysis and was subsequently mounted on a glass fiber on a Bruker Apex CCD diffractometer equipped with graphite-monochromated MoKα radiation. Refined cell-parameters and other crystal data are listed in Table 1⇓. Redundant data were collected for an approximate sphere of reciprocal space, and were integrated and corrected for Lorentz and polarization factors using the Bruker program SAINTPLUS (Bruker AXS Inc. 2001).
The structure was refined using the Bruker SHELXTL V. 6.10 package of programs, with neutral-atom scattering factors and terms for anomalous dispersion. Refinement included anisotropic-displacement parameters for all atoms. Table 2⇓ lists atom parameters, and Table 3⇓ lists selected interatomic distances.
Chemical analyses (Table 4⇓) were done in wavelength-dispersive spectroscopy mode with a CAMECA SX-50 electron-microprobe at the Ruhr-University-Bochum, Germany, at an acceleration voltage of 15 kV, a sample current of 15 nA, and a beam diameter of approximately 5 μm. Natural and synthetic materials were used as standards: natural andradite (Fe, Ca), natural spessartine (Mn), natural topaz (F), and synthetic AlPO4 (P). The analytical data were reduced and corrected using the PAP routine. Excellent agreement was obtained between analyzed Mn by EMPA and structure refinement: 1.44 apfu by the former method and 1.36 by the latter. The optimized formula (Wright et al. 2000) resulting from the two methods is given in Table 4⇓. The mean F-value is slightly higher than the theoretical maximum value when OH is not present (Table 4⇓). A likely explanation is the well known effect that the intensity of apatite FKα radiation depends on the crystal orientation relative to the electron beam (Stormer et al. 1993).
A sub-mm crystal fragment of the manganian apatite was prepared as a 0.074 mm thick parallel plate (polished on both sides) that contained the c axis. Polarized optical absorption spectra in the 390–1100 nm range were obtained at about one nm resolution with a home-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 infrared microscope containing a calcite polarizer. A pair of conventional 10× objectives was used as an objective and a condenser. Spectra were obtained through the central area of the sample, the clearest region with only minor inclusions. At this thickness, the crystal is pale blue with moderately weak pleochroism: E || c = darker blue; E ⊥ c = paler blue.
Results and discussion
Virtually all Mn in the Eibenstein an der Thaya sample resides at the Ca1 site, as the sites have the following occupancies: Ca1: Ca0.72(1)Mn0.28, Ca2: Ca0.96(1)Mn0.04. Hughes et al. (1991) used bond-valence theory to explain the site-preference of Mn for the Ca1 site in apatite. In the Austrian apatite, bond-valence sums for Mn2+ clearly illustrate the reason for the ordering of Mn2+ at the Ca1 site. Mn2+ at the Ca1 site has an incident bond-valence sum of 1.38 v.u., whereas Ca at the Ca2 site has an incident bond-valence sum of 1.12 v.u. (it must be noted that the bond-valence sums were calculated using the average M sites, as different sites for Ca and Mn were not distinguished). Clearly, the incident bond-valence sums are far too low at either site, but closer to the formal valence (2 v.u.) at the Ca1 site; thus it principally resides there.
High-spin Mn2+ involves no crystal-field stabilization energy, and only radius and site geometry should control Mn2+ substitution in the apatite structure. Indeed, average bond-length at the site that incorporates the smaller Mn2+ ion as a substituent for Ca (<Ca1–O>) decreases linearly with increasing Mn content (Fig. 1⇓).
Ercit et al. (1994) noted that the incorporation of Mn at Ca1 results in long-range positional disorder of the O2 and O3 sites; such disorder is also evident in our study, as exhibited in the atomic-displacement parameters of those atoms (Table 2⇑; note particularly the values of U11 and U22 for O3, and U33 for O2). Disordering of O2 about the mirror plane implies local symmetry-breaking to accommodate the movement of O2 off the mirror plane. Ercit et al. (1994) suggest such disorder results from the rotation of the phosphate tetrahedra approximately about the P–O1 bond, and the atomic-displacement parameters in the current study support such a contention.
The contradictory results of Ercit et al. (1994) and the present study should be addressed. The samples used in both studies contain relatively high amounts of Mn, yet the observed cation ordering differs; in the present study 82% of the Mn is ordered at the Ca1 site, whereas in Ercit et al. (1994) 64% of the Mn was so ordered. The essentially pure sample used in this study in contrast to the crystal of Ercit et al. (1994) that contained 0.11 apfu Fe and Zn substituents allows unequivocal assignment of the substituent Mn in the Eibenstein an der Thaya material. The difference between the two results may be the result of the inability to distinguish between Mn and Fe from X-ray diffraction data. However, it may be that ordering of Mn is different in the two samples as a result of different environmental conditions during crystal growth. Analysis of crystals that grew under known P-T-X conditions are required to separate these variables, and is of import in understanding accommodation of substituents in the apatite structure.
The optical absorption spectrum of Mn2+ in octahedral coordination is well-characterized and consists of two broad bands in the 450–600 nm range and a distinctive, sharp band near 408 nm in common minerals (Manning 1968). The weak band at about 404 nm in the E || c spectrum of Mn-rich fluorapatite (Fig. 2⇓) may be the corresponding band. The dominant band in Figure 2⇓ is in the 640 nm region with E || c > E ⊥ c. This band is also seen in spectra of apatite from the Hugo Mine, South Dakota, and Karelia, Russia (Gilinskaya and Mashkovtsev 1995; Rossman 2003). Gilinskaya and Mashkovtsev (1995) attribute the deep blue color of apatite from Karelia to Mn5+ at the P site. This interpretation is consistent with studies of well-characterized synthetic materials of the apatite structure that contain Mn5+ (Reinen et al. 1986; Dardenne et al. 1999). Because Mn5+ has intense absorption in the visible region of the spectrum, a small amount of Mn5+ will dominate the spectrum and the color of the mineral.
Blue color may also be associated with radiation damage in apatite (Gilinskaya and Mashkovtsev 1995). To determine if the color is radiation-induced, a fragment of fluorapatite was heated at 400 °C for 1 h along with a crystal of synthetic Sr5(P0.99Mn0.01)3Cl (Brixner 1973) and a crystal fragment of deepblue manganese-containing apatite from the Hugo Mine. None of these crystals faded as a result of the heating, indicating that their colors are not due to unstable radiation damage centers. There was an apparent slight change of color of the Eibenstein crystal fragment corresponding to the loss of a gray component. The change in color was slight, and the color remained pale blue after heating. All of these observations are consistent with the origin of color from Mn5+.
We can speculate that the 404 nm band (plus the other broad bands near 450 and 600 nm) in the fluorapatite spectrum are from Mn2+. If this is the case, it is surprising that these two bands are not significantly less intense (peak height) than the 404 nm band.
Estimation of the amount of Mn5+ at the P site
There is not enough information available to fully quantify the amount of Mn5+ in this crystal. However, limits on the Mn5+ content can be estimated from two independent methods: (1) The Mn content determined by EMPA is slightly higher (~0.08 apfu Mn) than that determined from site-scattering refinement at the Ca sites, and the P content determined from the EMPA is slightly lower (2%; ~5.88 apfu P) than for a fully occupied P site (6.00 apfu) (Table 4⇑). Therefore, an upper limit of the Mn5+ content at the P site is 5.5% of the total manganese. (2) The amount of Mn5+ may be calculated from the intensity of the 640 nm band in the optical spectrum. Although there is no established standard for the intensity of absorption of Mn5+ in apatite, it is possible to estimate the intensity from Mn5+ in a related compound. In this case, the calibration was derived from the spectrum of Sr5(P0.99Mn0.01)3Cl obtained from crystals prepared by Brixner (1973). The density of these crystals was calculated from the cell constants of Nötzold et al. (1994) assuming P = 3.0 atoms per formula unit. From these, the molar absorptivity for Mn5+ in this synthetic apatite structure was calculated to be 995. Assuming that the molar absorptivity applies to manganian fluorapatite, and assuming that all the intensity of the 640 nm (E || c) absorption is from Mn5+, the concentration of Mn5+ in the Eibenstein sample was calculated to be 0.25 wt%, or 2.6% of the total manganese (~P5.96Mn5+0.04).
We thank A. Prayer, Irnfritz, Lower Austria, for providing the apatite samples and A. Wagner, Vienna, Austria, for preparing them. This work was supported in part by NSF grants EAR-9627222 and EAR-9804768 to J.M.H., EAR-0003201 to J.M.H. and J.R., and EAR-0125767 to GRR. Thorough reviews were provided by Associate Editor J.M. Hanchar, P. Piccoli, and M. Gaft. F.C. Hawthorne provided a remarkably detailed review that considerably improved the presentation. The Instrumentation Laboratory of Miami University is gratefully acknowledged for maintaining the X-ray diffractometer.
Manuscript handled by John Hanchar
- Manuscript Received May 14, 2003.
- Manuscript Accepted October 26, 2003.