Mn-rich fluorapatite from Austria:
Crystal structure, chemical analysis, and spectroscopic investigations

John M. Hughes
Department of Geology, Miami University, Oxford, OH 45056, U.S.A.

Andreas Ertl
Institut für Mineralogie und Kristallographie, Geozentrum, Universität Wien, Althanstraße 14, A-1090 Vienna, Austria

Heinz-Jürgen Bernhardt
Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, D-44801 Bochum, Germany

George R. Rossman

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125-2500, U.S.A.

John Rakovan

Department of Geology, Miami University, Oxford, OH 45056, U.S.A.



   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 with 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 (Mo Ka). The Mn in the Eibenstein an der Thaya apatite is strongly ordered into the four Ca1 sites  (82% of the Mn occupies the Ca1 site), as the sites have the following occupancies:  Ca1: Ca0.72(1)Mn0.28,  Ca2: Ca0.96(1)Mn0.04. The variation of <Ca1-O1> with the amount of Mn substitution (by including previous structures) displays a monotonic decrease in average cation – oxygen distance with increased Mn (r2 = 1.00). The dominant band in the spectrum of fluorapatite from Eibenstein is in the 640 nm region with E || c > E perp c. The 640 nm band is attributed to Mn(V) at the P site because this band is also seen in spectra of apatite from other localities where the blue color was attributed to Mn(V) at the P site. This interpretation is consistent with studies of well-characterized synthetic materials of the apatite structure that contain Mn(V). Because Mn(V) has intense absorption in the visible region of the spectrum, if a small proportion of the total Mn is Mn(V) at the P site it would dominate the spectrum and the color of the mineral. To determine if the pale blue color is a radiation-induced color, a fragment of the fluorapatite crystal was heated at 400° C for 1 hour. 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 a component of Mn(V). Assuming that all the intensity of the 640 nm (E || c) absorption is from Mn(V), the concentration of Mn(V) in this fluorapatite was calculated to be 2.6% of the total manganese (~ 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.

13 MB tif file: Figure 1