Mn-rich fluorapatite from Austria; crystal structure, chemical analysis, and spectroscopic investigations

The crystal structure of a pale blue transparent Mn-rich fluorapatite (MnO:9.79 wt%) with the optimized formula approximately (Ca (sub 8.56) Mn (super 2+) (sub 1.41) Fe (super 2+) (sub 0.01) )P (sub 6) O (sub 24) F (sub 2.00) and space group P6 (sub 3) /m, a = 9.3429(3), c = 6.8110(2) Aa, Z = 2 has been refined to R = 2.05% for 609 unique reflections (MoKalpha ). The Mn in the Eibenstein an der Thaya, Austria apatite is strongly ordered at the Ca1 site: Ca1: Ca (sub 0.72(1)) Mn (sub 0.28) , Ca2: Ca (sub 0.96(1)) Mn (sub 0.04) . There is a linear variation in {Ca1-O} as a function of Mn content (r (super 2) = 1.00). The dominant band in the optical absorption spectrum of fluorapatite from Eibenstein is in the 640 nm region with E parallel to c > E perpendicular to c. The 640 nm band is attributed to Mn (super 5+) 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 Mn (super 5+) . Because Mn (super 5+) has intense absorption in the visible region of the spectrum, if a small proportion of the total Mn is Mn (super 5+) 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 degrees C for 1 hour, and the change in color was slight. All of these observations are consistent with the origin of color from Mn (super 5+) . Assuming that all the intensity of the 640 nm (Ec) absorption is from Mn (super 5+) , the concentration of Mn (super 5+) in this fluorapatite sample was calculated as 2.6% of the total manganese content ( approximately P (sub 5.96) Mn (super 5+) (sub 0.04) ). The calibration was estimated from the spectrum of the related compound Sr (sub 5) (P (sub 0.99) Mn (super 5+) (sub 0.01) ) (sub 3) Cl. The weak band at about 404 nm in the Ec spectrum may be the corresponding band for Mn (super 2+) in octahedral coordination.