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

Abstract

The crystal structure of a pale blue transparent Mn-rich fluorapatite (MnO: 9.79 wt%) with the optimized formula ~(Ca_(8.56)Mn^(2+)_(1.41)Fe^(2+)_(0.01))P_6O_24F_(2.00) and space group P6_3/m,ɑ = 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: Ca_(0.72(1))Mn_(0.28), Ca2: Ca_(0.96(1))Mn_(0.04). There is a linear variation in <Ca1-O> as a function of Mn content (r^2 = 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 Mn^(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^(5+). Because Mn^(5+) has intense absorption in the visible region of the spectrum, if a small proportion of the total Mn is Mn^(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° C for 1 hour, and the change in color was slight. All of these observations are consistent with the origin of color from Mn^(5+). Assuming that all the intensity of the 640 nm (E || c) absorption is from Mn^(5+), the concentration of Mn^(5+) in this fluorapatite sample was calculated as 2.6% of the total manganese content (~P_(5.96)Mn^(5+)_(0.04)). The calibration was estimated from the spectrum of the related compound Sr_5(P_(0.99)Mn^(5+)_(0.01))_3Cl. The weak band at about 404 nm in the E || c spectrum may be the corresponding band for Mn^(2+) in octahedral coordination.