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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 6 O 24 F 2.00  and space group P6 3 /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: Ca 0.72(1) Mn 0.28 , Ca2:Ca 0.96(1) Mn 0.04 . There is a linear variation in &lt;Ca1-O&gt; 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 &gt; 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 ) 3 Cl. The weak band at about 404 nm in the E || c spectrum may be the corresponding band for Mn 2+  in octahedral coordination.

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