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.
Abstract
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