Fluormacraeite, [(H 2 O)K]Mn 2 (Fe 2 Ti)(PO 4 ) 4 [OF](H 2 O) 10 ⋅ 4H 2 O, the first type mineral from the Plößberg pegmatite, Upper Palatinate, Bavaria, Germany
Year: 2025
Type: article
Abstract: Fluormacraeite, [(H 2 O)K]Mn 2 (Fe 2 Ti)(PO 4 ) 4 [OF](H 2 O) 10 ⋅ 4H 2 O, is a new monoclinic member of the paulkerrite group from the Plößberg pegmatite, Upper Palatinate, Bavaria, Germany. It was found in specimens of magnesium-bearing triplite. Associated minerals are spherical blue phosphosiderite, pink-coloured strengite micro-crystals, white fluorapatite globules, light-yellow leucophosphite, black–green rockbridgeite, and reddish-brown cacoxenite. Fluormacraeite occurs as isolated pale-yellow rhombic tablets, flattened on (010) with diameters in the range of 50 to 150 µm and thicknesses on the order of 10 to 30 µm. The crystal forms are {010}, {001}, and {111}. The calculated density for the empirical formula and single-crystal unit-cell volume is 2.39 g cm −3 . Optically, fluormacraeite crystals are biaxial (+), with α =1.610(3), β =1.620(3), and γ =1.644(3) (measured in white light). The calculated 2 V is 66.5°. The optical orientation is X = b , Y = c , and Z = a . The empirical formula from electron microprobe analyses and structure refinement is A 1 [K 0.14 (H 2 O) 0.76 ] Σ0.90 A 2 [K 0.79 (H 2 O) 0.21 ] Σ1.00 M 1 (Mn1.752+Mg 0.25 ) Σ2.00 M 2+ M 3 (Fe1.843+Al 0.13 Ti1.024+Mg 0.01 ) Σ3.00 (PO 4 ) 4.00 X [O 0.94 F 0.81 (OH) 0.25 ] Σ2.00 (H 2 O) 10 ⋅ 3.90H 2 O. Fluormacraeite has monoclinic symmetry with space group P 2 1 / c and unit-cell parameters a =10.546(2) Å, b =20.655(1) Å, c =12.405(1) Å, β =90.09(1)°, V =2702.1(6) Å 3 , and Z =4. The crystal structure was refined using synchrotron single-crystal data to w R obs =0.0559 for 5646 reflections with I > 3 σ ( I ). Fluormacraeite is isostructural with the paulkerrite-group minerals pleysteinite, macraeite, rewitzerite, hochleitnerite, fluor-rewitzerite, sperlingite, and paulkerrite, with ordering of K and H 2 O at different A sites ( A 1 and A 2) using the general formula A 1 A 2 M 1 2 M 2 2 M 3(PO 4 ) 4 X 2 (H 2 O) 10 ⋅ 4H 2 O. It is the F analogue of macraeite, with OF replacing O(OH) at the X 2 sites. The general crystal–chemical properties of the monoclinic paulkerrite-group minerals are compared.
Source: European Journal of Mineralogy
Language: English
Subfield: Geochemistry and Petrology
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,024
OpenAlex ID: W4408530571