Fluorbritholite-(Nd), Ca<sub>2</sub>Nd<sub>3</sub>(SiO<sub>4</sub>)<sub>3</sub>F, a new and key mineral for neodymium sequestration in REE skarns
Dan Holtstam, P. Casey, Luca Bindi, Hans‐Jürgen Förster, Andreas Karlsson, Oona Appelt
Abstract
Abstract Fluorbritholite-(Nd), ideally Ca 2 Nd 3 (SiO 4 ) 3 F, has been approved by the International Mineralogical Association (IMA2023–001) and constitutes a new member of the britholite group of the apatite supergroup. It occurs in skarn from the Malmkärra iron mine, Norberg, Västmanland (one of the Bastnäs-type deposits in Sweden), associated with calcite, dolomite, magnetite, lizardite, talc, fluorite, baryte, scheelite, gadolinite-(Nd) and other REE minerals. Fluorbritholite-(Nd) forms anhedral and small grains, rarely up to 250 μm across. They are brownish pink and transparent with a vitreous to greasy lustre. The mineral is brittle, with an uneven or subconchoidal fracture and lacks a cleavage. In thin section, the mineral is nonpleochroic, uniaxial (–). D calc = 4.92(1) g⋅cm −3 and n calc = 1.795. The empirical chemical formula from electron microprobe (WDS) point analyses is (Ca 1.62 Nd 0.97 Ce 0.83 Y 0.52 Sm 0.30 Gd 0.23 Pr 0.17 La 0.16 Dy 0.11 Er 0.03 Tb 0.03 Ho 0.01 Yb 0.01 ) Σ4.99 (Si 2.92 P 0.08 As 0.01 ) Σ3.01 O 12.00 [O 0.48 F 0.26 (OH) 0.14 Cl 0.10 Br 0.02 ] Σ1.00 . The crystal structure of fluorbritholite-(Nd) was refined from single-crystal X-ray diffraction data to R 1 = 0.043 for 704 unique reflections. It belongs to the hexagonal system, space group P 6 3 / m , with unit cell parameters a = 9.5994(3), c = 6.9892(4) Å and V = 557.76(5) Å 3 for Z = 2. Fluorbritholite-(Nd) and other britholite-group minerals are a major sink for neodymium in REE-bearing skarns of Bastnäs type.