Kentbrooksite from the Kangerdlugssuaq intrusion, east Greenland, a new Mn-REE-Nb-F end-member in a series within the eudialyte group: Description and crystal structure

Kentbrooksite, ideally (Na,REE)(15)(Ca,REE)(6)Mn3Zr3NbSi25O74F2 . 2H(2)O, is a new mineral from the Kangerdlugssuaq intrusion, East Greenland. The mineral occurs in alkaline pegmatitic bodies cutting pu laskite rocks and is found as anhedral to subhedral aggregates up to 2 cm. It is transparent, yellow-brown with white streak and vitreous luster. Hardness 5-6, brittle, uneven fracture and no cleavage. It is pyroelectric. The mineral is uniaxial negative, omega = 1.628(2) and epsilon = 1.623(2), nonpleochroic. D-meas = 3.10(4) g/cm(3), D-calc = 3.08 g/cm(3). The mineral is trigonal, space group R3m, a = 14.1686(2): c = 30.0847(4) Angstrom, Z= 3. The strongest reflections in the X-ray powder pattern are [d(Angstrom), Int, (hkl): 2.839, 100, (404); 2.961; 91, (315); 11.385, 43, (101); 7.088, 41, (110): 3.380, 37, (131); 4.295, 31, (205); and 5.652, 30, (202). The empirical formula, based on 3.77 (Zr + Hf + Nb + Ti) pfu, is (Na14.93REE0.44Y0.42K0.30Sr0.15)(Sigma 16.24)(Ca3.27Mn1.75REE0.62Na0.33)(Sigma 6.00)(Mn1.90Fe0.72Al0.13Mg0.05)(Sigma 2.80)(Nbo(0.55)Zr(0.12)Ti(0.10))(Sigma 0.77)Si0.60Zr2.81Hf0.06Ti0.13)(Sigma 3)[Si3O9)(2)(Si9O27)(2)O-2](F1.51Cl0.27OH0.22)(Sigma 2). 2.3H(2)O. IR spectrum is given. The structure has been refined from single-crystal X-ray diffraction data to R1 = 4.1%. Kentbrooksite be longs to the eudialyte group and has the framework characteristic for this group, consisting of three-membered and nine-membered rings of SiO4 tetrahedra cross-linked by Zr and M1 in octahedral coordination. Most important structural differences from e.g. eudialyte from the type locality (Ilimaussaq) are: Mn-[5] substitution for Fe-[4] in M2, Nb-[6] (M3) substitution for Si-[4] (M4), a high content of REE and F substitution for Cl. Kentbrooksite represents the Sigma Nb,REE,Mn,F end-member of a series within the eudialyte group.