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
Johnsen, Ole; Grice, Joel D.; Gault, Robert A.
European Journal of Mineralogy Volume 10 Number 2 (1998), p. 207 - 220
published: Mar 31, 1998
manuscript accepted: Oct 14, 1997
manuscript received: Feb 14, 1997
ArtNo. ESP147051002002, Price: 29.00 €
Abstract Kentbrooksite, ideally (Na,REE)15(Ca,REE)6Mn3Zr3NbSi25O74F2 · 2H2O, is a new mineral from the Kangerdlugssuaq intrusion, East Greenland. The mineral occurs in alkaline pegmatitic bodies cutting pulaskite 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, ω = 1.628(2) and 8 = 1.623(2), nonpleochroic. Dmeas = 3.10(4) g/cm3, Dcalc = 3.08 g/cm3. The mineral is trigonal, space group R3m, a - 14.1686(2), c = 30.0847(4) Å, Z= 3. The strongest reflections in the X-ray powder pattern are [d(λ), Int, (hkt)]: 2.839, 100, (404); 2.961, 91, (315); 11.385, 43, (101); 7.088, 41, (110); 3.380, 37, (131); 4.295, 34, (205); and 5.682, 30, (202). The empirical formula, based on 3.77 (Zr + Hf + Nb + Ti) pfu, is (Na0.44REE0.44Y0.42K0.30Sr0.15)Σ16.24(Ca3.27Mn1.78REE0.62Na0.33)Σ6.00(Mn1.90Fe0.72Al0.13Mg0.05)Σ2.8O (Nb0.55Zr0.12Ti0.10)Σ0.77Si0.60(Zr2.81Hf0.06Ti0.13)Σ3[(Si3O9)2(Si9O27)2O2](F1.51Cl0.27OH0.22)Σ2 · 2.3H2O. DR. spectrum is given. The structure has been refined from single-crystal X-ray diffraction data to R1 =4.1%. Kentbrooksite belongs 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 Ml in octahedral coordination. Most important structural differences from e.g. eudialyte from the type locality (Ilimaussaq) are: Mn substitution for Fe in M2, Nb (M3) substitution for Si (M4), a high content of REE and F substitution for Cl. Kentbrooksite represents the ΣNb,REE,Mn,F end-member of a series within the eudialyte group.