Abstract

Hundholmenite-(Y) occurs as pale brown, subhedral crystals up to 1 mm across, embedded in REE-bearing fluorite, in a granitic pegmatite at Hundholmen, Tysfjord, Nordland County, north Norway. Two other occurrences in the same area are the granitic pegmatites at Stetind and Lagmannsvik. The simplified formula, obtained from wavelength-dispersive EMP analyses and boron determination by ICP-AES, is (Y,REE,Ca,Na)15(Al,Fe3+)CaxAs1−x03+ (Si,As5+)Si6B3(O,F)48 (x = 0.78). The mineral is trigonal, R3m, with a = 10.675(6), c = 27.02(2) Å, V = 2667(5) Å3, Z = 3. The crystal structure was refined to R1 = 0.037 for 1720 observed reflections. Hundholmenite-(Y) is isostructural with okanoganite-(Y) and vicanite-(Ce), and the differences in site populations are discussed. The strongest seven reflections of the X-ray powder-diffraction pattern [dobs in Å, (I) (hkl)] are: 4.38 (33) (202), 3.114 (43) (214), 2.972 (100) (027), 2.947 (76) (125), 2.924 (66) (303, 033), 2.681 (36) (220) and 1.978 (37) (235). The mineral is optically uniaxial (−) with ω1.7578(5) and ε 1.7487(5). The Mohs hardness is ~5–6; Dcalc = 5.206(9) g/cm3.

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