Hundholmenite-(Y) from Norway – a new mineral species in the vicanite group: descriptive data and crystal structure

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,Fe 3+ )Ca x As 1− x 0 3+ (Si,As 5+ )Si 6 B 3 (O,F) 48 ( x = 0.78). The mineral is trigonal, R 3 m , with a = 10.675(6), c = 27.02(2) A, V = 2667(5) A 3 , Z = 3. The crystal structure was refined to R 1 = 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 [ d obs in A, ( 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 e 1.7487(5). The Mohs hardness is ~5–6; D calc = 5.206(9) g/cm 3 .