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

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–x3+ (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) Å 3, 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 o 1.7578(5) and e 1.7487(5). The Mohs hardness is ~5–6; Dcalc = 5.206(9) g/cm3.

Crystal structure of the OH-dominant gadolinite-(Y) analogue (Y,Ca)2(Fe,□)Be2Si2O8(OH,O)2 from Heftetjern pegmatite, Norway

“…The main accessory minerals are beryl, spessartine, gadolinite-(Y), cassiterite, scandian ixiolite, zircon, monazite-(Ce), Sc-bearing pyrochlore-group minerals, milarite and phenakite. A late mineral assemblage encountered mainly in vugs and fractures comprises bertrandite, triclinic titanite (Lussier et al, 2009), bohseite, bazzite, thortveitite (Raade et al, 2004), helvite, cascandite, scandiobabingtonite (Raade & Erambert, 1999), kristiansenite (Raade et al, 2002), heftetjernite (Kolitsch et al, 2010) and agakhanovite-(Y) (Hawthorne et al, 2014). Most minerals described below occur in vugs in amazonite and quartz and are associated with bertrandite, spessartine, Ce-enriched epidote, zircon, Mn-bearing hellandite-(Y) (Miyawaki et al, 2015), biotite, opal, violet and white yttrian fluorite and remnants of a metamict microlite-group mineral.…”

Proshchenkoite-(Y) from Russia — a new mineral species in the vicanite group: descriptive data and crystal structure

“…The crystal structure refinement of Boiocchi et al (2004) seems to indicate P > Si at the single tetrahedral site, but the estimated error is large, as could be expected for elements that are neighbours in the Periodic System. Their chemical data definitely imply Si > P, and in the hundholmenite-(Y) paper (Raade et al, 2007, Table 8), okanoganite-(Y) is defined with Si > P. The analytical results presented here indicate P > Si for our sample from Washington Pass. A mineral with P > Si at this particular site is, strictly speaking, a different species from one with Si > P, and should be regarded as the P analogue of okanoganite-(Y).…”

“…Since a correct distribution of elements at the five M sites is not easily made on the basis of site scattering values, it was suggested by Raade et al (2007) that the M1ÀM5 polyhedra should be treated as an entity with regard to nomenclature. (Raade et al, 2007). ** The sites As2 and Ca/Na are mutually exclusive.…”

“…In summary, the proshchenkoite-(Y) structure has, like the other members of the vicanite group, three distinct layers parallel to (001) (Raade et al, 2007):…”

“…THE vicanite group of Strunz and Nickel (2001) up to now includes three borosilicate mineral species: okanoganite-(Y) (Boggs, 1980;Boiocchi et al, 2004), vicanite-(Ce) (Maras et al, 1995;Ballirano et al, 2002) and hundholmenite-(Y) (Raade et al, 2007). The minerals are trigonal, R3m, with similar, layered structures.…”