Publication details
Allanite-(Y) from the Åskagen, Sweden
| Title in English | Allanite-(Y) from the Askagen, Sweden |
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| Authors | |
| Year of publication | 2024 |
| Type | Conference abstract |
| Citation | |
| Description | Allanite-(Y), ideally CaYAl2Fe2+(SiO4)(Si2O7)O(OH), is a valid species with the type locality in the Askagen pegmatite, Värmland, Sweden [1]. However, no structural characterisation of the material from the type locality has been done yet. The mineral has not been formally approved by IMA-CNMNC and published up to date. The mineral occurs as an accessory phase in the blocky zone of the NYF granitic pegmatite near Askagen, Värmland, Sweden. Allanite-(Y) form rims around altered crystals of thalénite-(Y). Allanite-(Y) replaced primary thalénite-(Y) during an episode of early post-magmatic hydrothermal activity. Other associated minerals are iimoriite-(Y) and allanite-(Nd). We characterised samples from the type locality using electron microprobe analysis, single-crystal X-ray diffraction, Raman spectroscopy, and Mössbauer spectroscopy. Allanite-(Y) forms euhedral crystals with size up to 1 mm, black with a vitreous lustre, conchoidal fracture and greyish brown streak. Allanite-(Y) has the Mohs hardness of ca. 6, the calculated density of 3.948 g.cm-3 and is biaxial (-) with ? = 1.760(3), ß = 1.799(2) and ? = 1.784(3) at 589 nm light, pleochroism is weak pale yellowish brown. Allanite-(Y) has monoclinic symmetry, with the space group P21/m, a = 8.8520(8) A, b = 5.6959(5) A, c = 10.0543(9) A, ß = 115.510(2)°, V = 457.52(7) A3 and Z = 2. The allanite-(Y) structure was refined to R1 = 3.83 % using 2202 unique reflections collected with MoK? X-ray source using the SHELXL-2013 program [2]. For the refinement we employed neutral scattering factors for cations and ionized scattering factors for oxygens [3]. Occupancies of A1-, A2-, M1- and M3- cation sites were refined as free variables. Occupancies of A(2) and M(1,3) sites were refined using two major elements located in the respective sites, i.e., Nd vs Ca with fixed Y at the A2 site, and Al vs Fe at the M1, M3 sites. The hydrogen atom was located in a difference Fourier map. The formula of allanite-(Y) was calculated on the basis of M+T = 6 (apfu), the amount of OH was calculated by charge balance. Fe3+ was determined from the structure with respect to the results of Mössbauer spectroscopy. During the structural refinement, all sites evinced full occupancies with the resulting formula: A1(Ca0.920Mn0.073Na0.007)?1.000A2(Y0.322Ca0.239Nd0.111Sm0.100Gd0.091Dy0.046Ce0.027?0.018Pr0.012Tb0.014Er0.009Ho0.005Yb0.004La0.002)?1.000M1(Al0.916Fe3+0.080Ti0.004)?1.000M2(Al1.000)M3(Fe2+0.922Al3+0.052Mg0.025)?0.999T1(Si1.000)T2(Si1.000) T3(Si0.956Al0.044)?1.000O11.856[(OH)1.000]. Allanite-(Y) belongs to the allanite group of the epidote supergroup. The closest end-member compositions of valid allanite group species are allanite-(Ce), allanite-(La) and allanite-(Nd) related via the simple substitutional mechanism Y - REE. |