2024/09/20 by Adam Pieczka, Bożena Gołębiowska, Adam Włodek +6 · 1 voice
Materials Science · Chemistry · Earth and Planetary Sciences · #Crystal Structures and Properties #Inorganic Fluorides and Related Compounds #Mineralogy and Gemology Studies
paper · doi:10.2138/am-2024-9453
openalex publication_date 2024/09/20 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/22
Abstract Allanite-(Sm), the third samarium mineral to be discovered, was studied using electron microprobe analysis and single-crystal X-ray diffraction. It is isostructural with monoclinic epidote-supergroup minerals and belongs to the allanite group. The mineral was found in a granitic pegmatite in a serpentine quarry near Jordanów Śląski in Lower Silesia, SW Poland. It occurs as subhedral to anhedral crystals with sizes of up to ∼150 μm accompanied by allanite-(Nd), allanite-(Y), clinozoisite, hingganite-(Y), and xenotime-(Y). Allanite-(Sm) is yellowish-brown and shows a white streak, a vitreous luster, and a Mohs hardness of ∼6. It is brittle, with conchoidal fracture and imperfect cleavage. The calculated density is 3.842 g·cm−3, and the calculated mean refractive index is close to 1.737. The holotype crystal contains (in wt%) 33.08(11) SiO2, 0.08(3) TiO2, 21.15(9) Al2O3, 0.45(6) Sc2O3, 0.48 Fe2O3(calc), 0.02(1) V2O3, 2.49(31) Y2O3, 0.18(4) La2O3, 0.95(9) Ce2O3, 0.34(6) Pr2O3, 3.52(31) Nd2O3, 6.12(23) Sm2O3, 0.06(2) EuO, 3.25(21) Gd2O3, 0.34(2) Tb2O3, 0.94(7) Dy2O3, 0.15(4) Ho2O3, 0.16(3) Er2O3, 0.07(6) Tm2O3, 0.06(4) Yb2O3, 0.28(4) MnO, 8.38 FeO(calc), 0.13(2) MgO, 13.71(15) CaO, and 1.95 H2O(+)calc; the total is 98.04. The empirical EPMA + SREF formula is (AlCa0.966Mn0.022Fe0.0102+)Σ0.998A2(Ca0.372Sm0.192Y0.121Nd0.114Gd0.098Ce0.032Dy0.028Pr0.011Tb0.010La0.006Er0.005Ho0.004Eu0.002Tm0.002Yb0.002)Σ0.998Al0.998M1Al0.998M2(M3Fe0.6282+Al0.275Sc0.036Fe0.0333+Mg0.018Ti0.005V0.0023+)Σ0.998 (T1–T3Si3.012O11)O(OH), the simplified formula is Ca(Sm,REE,Ca)Al2(Fe2+,Al,Fe3+)(Si2O7)(SiO4)O(OH), and the ideal formula is CaSm(Al2Fe2+)(Si2O7)(SiO4)O(OH), which requires (in wt%) SiO2 30.37, Al2O3 17.18, Sm2O3 29.38, FeO 12.11, CaO 9.45, and H2O 1.52; the total is 100. Allanite-(Sm) is monoclinic, space group P21/m, with the following unit-cell parameters: a = 8.8923(6) Å, b = 5.7005(3) Å, c = 10.1280(8) Å, β = 115.445(9)°, V = 463.59(6) Å3, Z = 2, and a:b:c = 1.5599:1:1.7767. The T site is exclusively occupied by Si, the A1 site is occupied by Ca and Mn, and the A2 site is mainly occupied by rare earth elements (REE), of which Sm is dominant. The M1 and M2 sites are occupied by Al and the M3 site by divalent cations, of which Fe2+, characteristic of allanite-type species, is dominant. Allanite-(Sm) and the associated allanite-(Y), allanite-(Nd), and hingganite-(Y) are secondary minerals precipitated from Ca-rich hydrothermal fluid during the metasomatic alteration of the Jordanów Śląski pegmatite. They accumulated REE released by the alteration of nearby xenotime-(Y). The disequilibrium rapid crystallization of REE-selective allanite and hingganite phases in an environment strongly enriched in heavy REE (HREE) relative to light REE (LREE), as well as small-scale heterogeneities in REE abundances in the fluid, resulted in the formation of exotic allanite species, including allanite-(Sm).