Allanite-(Sm), CaSm(Al2Fe2+)(Si2O7)(SiO4)O(OH), the third samarium mineral from Jordanów Śląski, Lower Silesia, Poland

Abstract

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 A1(Ca0.966Mn0.022Fe2+0.010)Σ0.998A2(Ca0.372Sm0.192Y0.121Nd0.114Gd0.098Ce0.032Dy0.028Pr0.011Tb0.010La0.006Er0.005Ho0.004Eu0.002Tm0.002Yb0.002)Σ0.998M1Al0.998M2Al0.998M3(Fe2+0.628Al0.275Sc0.036Fe3+0.033Mg0.018T i0.005V3+0.002)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), b = 5.7005(3), c = 10.1280(8) Å, β = 115.445(9)º, 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 structures 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 crystallization of exotic allanite species, including allanite-(Sm).