Abstract

Agakhanovite-(Y), ideally (YCa)□2KBe3Si12O30, is a new milarite-group mineral from the Heftetjern pegmatite, Tørdal, southern Norway. Crystals are prismatic along [001], and show the forms {100} and {100}. Agakhanovite-(Y) is colorless with a white streak and a vitreous luster, and does not fluoresce under ultraviolet light. There is no cleavage or parting, and no twinning was observed. Mohs hardness is 6, and agakhanovite-(Y) is brittle with a conchoidal fracture. The calculated density is 2.672 g/cm3. Optical properties were measured with the Bloss spindle stage for the wavelength 590 nm using a gel filter. Agakhanovite-(Y) is uniaxial (−) with indices of refraction ω = 1.567, ɛ = 1.564, both ±0.002; the calculated birefringence is 0.003 and it is non-pleochroic. Agakhanovite-(Y) is hexagonal, space group P6/mcc, a = 10.3476(2), c = 13.7610(3) Å, V = 1276.02(9) Å3, Z = 2, c:a = 1.330. The seven strongest lines in the X-ray powder-diffraction pattern are as follows: d (Å), I, (hkl): 2.865, 100, (1̄24); 3.287, 96, (1̄31); 4.134, 84, (1̄22); 6.877, 56, (002); 2.986, 43, (030); 4.479, 38, (020); 2.728, 36, (024). Chemical analysis by electron microprobe gave SiO2 69.56, Al2O3 0.35, Y2O3 9.69, Yb2O3 0.15, FeO 0.02 CaO 5.75, Na2O 0.07, K2O 4.52, BeO(calc) 7.06, H2O(calc) 1.74, sum 98.91 wt%. The H2O content was determined by crystal-structure analysis. On the basis of 30 anions, the empirical formula is (Y0.89Yb0.01Ca1.06)∑1.96(H2O)0.92Na0.02K1.00(Be2.93Al0.07)∑3.00Si12.02O30. The crystal structure of agakhanovite-(Y) was refined to an R1 index of 1.9% based on 660 unique observed reflections collected on a three-circle rotating-anode (MoKα X-radiation) diffractometer equipped with multilayer optics and an APEX-II detector. In the end-member structure of agakhanovite-(Y), the A site is occupied equally by Y and Ca, and the B site is vacant; agakhanovite-(Y) is the Y-analog of oftedalite: ScCa□2KBe3Si12O30, and the Y-Ca-Be analog of klöchite, (Fe2+Fe3+)□2KZn3Si12O30.

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