Agakhanovite-(Y), ideally (YCa):square 2KBe3Si12O30, a new milarite-group mineral from the Heftetjern pegmatite, Tordal, Southern Norway: Description and crystal structure

作者：Hawthorne Frank C^*; Abdu Yassir A; Ball Neil A; Cerny Petr; Kristiansen Roy

来源：American Mineralogist, 2014, 99(10): 2084-2088.

DOI：10.2138/am-2014-4880

• Agakhanovite-(Y)
• new mineral species
• milarite-group mineral
• Heftetjern pegmatite, Tordal, southern Norway
• crystal structure
• electron microprobe analysis
• optical properties

Agakhanovite-(Y), ideally (YCa)square 2KBe3Si12O30, is a new milarite-group mineral from the Heftetjern pegmatite, Tordal, 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/cm(3). 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 omega = 1.567, epsilon = 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) angstrom, V = 1276.02(9) angstrom(3), Z = 2, c:a = 1.330. The seven strongest lines in the X-ray powder-diffraction pattern are as follows: d (angstrom), I, (hkl): 2.865, 100, ((1) over bar 24); 3.287, 96, ((1) over bar 31); 4.134, 84, ((1) over bar 22); 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 (Y0.89Yb0.01 Ca-1.06)(Sigma 1.96)(H2O)(0.92)Na0.02K1.00(Be2.93Al0.07)(Sigma 3.00)Si12.02O30. The crystal structure of agakhanovite-(Y) was refined to an RI index of 1.9% based on 660 unique observed reflections collected on a three-circle rotating-anode (MoK alpha 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 square 2KBe3Si12O30, and the Y-Ca-Be analog of klochite, (Fe2+Fe3+)square 2KZn3Si12O30.

• 出版日期2014-10