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

Abstract

Agakhanovite-(Y), ideally (YCa)o 2 KBe 3 Si 12 O 30 , 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/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 w = 1.567, e = 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) Å 3 , 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, (124); 3.287, 96, (131); 4.134, 84, (122); 6.877, 56, (002); 2.986, 43, (030); 4.479, 38, (020); 2.728, 36, (024). Chemical analysis by electron microprobe gave SiO 2 69.56, Al 2 O 3 0.35, Y 2 O 3 9.69, Yb 2 O 3 0.15, FeO 0.02 CaO 5.75, Na 2 O 0.07, K 2 O 4.52, BeO(calc) 7.06, H 2 O(calc) 1.74, sum 98.91 wt%. The H 2 O content was determined by crystal-structure analysis. On the basis of 30 anions, the empirical formula is (Y 0.89 Yb 0.01 Ca 1.06 ) S1.96 (H 2 O) 0.92 Na 0.02 K 1.00 (Be 2.93 Al 0.07 ) S3.00 Si 12.02 O 30 . The crystal structure of agakhanovite-(Y) was refined to an R 1 index of 1.9% based on 660 unique observed reflections collected on a three-circle rotating-anode (MoKa 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: ScCao 2 KBe 3 Si 12 O 30 , and the Y-Ca-Be analog of klöchite, (Fe 2+ Fe 3+ )o 2 KZn 3 Si 12 O 30 .

Beryllium minerals as monitors of geochemical evolution from magmatic to hydrothermal stage; examples from NYF pegmatites of the Třebíč Pluton, Czech Republic

“…Secondary milarite III after helvite-danalite or phenakite with Al~1 apfu, low Fe, Mn and Sc ≤ 0.1 apfu and REE-free is similar to secondary milarite from granitic pegmatites (Černý 2002) but also to primary milarite I from the Velká skála pegmatite (Novák et al 2017). Agakhanovite-(Y) from the type locality Heftetjern, Norway, fills small miarolitic cavities as a product of late hydrothermal solutions (Hawthorne et al 2014). Hence, the chemical composition of milarite-group minerals, in particular, reflects local enrichment of some elements than its paragenetic position within certain granitic pegmatite.…”

Crystal structure of the OH-dominant gadolinite-(Y) analogue (Y,Ca)2(Fe,□)Be2Si2O8(OH,O)2 from Heftetjern pegmatite, Norway

“…The main accessory minerals are beryl, spessartine, gadolinite-(Y), cassiterite, scandian ixiolite, zircon, monazite-(Ce), Sc-bearing pyrochlore-group minerals, milarite and phenakite. A late mineral assemblage encountered mainly in vugs and fractures comprises bertrandite, triclinic titanite (Lussier et al, 2009), bohseite, bazzite, thortveitite (Raade et al, 2004), helvite, cascandite, scandiobabingtonite (Raade & Erambert, 1999), kristiansenite (Raade et al, 2002), heftetjernite (Kolitsch et al, 2010) and agakhanovite-(Y) (Hawthorne et al, 2014). Most minerals described below occur in vugs in amazonite and quartz and are associated with bertrandite, spessartine, Ce-enriched epidote, zircon, Mn-bearing hellandite-(Y) (Miyawaki et al, 2015), biotite, opal, violet and white yttrian fluorite and remnants of a metamict microlite-group mineral.…”

Heflikite, ideally Ca2(Al2Sc)(Si2O7)(SiO4)O(OH), the first scandium epidote-supergroup mineral from Jordanów Śląski, Lower Silesia, Poland and from Heftetjern, Tørdal, Norway

“…The main accessory minerals are beryl, spessartine, allanite-(Ce), uedaite-(Ce), gadolinite-(Y), cassiterite, Sc-rich ixiolite, zircon, monazite-(Ce), Sc-bearing pyrochlore-supergroup minerals, milarite and phenakite. A latestage mineral assemblage, present mainly in vugs and fractures, comprises agakhanovite-(Y), bazzite, bertrandite, bohseite, cascandite, Ce-La-Sc-enriched epidote-allanite minerals, Y-rich fluorite, heftetjernite, Mn-bearing hellandite-(Y), Sc-rich helvine, triclinic Ca-hingganite-(Y), Sc-rich garnet, oftedalite, rynersonite, scandiobabingtonite, thortveitite, triclinic titanite and the unnamed new species of OH-dominant analogue of gadolinite-(Y), (Y,Ca) 2 (Fe,□)Be 2 Si 2 O 8 (OH,O) 2 (Raade and Erambert, 1999;Raade and Bernhard, 2003;Raade and Kristiansen, 2003;Raade et al, 2002Raade et al, , 2004Cooper et al, 2006Cooper et al, , 2019Lussier et al, 2009;Kolitsch et al, 2010;Hawthorne et al, 2014;Miyawaki et al, 2015;Chukanov et al, 2017;Raade, 2020;Steffenssen et al, 2020). Heflikite is a part of this late-stage mineral assemblage.…”