Eirikite, ideally, KNa6[Be2(Si15Al3)∑= 18O39F2], is a new mineral species from the Vesle Arøya island in the Langesundsfjord district, Larvik, Vestfold, Norway. It is the potassium analogue of leifite, NaNa6[Be2(Si15Al3)∑=18O39F2], and telyushenkoite, CsNa6[Be2(Si15Al3)∑= 18O39F2]. An average of five electron microprobe analyses gave Na2O 13.38, K2O 3.13, Al2O3 11.08, SiO2 66.03, F 2.70, BeO 3.65 (calc.), O = F −1.14, total 98.83 wt. %, resulting in the empirical formula K0.91Na5.92[Be2(Si15.07Al2.98)∑=18.05]O39(F1.95O0.05); infrared spectroscopy indicates the absence of OH and H2O.

Eirikite occurs as both finely or coarsely fibrous, monomineralic aggregates and bundles up to 3 cm across, wholly or partly filling voids between larger microcline crystals. Individual acicular-prismatic crystals show the forms {101̅0}and {0001}. Eirikite also occurs as aggregates up to 100 cm3 in volume, built of interlocking, radiating fibrous groups, each up to 5 mm across, intergrown with large amounts of small zircon crystals and penetrated by aegirine crystals. Polylithionite, albite, eudialyte, catapleiite and pyrochlore can be found within these aggregates. Eirikite is white to colourless with a white streak, a vitreous to silky lustre, Mohs hardness of 6; it is brittle with an uneven to conchoidal fracture, and with a good {101̅0} cleavage. The mineral is uniaxial positive, ω=1.517(1) and ε = 1.521(1), and nonpleochroic; Dmeas = 2.59(1) g/cm3, Dcalc = 2.577 g/cm3 (from the empirical chemical formula) and 2.584 g/cm3 (from the crystal structure determination).

Eirikite is trigonal, Pm1 (no. 164), with a = 14.3865(9) Å, c = 4.8733(4) Å, V = 873.5(1) Å3, and Z = 1, refined from X-ray powder diffraction data. The five strongest lines in the X-ray powder pattern are [d (in Å) (I/I0)(hkl)]: 4.710(29)(120), 4.153(21)(030), 3.386(70)(211), 3.161(100)(031), and 2.466(31)(231). The crystal structure, which was determined from single-crystal X-ray intensity data and refined to R1(F) = 1.99 %, is a framework of corner-sharing (Si,Al)O4 and BeO3F tetrahedra (<Be–(O,F)> = 1.610 Å), leading to chain and ring units. Voids in the framework are occupied by K (<K–O>= 3.105 Å), and seven-coordinated Na atoms (<Na–(O,F)> = 2.542 Å). The B site, which is vacant in telyushenkoite and partly occupied by water molecules in leifite, is nearly vacant in eirikite.

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