JORGENSENITE, NA-2(SR,BA)(14)NA2AL12F64(OH,F)(4), A NEW ALUMINOFLUORIDE MINERAL FROM IVIGTUT, GREENLAND

Jorgensenite, Na-2(Sr,Ba)(14)Na2Al12F64(OH,F)(4), is a new aluminofluo ride mineral from the Ivigtut cryolite deposit at Ivigtut, Greenland. It occurs in fissure fillings within the main cryolite mass, and in cr ystal-lined cavities containing fan-shaped aggregates (up to 2 mm acro ss) of jorgensenite overgrown by jarlite. It also occurs as grains up to 10 mm across with stenonite. It is veined by stenonite, but also ve ins and cements irregular fragments of stenonite. Jorgensenite is tran sparent, colorless to white, with a colorless to white streak, a vitre ous luster and an uneven fracture; it shows no fluorescence. It is bri ttle, H 3 1/2-4, D-meas 3.89(1) g/cm(3), D-calc 3.94 g/cm(3). In plane -polarized light, it is colorless, nonpleochroic, biaxial negative, al pha 1.436(1), beta 1.442(1), gamma 1.442(1), 2V 0-5 degrees, beta = b, and without visible dispersion. Crystals are elongate along the 6 axi s; no twinning was observed. Jorgensenite is monoclinic, space group C 2/m, with the following unit-cell dimensions, refined from powder X-ra y-diffraction data: a 16.046(1), b 10.971(1), c 7.281(1) Angstrom, bet a 101.734(6)degrees, V 1254.9(3) Angstrom(3), Z = 1. The axial ratios, calculated from the refined cell-parameters, are a:b:c = 1.4634:1:0.6 631. The ten strongest reflections in the X-ray powder-diffraction pat tern [d in Angstrom(I,hkl)] are: 7.844(8,200), 3.643(9,311), 3.453(<10 ,11(2)over bar>), 3.193(10,420), 3.112(<9,42(1)over bar>), 2.998(5,330 ), 2.989(9,022), 2.220(<8,44(1)over bar>), 2.173(9,150), and 2.001(<8, 80(1)over bar>). Chemical analysis by electron-microprobe and analytic al SEM gave Na 3.25, K 0.30, Mg 0.38, Ca 0.28, Sr 32.76, Ba 8.63, Al 1 0.97, F 42.50, H2Ocalc 1.22, sum 100.29 wt%; the presence of OH in the structure was confirmed by microbeam infrared spectroscopy. The formu la unit, calculated on the basis of 12 Al atoms per formula unit, is . 23Na0.63)(Na1.54Mg0.46)Al12F64[(OH)(2.00)F-2.00]. Jorgensenite is isos tructural with jarlite, ideally Sr,Na,square)(14)(Mg,square)(2)Al12F64 (OH,H2O)(4), and is named after Vilhelm Jorgensen (1844-1925), cofound er (with G.A. Hagemann) of the cryolite factory at Ivigtut in 1870.