Abstract The new mineral, illoqite-(Ce), with the ideal formula Na 2 NaBaCeZnSi 6 O 17 , has been discovered in the Taseq Slope, Ilímaussaq Alkaline Complex, Southern Greenland. Illoqite-(Ce) occurs in a hyperalkaline ussingite vein closely related to one of the largest ussingite veins in the Ilímaussaq complex. The associated minerals are aegirine, arfvedsonite, a britholite-group mineral, epistolite, chkalovite, lueshite, Mn-rich pectolite group member and steenstrupine-(Ce). Illoqite-(Ce) crystallises as either single euhedral crystals up to 150 μm in size or radiating aggregates consisting of a few or many crystals. The aggregates are up to 200 μm in diameter. Illoqite-(Ce) can occur as scattered small groups of crystals or aggregates, but sometimes they occur in high concentrations creating clusters or bands almost completely consisting of illoqite-(Ce). The empirical formula on the basis of 17 anions is Na 2.00 Na 1.00 (Ba 0.59 Sr 0.32 Ca 0.04 Na 0.03 ) Σ0.98 (Ce 0.68 La 0.31 Nd 0.09 Pr 0.04 ) Σ1.12 (Zn 0.42 Fe 0.34 Li 0.14 Mn 0.09 ) Σ0.99 Si 5.97 O 17 , with the simplified formula being Na 2 Na(Ba,Sr)(Ce,La,Nd)(Zn,Fe,Li)Si 6 O 17 . Illoqite-(Ce) exhibits sector zoning between elements Sr and Ba. The crystal structure was determined using single-crystal X-ray diffraction data and refined to R 1 = 2.46% using 1902 unique reflections. Illoqite-(Ce) is orthorhombic, Pcca , with a = 14.5340(7), b = 5.2213(1), c = 19.8270(4) Å, V = 1507.25(6) Å 3 and Z = 4. The strongest lines of the powder X-ray diffraction pattern [ d , Å ( I , %) ( hkl )] are: 7.266 (79) (200), 4.688 (44) (104), 4.241 (64) (210), 3.486 (79) (114), 3.340 (52) (312), 2.986 (67) (410), 2.882 (100) (314) and 2.789 (44) (016). Illoqite-(Ce) is a new member of the nordite supergroup and is named after the Greenlandic word illoq, meaning cousin, in allusion to the mineral's close relation to nordite-(Ce).
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