Helvine-group minerals from Norwegian granitic pegmatites and some other granitic rocks: Cases of significant Sc and Sn contents

Helvine-group minerals from two granitic pegmatites have disparate compositions, from nearly pure helvine (Ågskardet, northern Norway; Devonian) to helvine close to ternary compositions (Heftetjern, southern Norway; Precambrian). Metagranite from Høgtuva (northern Norway; Precambrian with Caledonian metamorphic overprint) contains Zn-rich danalite. The Ågskardet helvine contains up to 0.46 wt.% SnO2, and the Heftetjern ternary helvine shows a maximum of 1.74 wt.% Sc2O3. Helvine minerals were also analyzed from three occurrences connected to peralkaline granite (ekerite) of the Permian Oslo Rift. Nearly pure genthelvite (99.19 mol.%) occurs in miarolitic cavities at Gjerdingselva. Two mineralogically different granitic pegmatites derived from the same ekerite pluton in the southern part of the Oslo Rift show quite distinct helvine compositions, from nearly continuous solid solution between helvine and genthelvite in crystals with oscillatory zonation (Rundemyr) to solid solutions midway between danalite and genthelvite (Bakstevalåsen). The Rundemyr crystals have a maximum SnO2 content of 1.28 wt.%. The incorporation of minor elements (Ca, Mg, Al, Sn, Sc) in helvine-group minerals is discussed with emphasis on their chalcophilicity characteristics. For stereochemical reasons, Sn in helvine minerals must be tetravalent, even if Sn2+ is more chalcophile than Sn4+.