OCCURRENCE AND ORIGIN OF DJERFISHERITE FROM THE ELWIN BAY KIMBERLITE, SOMERSET-ISLAND, NORTHWEST-TERRITORIES

This paper focuses on a new occurrence of djerfisherite [approximately K-6(Na,Li)(Fe,Ni,Cu)(24)S26Cl] from the Elwin Bay kimberlite on Somerset Island. The small (up to 10 mu m), subhedral grains of djerfisherite occur individually and as clusters in the groundmass, suggesting that they are late primary magmatic phases of the kimberlite. Based on 84 compositions obtained with an electron microprobe, compositional ranges (weight percent) are: K (8.78-9.94), Fe (34.76-47.35), Ni (3.92-18.88), Cu (0.09-4.49), S (31.81-34.70), and Cl (1.25-1.57). Grain-to-grain compositional variation exists, as does variation within single zoned grains, characterized predominantly by a Fe-rich core and a Ni-rich rim. The analytical data indicate that Cl is an essential component of the Elwin Bay djerfisherite, as it is consistently present at a level of one atom per formula unit. The compositions also show a strong relation between the total number of transition-element cations (Fe, Ni, Cu) per formula unit and the number of d-shell electrons (DSE). There are two types of substitution: (i) 2Ni double left right arrow (1 + x)Fe + (1 - x)Cu, where the value of x approaches 1, in the Early and Late stages, when the evolving composition of the magma controls the composition of the djerfisherite, and (ii) Fe double left right arrow 0.5 Cu + 0.75, Ni double left right arrow 1.5 Cu + 0.25, and Ni double left right arrow 2.5 - 3Fe, reactions that stabilize the DSE at 6.50 in the Intermediate stage. The stability of djerfisherite depends in part on fixing d-shell electrons in the cubic clusters of transition elements. The formula of the putative iron end-member of the Elwin Bay djerfisherite is approximately K6Fe22,75S26Cl.