Sorosilicate (Si2O7 groups), allanite group
Specific gravity: 4.22
Common impurities: Al
At the type locality, the Neprimetnyi pegmatite, Mount Ulyn Khuren, Khaldzan Buragtag massif, Myangad District, Khovd Province, Mongolia, ferriallanite-(Ce) is of metasomatic origin and represents a major product of the pseudomorphism of rare-earth-element-rich minerals of the eudialyte group. It occurs as an aggregate of subhedral grains from 0.3 x 0.5 to 1 x 2 mm within masses of zircon, quartz and kainosite-(Y) and is associated with aegirine, fergusonite-(Y) beta, yttrium-rich and normal ilvaite, hingganite-(Ce), neodymium-rich allanite-(Ce), magnetite, fayalite and fluorite (CM 40.1641-1648)
At the Bastnäs Mines, Riddarhyttan, Skinnskatteberg, Västmanland County, Sweden, quartz banded hematite ore occurs adjacent to a magnetite skarn ore, the latter replacing a dolomite layer. The cerite ore was worked in two small mines, where it formed a restricted layer in the skarn zone, composed mainly of cerite-(Ce), tremolitic amphibole, allanite-(Ce), bastnäsite-(Ce), törnebohmite-(Ce) and talc. Sulphide minerals, dominantly chalcopyrite, bismuthinite and molybdenite, are closely associated with the lanthanide minerals, and usually are interstitial to them.
Three samples were analysed.
Sample 1 consists of coarse-grained ferriallanite-(Ce) forming euhedral crystals up to 8 mm long, together with bastnäsite-(Ce)–bastnäsite-(La) and quartz. Bismuthinite, chalcopyrite and molybdenite occupy smaller voids between the ferriallanite-(Ce) crystals.
Sample 2 consists of rounded aggregates of fine-grained cerite-(Ce) with lesser amounts of bastnäsite-(Ce), surrounded by margins of anhedral, irregular ferriallanite-(Ce) grains. Törnebohmite-(Ce) and a new sorosilicate mineral species are closely associated with the ferriallanite-(Ce). Patches of fluocerite-(Ce), partly altered to cerianite and bastnäsite-(La), occur sporadically in the cerite-bearing assemblage.
Sample 3 consists of euhedral to subhedral crystals of ferriallanite-(Ce) up to 1 mm long, together with cerite-(Ce) crystals, in massive bismuthinite with subordinate chalcopyrite and molybdenite (CM 41.1233-1240).
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