CALCIO-ANCYLITE (CE) FROM ILIMAUSSAQ AND NARSSARSSUK, GREENLAND, KOLA-PENINSULA AND POLAR URALS, RUSSIA - ANCYLITE (CE) CALCIO-ANCYLITE (CE), AN ISOMORPHOUS SERIES

Calcio-ancylite-(Ce) is a rare species with an interesting history. Wh en the mineral was first described, it was not named and its type loca lity was not specified; when found for the second time it was not reco gnized and a hen the name ''calcio-ancylite'' was used for the first t ime, it was for a calcium-rich ancylite. Since its proper definition, it has only been described from a few localities. The present studies, including microprobe and X-ray diffraction of a large selection of sp ecimens of the ancylite family of minerals, have identified calcio-anc ylite-(Ce) at five new localities, in different. parageneses: as a com ponent of pseudomorphs after steenstrupine in an albite vein, Lilleelv , Ilimaussaq complex, Greenland; in a single specimen from a feldspar- natrolite vein genetically connected with Narssarssuk, Greenland; in p erovskite nests in pyroxenites of the Afrikanda massif, Kola peninsula , Russia, where the crystals are zoned, with cores of ancylite-(Ce) an d border zones of calcio-ancylite-(Ce); and in alpine-type veins of bo th Dodo and Puiva rock crystal deposits, Polar Urals, Russia. Ancylite -(Ce) -calcio-ancylite-(Ce) forms a, probably continuous, isomorphous series. The syst em is complex: the Ca/Sr ratio varies in all series; Sr-free calcio-ancylite-(Ce) is known from several localities, but all ancylite-(Ce) analyses show the presence of some Ca; intermediate mem bers of this series are characterized by (Sr+Ca)/Sigma REE approximate to 0.5 whereas this ratio varies from approximate to 0.35 to 1.0 for end-close Ca/Sr members; the Ca/Sr ratio has a profound influence on t he REE composition since Ca-rich phases of the system are more Nd-rich and La-poor in comparison with Sr-rich members. Ancylite-(Ce) and cal cio-ancylite-(Ce) X-ray powder diagrams are similar, but sufficiently different to allow a clear distinction between these two minerals: the strong (110) and (011) reflections are the most sensitive to changes in the Ca/Sr ratio, the Ca-rich members having the smallest d-values. So in spite of the fact that calcio-ancylite-(Ce) is a rare mineral, i t is found under a wide range of conditions: calcio-ancylite-(Ce) is n ow known as a late mineral of granitoids, from altered zones of metamo rphic rocks, from alpine veins, from high-alkaline rocks and from ultr abasic alkaline massifs. Sr-free calcio-ancylite-(Ce) is typical for g ranitoids; Sr-rich calcio-ancylite-(Ce) with transition to ancylite(Ce ) is found in alkaline complexes.