Crandallite, goyazite and gorceixite from the Kovdor massif, Russia

Crandallite group minerals from hydrothermal assemblages occurring in the c arbonatite-phoscorite complex of the Kovdor alkaline-ultrabasic massif have been studied. Goyazite, gorceixite and crandallite occur in cavities of do lomitic carbonatite veins; crandallite was identified also in the crust of weathering and in a carbonate-fluorapatite breccia. A natural evolution of species-forming cations in the Sr-Ba-Ca order is shown on the basis of an a nalysis of spatial relationships of minerals in hydrothermal assemblages. C rystals of the studied phosphates are characterized by a similar morphology of faces. Two generations of goyazite are distinguished. Zoned fine bladed crystals of goyazite-II have an inner zone composed of (Sr0.69Ba0.25Ca0.01 )(Sigma 0.95)Al-2.83[PO3(O-1/2(OH)(1/2))](2) (OH)(5.39) and a 5 mu m extern al zone of (Ba0.62Sr0.23Ca0.14)(Sigma 0.99)Al-3.26[PO4] [PO3OH](OH)(6.76). IR-spectroscopic data are given. Chemically all the described minerals are unique due to the lack of REE impurities in the first cationic position (al l analyses yielded REE contents lower than the sensitivity of the electron microprobe). X-ray diffraction patterns indicated the absence of mechanic i mpurities. These facts made it possible to obtain X-ray diffraction pattern s and unit-cell parameters that approach JCPDS: for crandallite (space grou p R (3) over bar m) a = b = 7.009(9) Angstrom, c = 16.15(2) Angstrom, for g oyazite (R (3) over bar m) a = b = 7.021(6) Angstrom, c = 16.64(2) Angstrom , and for gorceixite (Cm) a = 12.20(4) Angstrom, b = 7.048(8) Angstrom, c = 7.049(9) Angstrom, beta = 125.20 degrees(1). The chemical composition and eel-parameters of the gorceixite are practically identical to those defined for gorceixite from Rapid-Creek formation, Yukon Territory, Canada. The pr esence of crandallite group minerals can serve as a possible criterion for the search of secondary scandium concentrations. Scandium phosphates can be produced during low-temperature stages of hydrothermal mineralization.