The first occurrence of ewaldite and donnayite-(Y), two rare carbonate minerals, in the Narssarssuk pegmatite, South Greenland

Ewaldite and donnayite-(Y) have recently been found, epitaxially inter-grown, in crystals from the Narssârssuk pegmatite, South Greenland. They form decidedly hemimorphic and characteristically cone-shaped, individual crystals up to 1.1 mm in diameter and 0.8 mm thick, and aggregates of few crystals grown together in random orientation. These crystals show the forms {0001}, {0001} and {1124}, with overgrowths of thin, hexagonal plates covering the {0001} faces in a three-fold parallel array. They are colourless to white, transparent to translucent. The ewaldite-donnayite-(Y) crystals are found on calcite, with associated minerals being ashcroftine-(Y), ancylite-(Ce), and fluorite along with aegirine, elpidite, feldspar (orthoclase), graphite and polylithionite. The Narssârssuk ewaldite is uniaxial negative. n e and n ω , for λ = 589 nm, are: 1.552(0.5) and 1.653(0.5) respectively. The empirical formulae, based on microprobe analyses, of the Narssârssuk ewaldite and donnayite-(Y) are: (Ba 2 . 3 7 Sr 0 . 6 0 ) Σ 2 . 9 7 Na 1 . 0 6 Ca 0 . 8 9 (Y 0 . 3 8 Ce 0 . 2 4 Nd 0 . 1 8 La 0 . 0 8 Dy 0 . 0 6 Pr 0 . 0 6 Sm 0 . 0 4 Er 0 . 0 3 ) Σ 1 . 0 7 (CO 3 ) 6 .3H 2 O, and (Sr 2 . 0 9 Ba 1 . 0 1 ) Σ 3 . 1 0 Na 1 . 1 1 Ca 1 . 0 0 (Y 0 . 4 0 Ce 0 . 1 6 Nd 0 . 0 7 La 0 . 0 6 Dy 0 . 0 7 Pr 0 . 0 5 Er 0 . 0 6 Yb 0 . 0 3 ) Σ 0 . 9 0 (CO 3 ) 6 .3H 2 O, respectively, CO 2 and H 2 O being calculated by stoichiometry, and their presence confirmed by IR spectroscopy. The crystal lattice parameters of ewaldite are a = 5.294(2) A, and c = 12.666(4) A, presumed space group P63mc, and those of donnayite-(Y) are a = 5.20(1) A, and c = 18.35(4) A, presumed space group R3m.