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

摘要

Ewaldite and donnayite-(Y) have recently been found, epitaxially inter-grown, in crystals from the Narssarssuk 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}, {000 1-bar} and {11 2-bar 4-bar}, 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 Narssarssuk ewaldite is uniaxial negative. N_(epsilon) and n_(omega), for lambda = 589 nm, are: 1.552(0.5) and 1.653(0.5) respectively. The empirical formulae, based on microprobe analyses, of the Narssarssuk ewaldite and donnayite-(Y) are: (Ba_(2.37)Sr_(0.60))_(sum2.97)Na_(1.06)Ca_(0.89)(Y_(0.38)Ce_(0.24)Nd_(0.18)La_(0.08)Dy_(0.06)Pr_(0.06)Sm_(0.04)Er_(0.03))_(sum1.07)(CO_3)_6 centre dot 3H_2O, and (Sr_(2.09)Ba_(1.01))_(sum3.10)Na_(1.11)Ca_(1.00)(Y_(0.40)Ce_(0.16)Nd_(0.07)La_(0.06)Dy_(0.07)Pr_(0.05)Er_(0.06)Yb_(0.03))(sum0.90)(CO_3)_6 centre dot 3H_2O, respectively, CO_2 and H_2O 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 P6_3mc, and those of donnayite-(Y) are a = 5.20(1)A, and c = 18.35(4) A, presumed space group R3m.