Abstract

Zoned crystals of carbocernaite occur in hydrothermally reworked burbankite-fluorapatite-bearing calcite carbonatite at Bear Lodge, Wyoming. The mineral is paragenetically associated with pyrite, strontianite, barite, ancylite-(Ce), and late-stage calcite, and is interpreted to have precipitated from sulfate-bearing fluids derived from an external source and enriched in Na, Ca, Sr, Ba, and rare-earth elements (REE) through dissolution of the primary calcite and burbankite. The crystals of carbocernaite show a complex juxtaposition of core-rim, sectoral, and oscillatory zoning patterns arising from significant variations in the content of all major cations, which can be expressed by the empirical formula (Ca0.43–0.91Sr0.40–0.69REE0.18–0.59Na0.18–0.53Ba0–0.08)∑1.96–2.00(CO3)2. Interelement correlations indicate that the examined crystals can be viewed as a solid solution between two hypothetical end-members, CaSr(CO3)2 and NaREE(CO3)2, with the most Na-REE-rich areas in pyramidal (morphologically speaking) growth sectors representing a probable new mineral species. Although the Bear Lodge carbocernaite is consistently enriched in light REE relative to heavy REE and Y (chondrite-normalized La/Er = 500–4200), the pyramidal sectors exhibit a greater degree of fractionation between these two groups of elements relative to their associated prismatic sectors. A sample approaching the solid-solution midline [(Ca0.57Na0.42)∑0.99(Sr0.50REE0.47Ba0.01)∑0.98(CO3)2] was studied by single-crystal X-ray diffraction and shown to have a monoclinic symmetry [space group P11m, a = 6.434(4), b = 7.266(5), c = 5.220(3) Å, γ = 89.979(17)°, Z = 2] as opposed to the orthorhombic symmetry (space group Pb21m) proposed in earlier studies. The symmetry reduction is due to partial cation order in sevenfold-coordinated sites occupied predominantly by Ca and Na, and in tenfold-coordinated sites hosting Sr, REE, and Ba. The ordering also causes splitting of carbonate vibrational modes at 690–740 and 1080–1100 cm–1 in Raman spectra. Using Raman micro-spectroscopy, carbocernaite can be readily distinguished from burbankite- and ancylite-group carbonates characterized by similar energy-dispersive spectra.

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