Neptunite from the Woodsreef Serpentinite, New South Wales: A new occurrence

Neptunite has been identified in a tectonic inclusion (75×44 m) occurring near the western faulted margin of the Woodsreef Serpentinite in northern New South Wales. The serpentinite is one of a series of ultramafic bodies which forms the Great Serpentinite Belt of eastern Australia. Major mineralogy of the rock, which is best described as a foliated amphibolite, includes a sodic amphibole, a sodic pyroxene and albite. Neptunite is restricted to leucocratic, albite-rich layers in the inclusion, where it occurs as dark-reddish brown crystals up to 7 mm long. Its concentration is generally less than 1%. Under the microscope, neptunite grains are euhedral and subhedral, deep red and orange in colour, with a marked pleochroism. The refractive indices are: α=1.69, β=1.70, γ=1.73; 2 V=39° (+). The average chemical composition, determined by microprobe analysis and neutron activation analysis, corresponds to the formula Li_1.2Na_2.2K_0.8Fe_1.7Mn_0.1Mg_0.2Ti₂Si₈O₂₄. The neptunite from Woodsreef thus occupies a place nearest to the neptunite end-member in the series neptunite-mangan-neptunite. The powder diffraction data gave, after a least squares refinement, a=16.43, b=12.51, c=10.00 Å, β=115.32°. The origin of the rock containing neptunite is briefly discussed.