SummaryAnalyses of glauconites and celadonites from continental sedimentary rocks and sea-floor basalts using X-ray diffraction, electron probe microanalysis, infra-red spectroscopy, and X-ray fluorescence spectroscopy are reported. The minerals are shown to be distinct species; each an isomorphous replacement series, glauconite having an average half unit cell of K0·85(Fe3+, Al3+)1·34 (Mg2+, Fe2+)0·66(Si3·76Al0·24)O10(OH)2 whereas celadonite approaches the ideal half unit cell of K(Fe3+, Al3+)(Mg2+, Fe2+)Si4O10(OH)2. Considerable Fe3+Alvi interchangeability occurs in the octahedral layer in both minerals and considerable substitution of aluminium in the tetrahedral layer of glauconites results in the more disordered 1Md type of structure compared with the more highly ordered 1M structure of celadonites. Some mixed layer glauconite-smectites and celadonites were also examined and could be distinguished from true glauconites and celadonites by chemical analysis, XRD, and IR techniques. It is proposed that the terms ‘glauconite’ and ‘celadonite’ should be used only for those minerals containing less than 5% interlayering.

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Carbonates of the magnesite-siderite series have been found and analysed in carbonatites from the Lueshe, Newania, Kangankunde, and Chipman Lake complexes. This series has been represented until now only by a few X-ray identifications of magnesite and three published analyses of siderite and breunnerite (magnesian siderite). Most of the siderite identified in carbonatites in the past has proved to be ankerite, but the new data define the complete solid-solution series from magnesite to siderite. They occur together with dolomite and ankerite and in one rock with calcite. The magnesites, ferroan magnesites and some magnesian siderites may be metasomatic/hydrothermal in origin but magnesian siderite from Chipman Lake appears to have crystallized in the two-phase calcite + siderite field in the subsolidus CaCO3-MgCO3-FeCO3 system. Textural evidence in Newania carbonatites indicates that ferroan magnesite, which co-exists with ankerite, is a primary liquidus phase and it is proposed that the Newania carbonatite evolved directly from a Ca-poor, Mg-rich carbonatitic liquid generated by partial melting of phlogopite-carbonate peridotite in the mantle at pressures >32 kbar.K EYWO R D S: carbonate, carbonatite complexes, magnesite, siderite, breunnerite.

The drill core of Funafuti Atoll, collected by The Royal Society Coral Reef Expedition in 1896-1898, has been dated using high-precision 14 C and Sr isotope measurements determined by mass spectrometry. The core top is a Holocene reef deposit, reflecting 26.4 m of reef growth since 8 ka, and is underlaid by Pleistocene reef-lagoonal deposits separated by a series of inferred hiatuses at 26.4 m, ca. 30 m, ca. 65 m and ca. 80 m (ca. 0.1 Ma, ca. 0.2 Ma, ca. 0.3 Ma and ca. 0.6 Ma, respectively), reflecting atoll carbonate deposits associated with late Quaternary sea-level changes. In turn, the underlying sequence below ca. 80 m comprises an upper section of additional Pleistocene reef-lagoonal deposits, having a high accumulation rate greater than 100 m Ma −1 , and a heavily dolomitized lower section. Sr isotope data in the lower section provide maximum (oldest) ages for dolomite formation and, depending on the extent of the isotopic memory from precursor CaCO 3 , reflect ongoing diagenetic alteration from ca. 2.3 Ma (i.e. the oldest dolomite) or in one or more discrete diagenetic events at ca. 1 Ma, and possibly at ca. 2 Ma. This sequence reflects interaction between atoll carbonate sedimentation, tectonic subsidence and eustatic sea-level change.

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