| Reference Type | Journal (article/letter/editorial) |
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| Title | Orbicular rhythmic layering in the palabora carbonatite, South Africa |
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| Journal | Geological Magazine |
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| Authors | Moore, Alan C. | Author |
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| Year | 1984 (January) | Volume | 121 |
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| Issue | 1 |
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| Publisher | Cambridge University Press (CUP) |
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| DOI | doi:10.1017/s0016756800027941Search in ResearchGate |
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| Generate Citation Formats |
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| Mindat Ref. ID | 252307 | Long-form Identifier | mindat:1:5:252307:5 |
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| GUID | 0 |
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| Full Reference | Moore, Alan C. (1984) Orbicular rhythmic layering in the palabora carbonatite, South Africa. Geological Magazine, 121 (1) 53-60 doi:10.1017/s0016756800027941 |
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| Plain Text | Moore, Alan C. (1984) Orbicular rhythmic layering in the palabora carbonatite, South Africa. Geological Magazine, 121 (1) 53-60 doi:10.1017/s0016756800027941 |
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| In | (1984, January) Geological Magazine Vol. 121 (1) Cambridge University Press (CUP) |
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| Abstract/Notes | AbstractThe earliest stage of magmatic activity within the Palabora carbonatite was marked by the intrusion of phosphate-bearing pyroxenite. In good exposures in the north, large-scale (2m diameter) orbicular structures are found. These consist of regularly-spaced alternating dark layers (phlogopite-rich) and light layers (diopside-plus apatite-rich) which, in hand specimen, are very similar to the ‘inch-scale’ planar layering which has been described in layered mafic instrusions. One of the purposes of this paper is to describe these unique features which are currently being destroyed by mining, as they form economic phosphate concentrations.The resemblance of the Palabora orbicules to Liesegang rings has led to the development of a qualitative model whereby the orbicules are regarded as having been formed by concentric periodic precipitation around central cores within dynamically quiet, isolated pockets of largely liquid magma. The controlling parameters are interpreted as being the rates of growth of the constituent minerals and the rates of diffusion of the elements crucial to their growth, i.e. K+, Al3+ and (OH)- for dark layers, and Ca2+ and P5+ for light layers.The presence of these spectacular structures with their delicate layering indicates that at the time of crystallization of the pyroxenites relatively non-turbulent conditions prevailed, and diopside, phlogopite and apatite crystallized essentially coevally. Hence, the vertical large-scale banding in the pyroxenite may also be a function of diffusion controlled processes rather than being caused by separate magma pulses. |
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