Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Rare-earth phosphate with neodymium the dominant rare-earth element.
Chemical Formula: (Nd,La,Ce)PO4
Method(s) of Verification: Central Wales - EMPA (Cooper et al., 1983; Read et al., 1987).
- Sedimentary: allogenic (detrital)
- Sedimentary: diagenetic
Authigenic monazite grains, separated from the host Lower Palaeozoic mudrock prior to geochemical analysis. © J.A. Evans, NIGL.
Introduction: monazite is a group name for the minerals monazite-(Ce), monazite-(La) and monazite-(Nd), defined by the most abundant rare-earth element (REE) present as in the following examples: Monazite-(Ce): Ce>La & Nd e.g. [Ce0.5La0.25Nd0.2Th0.05(PO4)]; Monazite-(La): La>Ce & Nd e.g. [La0.5Ce0.25Nd0.2Th0.05(PO4)]; Monazite-(Nd): Nd>La & Ce e.g. [Nd0.5La0.25Ce0.2Th0.05(PO4)] Because monazite, like many rare-earth minerals, commonly carries radiogenic isotopes of lead (Pb), uranium (U) and thorium (Th), it has potential as a radiometric dating tool and recent research has borne this out (Evans & Zalasiewicz, 1996; Evans et al., 2002). This work is important as it has shown that nodular monazite occurring in sedimentary rocks and formed during their diagenesis can be employed to date such rocks - a particularly useful tool in the case of poorly fossiliferous or unfossiliferous rocks.
Occurrence in Wales: there is only one confirmed occurrence of monazite-(Nd) from Wales, although a better picture of its distribution will be possible as more analyses become available. It occurs as a detrital mineral in sedimentary rocks and authigenic nodules in Lower Palaeozoic sedimentary rocks across Wales (Cooper et al., 1983; Read et al., 1987).
- Central Wales: authigenic nodules of monazite up to 2 mm in size are locally abundant in the Lower Palaeozoic sedimentary rocks of Central Wales, a phenomenon only recognised relatively recently due to their concentration in heavy mineral suites obtained by panning of stream sediments (Cooper et al., 1983). In some areas the concentration is estimated to be around 10 million nodules per cubic metre of rock (Evans et al., 2002). The nodules are most abundant in hemipelagic mudstones and it has been suggested that their abundance in such rocks is due to migration of light rare earth elements (REE) up from underlying coarser turbidite units (Milodowski & Zalasiewicz, 1991). There is also evidence for their nucleation around detrital monazite grains. Analyses of the highly-zoned nodules indicate that the cores are enriched in heavy REE (in one case with a monazite-(Nd) composition) while light REE are predominant in the rims. The nodules retain traces of the original bedding fabric but have undergone rotation during regional cleavage formation: they are interpreted as having formed during burial-related diagenesis of the host-rocks. Their potential as a geochronological tool has been discussed above.
- Cooper, D.C., Basham, I.R. & Smith, T.K., 1983. On the occurrence of an unusual form of monazite in panned stream sediments in Wales. Geological Journal, 18, 121-127.
- Evans, J.A., Zalasiewicz, J.A.,Fletcher, I, Rasmussen, B. & Pearce, N.J.G., 2002. Dating diagenetic monazite in mudrocks: constraining the oil window. Journal of the Geological Society of London, 159, 619-622.
- Milodowski, A.E. & Zalasiewicz, J.A., 1991. Redistribution of rare earth elements during diagenesis of turbidite/hemipelagite mudrock sequences of Llandovery age from Central Wales. In: Morton, A.C., Todd, S.P. & Haughton, P.D. (eds): Developments in Sedimentary Provenance Studies. Geological Society, London, Special Publications, 57, 101-124.
- Read, D. Cooper, D.C & McArthur, J.M., 1987. The composition and distribution of nodular monazite in the Lower Palaeozoic rocks of Great Britain. Mineralogical Magazine, 51, 271-280.