Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Rare-earth phosphate with cerium the dominant rare-earth element.
Chemical Formula: (Ce,La,Nd)PO4
Method(s) of Verification: Central Wales - EMPA (BGS, Keyworth); Fron Oleu - EMPA (Starkey & Robinson, 1992).
- Hydrothermal: alpine type veins
Orange monazite crystal (1 mm) associated with quartz from Fron Oleu, near Prenteg, Gwynedd. I.E. Jones Collection. Photo M.P. Cooper, © National Museum of Wales.
Orange monazite from Hendre Quarry, Glyn Ceiriog, Clwyd. N. Hubbard Collection. © National Museum of Wales.
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 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)]. Monazite-(Ce) may occur in a range of settings. It is a minor accessory primary mineral in certain igneous rocks and is a frequent detrital mineral in clastic sedimentary rocks derived from their erosion. It may also occur in sediments in a nodular or concretionary form. It occurs in some pegmatites, notably those enriched in rare-earths, and is also known from alpine fissure-type veins. Because monazite, like many rare-earth minerals, commonly carries radiogenic isotopes of Pb, U and 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: monazite has three modes of occurrence in Wales. Firstly, it occurs as a primary mineral in certain igneous rocks, for example in rhyolitic ashflow tuffs on Ramsey Island, Pembrokeshire (R.E. Bevins, unpublished data). Secondly it is widespread both as detritus and as authigenic nodules in Lower Palaeozoic sedimentary rocks across Wales (Cooper et al., 1983; Read et al., 1987), while thirdly crystalline monazite occurs as a minor component of Alpine Fissure-type assemblages at certain localities in North Wales, listed below.
- 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 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.
- Fron Oleu, Prenteg, Tremadog, Gwynedd: honey-brown monazite crystals to 1.5 mm, associated with albite and apatite, occur in alpine fissure-type veins hosted by metasediments closely adjacent to the contact of a major dolerite sill (Starkey & Robinson, 1992).
- Hendre Quarry, Glyn Ceiriog, Clwyd: small (0.5 to occasionally 1 mm) pinkish to yellow rhombic monazite crystals occur with anatase and xenotime-(Y) in an alpine fissure-type assemblage on joints in dolerite at this disused quarry (Starkey et al., 1991).
- 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., . U-Pb, Pb-Pb and Sm-Nd dating of authigenic monazite: Implications for the diagenetic evolution of the Welsh Basin. Earth & Planetary Science Letters, 144, 421-433.
- 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.
- Starkey, R.E. & Robinson, G.W., 1992. Famous mineral localities, Prenteg, Tremadog, Gwynedd, Wales. Mineralogical Record, 23, 391-399.
- Starkey, R.E., Hubbard, N. & Bayley, M.P., 1991. Mineralization at Hendre Quarry, Glyn Ceiriog, Clwyd, Wales. U.K. Journal of Mines and Minerals, No. 10, 48-51.