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Brinrobertsite: a new R1 interstratified pyrophyllite/smectite-like clay mineral: characterization and geological origin

  • H. Dong (a1), D. R. Peacor (a2), R. J. Merriman (a3) and S. J. Kemp (a3)


Brinrobertsite, an ordered, mixed-layered, dioctahedral pyrophyllite-smectite (P/S), occurs in a metabentonite in the Ordovician Nant Ffrancon Formation near Bangor, N Wales. It comprises ~30% of the metabentonite, in association with quartz (~50%) and chlorite (clinochlore; 20%) which replaced glass shards and fine-grained glass matrix. Transmission electron microscopy (TEM) images show sequences of dominant ~24 Å 001 lattice fringes inferred to correspond to 2:1 layers with alternate pyrophyllite-like (low-charge) and smectite-like (higher-charge) interlayers (i.e. R1 ordering). The hk0 diffraction patterns are mostly hexanets with some spotty circles, implying that layers are largely coherently related, but with some turbostratic stacking. Collective data show that d 100 = 5.2, b = 9.1, and d 001 = 24–25 Å, assuming monoclinic or pseudomonoclinic symmetry. The composition, as determined by energy dispersive spectral analysis, is (Na0.22K0.07Ca0.06)(Al3.81Mg0.08Fe0.08)(Si7.84 Al0.16)O20(OH)4·3.54H2O, as consistent with the sum of the compositions of pyrophyllite-like and smectite-like units. Water content was determined by DTA/TGA analysis. The powder diffraction patterns have a principal peak with d 001 = 24.4 Å. Patterns of air-dried and glycol-saturated brinrobertsite, including Na- and Ca-saturated and untreated samples, were modelled satisfactorily as R1-ordered P/S by the program NEWMOD-for-Windows. The unique composition of brinrobertsite relative to R1 IS, which is ubiquitous in metabentonites, was caused by leaching of alkalis and alkaline-earth elements by hydrothermal fluids associated perhaps with a nearby intrusion, as demonstrated by bulk-chemical analyses of the metabentonite. The crystal structure is modelled as having Al/Si distributions symmetrical by reflection across interlayers. This causes all 2:1 layers to be equivalent in having one tetrahedral sheet with little or no Al, and the other with significant Al substitution, giving rise to alternate high- and low-charge interlayers. Geological evidence suggests that brinrobertsite is a back-reacted product of hydrothermal alteration in the sequence: glass → pyrophyllite → brinrobertsite.


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Altaner, S.P. and Ylagan, R.F. (1997) Comparison of structural models of mixed-layer illite/smectite and reaction mechanisms of smectite illitization. Clays and Clay Minerals, 45, 517533.
Bevins, R.E. and Merriman, R.J. (1988) Co-existing prehnite-actinolite and prehnite-pumpellyite facies assemblages in the Tal y Fan Metabasite Intrusion, North Wales: Implications for Caledonian metamorphism. Journal of Metamorphic Geology, 6, 1739.
British Geological Survey (1985) Bangor. Sheet 106. Solid Edition. 1:50,000 (Keyworth, Nottingham, UK).
Brown, G. (1984) Crystal structures of clay minerals and related phyllosilicates. Pp. 221240 in: Clay Minerals: their Structure, Behaviour and Use (Fowden, L., Barrer, R.M. and Tinker, P.B., editors). Philosophical Transactions of the Royal Society, A311. The Royal Society, London.
Dong, H., Peacor, D.R. and Freed, R.L. (1997) Phase relations among smectite, R1 illite/smectite and illite. American Mineralogist, 82, 379391.
Eberl, D.D. (1979) Reaction series for dioctahedral smectite: The synthesis of mixed-layer pyrophyllite/smectite. Pp. 375383 in: Proceedings of the International Clay Conference, Oxford, 1978 (Mortland, M.M. and Farmer, V.C., editors). Elsevier, Amsterdam.
Eslinger, E. and Pevear, D.R. (1988) Clay Minerals for Petroleum Geologists and Engineers. Society of Economic Paleontologists and Mineralogists, Tulsa, Oklahoma, USA.
Frey, M. (1987) The reaction-isograd kaolinite + quartz = pyrophyllite + H2O, Helvetic Alps, Switzerland. Schweizerische Mineralogische und Petrographische Mitteilungen, 67, 111.
Grim, R.E. (1968) Clay Mineralogy, 2nd edition. McGraw-Hill, New York.
Howells, M.F., Reedman, A.J. and Campbell, S.D.G. (1991) Ordovician (Caradoc) marginal basin volcanism in Snowdonia (north-west Wales). HMSO for the British Geological Survey, London.
Jiang, W.-T., Peacor, D.R., Merriman, R.J. and Roberts, B. (1990) Transmission and analytical electron microscopic study of mixed-layer illite/smectite formed as an apparent replacement product of diagenetic illite. Clays and Clay Minerals, 38, 449468.
Kim, J.-W., Peacor, D.R., Tessier, D. and Elsass, F. (1995) A technique for maintaining texture and permanent expansion of smectite interlayers for TEM observations. Clays and Clay Minerals, 43, 5157.
Kodama, H. (1958) Mineralogical study on some pyrophyllites in Japan. Mineralogical Journal, 2, 236244.
Li, G., Peacor, D.R., Merriman, R.J. and Roberts, B. (1994) The diagenetic to low grade metamorphic evolution of matrix white micas in the system muscovite-paragonite in a mudrock from Central Wales, U.K. Clays and Clay Minerals, 42, 369381.
Merriman, R.J. and Roberts, B. (1985) A survey of white mica crystallinity and polytypes in pelitic rocks of Snowdonia and Llyn, N. Wales. Mineralogical Magazine, 49, 305319.
Merriman, R.J. and Peacor, D.R. (1999) Very low-grade metapelites; mineralogy, microfabrics and measuring reaction progress. Pp. 1060 in: Low – Grade Metamorphism (Frey, M. and Robinson, D., editors). Blackwell Sciences Ltd., Oxford, UK.
Merriman, R.J., Roberts, B. and Peacor, D.R. (1990) A transmission electron microscope study of white mica crystallite size distribution in a mudstone to slate transitional sequence, North Wales, U.K. Contributions to Mineralogy and Petrology, 106, 2740.
Moore, D.M. and Reynolds, R.C. Jr. (1997) X-ray Diffraction and the Identification and Analysis of Clay Minerals, 2nd edition. Oxford University Press, Oxford, UK, 332 pp.
Nadeau, P.H. (1999) The fundamental particle model: a clay mineral paradigm. Pp. 1319 in: Clays for our Future. Proceedings of the 11th International Clay Conference, Ottawa, Canada, 1997 (Kodama, H., Mermut, A.R. and Torrance, J.K., editors). Published by ICC97 Organizing Committee, Ottawa, Canada.
Nadeau, P.H., Wilson, M.J., McHardy, W.J. and Tait, J.M. (1984) Interstratified clays as fundamental particles. Science, 225, 923925.
Reynolds, R.C. and Reynolds, R.C. (1996) Description of Newmod-for-Windows . The calculation of one-dimensional X-ray diffraction patterns of mixed layered clay minerals. R.C. Reynolds, 8 Brook Road, Hanover, NH 03755, USA.
Roberts, B. and Merriman, R.J. (1985) The distinction between Caledonian burial and regional metamorphism in metapelites from North Wales: an analysis of isocryst patterns. Journal of the Geological Society of London, 142, 615624.
Roberts, B. and Merriman, R.J. (1990) Cambrian and Ordovician metabentonites and their relevance to the origins of associated mudrocks in the northern sector of the Lower Palaeozoic Welsh marginal basin. Geological Magazine, 127, 3143.
Roberts, B., Merriman, R.J., Hirons, S.R., Fletcher, C.J.N. and Wilson, D. (1996) Synchronous very low grade metamorphism, contraction and inversion in the central part of the Welsh Lower Palaeozoic Basin. Journal of the Geological Society, London, 153, 277286.
Środoń, J. (1999) Nature of mixed-layer clays and mechanisms of their formation and alteration. Annual Reviews of Earth and Planetary Science, 27, 1953.
Tillick, D.A., Peacor, D.R. and Mauk, J.L. (2001) Genesis of dioctahedral phyllosilicates during hydrothermal alteration of volcanic rocks: I. The Golden Cross epithermal ore deposit, New Zealand. Clays and Clay Minerals, 49, 126140.


Brinrobertsite: a new R1 interstratified pyrophyllite/smectite-like clay mineral: characterization and geological origin

  • H. Dong (a1), D. R. Peacor (a2), R. J. Merriman (a3) and S. J. Kemp (a3)


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