Skip to main content Accessibility help

Chlorite interstratified with a 7 Å mineral: an example from offshore Norway and possible implications for the interpretation of the composition of diagenetic chlorites

  • S. Hillier (a1) and B. Velde (a1)


X-ray diffraction (XRD) patterns of a pore-lining diagenetic chlorite (14 Å) from a reservoir sandstone, offshore Norway, show broad odd-order and sharp even-order basal reflections indicating that it contains 7 Å layers. Using NEWMOD, simulated XRD patterns with 15% 7 Å serpentine layers and a maximum crystallite thickness of 30 layers match the natural mineral well. Microprobe analyses of the 7 Å-14 Å mineral indicate that it is Fe-rich and aluminous suggesting that it is interstratified berthierine-chamosite. Apparent octahedral vacancies, however, suggest a significant dioctahedral component, and an alternative interpretation is interstratified kaolinite-chlorite. Indeed, chemical analyses of the mineral suggest a mixture of chlorite with 15% kaolinite, precisely the proportion of 7 Å layers indicated by XRD. Two other examples from the literature, previously identified as diagenetic chlorite, are probably also 7 Å-14 Å interstratified minerals, and the proportion of 7 Å layers indicated by XRD is also correlated with their structural formulae, if the 7 Å layers are, in fact, kaolinitic. This type of interstratification could explain why Fe-rich diagenetic chlorites appear to be compositionally distinct from metamorphic chlorites. The structure and chemistry of the Norwegian chlorite tend to support the idea that pore-lining chlorites form early in the diagenetic history, inhibiting the precipitation of later diagenetic minerals, and hence preserving abnormally high porosity at greater depths.



Hide All
Ahn, H.J. & Peacor, D.R. (1985) Transmission electron microscopic study of chlorite in Gulf Coast argillaceous sediments. Clays Clay Miner., 33, 228–236.
Bailey, S.W. (1988) Odinite, a new dioctahedral-trioctahedral Fe3+-rich 1:1 clay mineral. Clay Miner., 23, 237–;247.
Brindley, G.W. & Gillery, F.H. (1953) A mixed-layer kaolin-chlorite structure. Clays Clay Miner., 2, 349–353.
Burton, J.H., Krinsley, D.H. & Pye, K. (1987) Authigenesis of kaolinite and chlorite in Texas Gulf coast sediments. Clays Clay Miner., 35, 291–296.
Dean, R.S. (1983) Authigenic trioctahedral clay minerals coating Clear Water Formation sand grains at Cold Lake, Alberta, Canada. Program & Abstracts 20th Ann. Meet., The Clay Minerals Society, Buffalo, New York, p. 79.
Hillier, S.J. & Velde, B. (1991) Octahedral occupancy and the chemical composition of diagenetic (low temperature) chlorites. Clay Miner., 26, 149–168.
Howhr, J., Eslinger, E.V., Hower, M.E. & Perry, E.A. (1976) Mechanism of burial metamorphism of argillaceous sediments: 1. Mineralogical and chemical evidence. Bull. Geol. Soc. Amer., 87, 725–737.
Hughes, C.R. (1989) The application of analytical transmission electron microscopy to the study of oolitic ironstones: a preliminary study. Pp. 121–;131 in: Phanerozoic Ironstones(Young, J.P. and Taylor, W.E.G., editors). Geological Society Special Publication 46.
Humprheys, B.A., Smith, S.A. & Strong, G.E. (1989) Authigenic chlorite formation in late Triassic sandstones from the Central Graben, North Sea. Clay Miner., 24, 427–;444.
Jackson, M.L. & Abdel-Kader, F.H. (1978) Kaolinite intercalation procedure for all sizes and types with X-ray diffraction spacing distinctive from other phyllosilicates. Clays Clay Miner., 26, 81–87.
Moore, D.M. & Hughes, R.E. (1991) Characteristics of chlorite interlayered with a 7 A mineral as found in sandstone reservoirs. Program & Abstracts, 28th Ann. Meet, The Clay Minerals Society, Houston, Texas, p.115
Nelson, B.W. & Roy, R. (1958) Synthesis of chlorites and their structural and chemical constitution. Am. Miner., 43, 707–725.
Odin, S.G. (1988) Green Marine Clays. Developments in Sedimentology 45. Elsevier, Amsterdam.
Pittman, E.D. & Lumsden, D.N. (1968) Relationship between chlorite coatings on quartz grains and porosity, Spiro Sand, Oklahoma. J. Sed. Pet., 38, 668–670.
Reynolds, R.C. (1988) Mixed-layer chlorite minerals. Pp. 601–;629 in: Hydrous Phyllosilicates (Exclusive of Micas) (Bailey, S.W., editor). Reviews in Mineralogy 19. Mineralogical Society of America, Washington, DC.
Tillman, R. W. & Almon, W.R. (1979) Diagenesis of Frontier Formation offshore bar sandstones Spearhead ranch field, Wyoming. Pp. 337–;378 in: Aspects of Diagenesis (Scholle, P.A. & Schluger, P.R., editors). SEPM Special Publication 26.
Walker, J.R. & Thompson, G.R. (1990) Structural variations in chlorite and illite in a diagenetic sequence from the Imperial Valley, California. Clays Clay Miner., 38, 315–321.
Whittle, C.K. (1986) Comparison of sedimentary chlorite compositions by X-ray diffraction and analytical TEM. Clay Miner., 21, 937–947.
Wiewiora, A. & Weiss, Z. (1990) Crystallochemical classifications of phyllosilicates based on the unified system of projection of chemical composition: II. The chlorite group. Clay Miner., 25, 83–92.


Full text views

Total number of HTML views: 0
Total number of PDF views: 0 *
Loading metrics...

Abstract views

Total abstract views: 0 *
Loading metrics...

* Views captured on Cambridge Core between <date>. This data will be updated every 24 hours.

Usage data cannot currently be displayed