Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (5) Citing Articles via Google Scholar Google Scholar Articles by Peacor, D. R. Articles by Yan, Y. Search for Related Content GeoRef GeoRef Citation

TRANSMISSION AND ANALYTICAL ELECTRON MICROSCOPY EVIDENCE FOR HIGH Mg CONTENTS OF 1M ILLITE: ABSENCE OF 1M POLYTYPISM IN NORMAL PROGRADE DIAGENETIC SEQUENCES OF PELITIC ROCKS

¹ Department of Geological Sciences, The University of Michigan, Ann Arbor, Michigan, 48109-1063, USA
² Departamento de Ciencias de la Tierra, Cristalografía y Mineralogía, Universidad de Zaragoza, 50.009 Zaragoza, Spain
³ Department of Geology, Miami University, Oxford, OH 45056, USA
⁴ Daveyhurst, c/- Croesus Mining NL, 39 Porter St., Kalgoorlie, WA 6430, Australia
⁵ 25 Harris Street, #23, Acton MA, 01720, USA

^* E-mail address of corresponding author: drpeacor{at}umich.edu

The normal prograde diagenetic and low-grade metamorphic sequence of dioctahedral clay minerals including illite-rich I-S and illite, as observed by TEM, proceeds from a partially disordered 1M_d stacking sequence to 2M₁; i.e. 1M does not normally occur as an intermediate polytype. Examples of 1M illite stacking sequences have been studied, however, from the Golden Cross gold deposit, New Zealand, the Broadlands-Ohaaki geothermal system, New Zealand, the Potsdam Sandstone, New York, and the Silverton Caldera, Colorado. Specific clay-mineral packets identified by TEM techniques as 1M illite were found to have anomalously high Mg contents. The Broadlands illite provides the most definitive data, as separate packets of 1M and 2M₁ illite coexist. Average compositions for 1M and 2M₁ illite are (K_1.66 Ca_0.04)_1.70 (Al_3.32 Fe_0.31 Mg_0.57 Mn_0.06) _4.26 (Si_6.43 Al_1.57) ₈ O₂₀ (OH)₄ and (K_1.57 Na_0.31 Ca_0.03)_1.91 (Al_3.58 Fe_0.05 Mg_0.29 Mn_0.01)_3.93 (Si_6.70 Al_1.30)₈ O₂₀ (OH)₄, respectively. In addition, 1M_d illite, which is the polytype occurring in the common 1M_d to 2M1 prograde sequence, is relatively Mg poor, but coexists with Mg-rich illite in the Silverton Caldera sample.

These data confirm that 1M stacking is caused by compositional anomalies, and thus explain the lack of the 1M stacking sequence in normal diagenetic sequences in pelitic rocks, as most illite in such environments has a relatively small phengitic component. The parameter z, a measure of the corrugation of the oxygen sheets, may be the key parameter reflecting the polytypic state of dioctahedral and trioctahedral micaceous minerals. Such composition-determined relations may be related to the occurrence of 1M polytypism in glauconite and celadonite, both dioctahedral 2:1 clay minerals having large Mg or Fe octahedral-cation components, and in trioctahedral micas. Insofar as the 1M stacking sequence does not have the same composition as 2M₁ material, these data confirm that the different varieties of illite are not polytypes, sensu stricto.

Key Words: Illite • Polytypism • Transmission Electron Microscopy

This article has been cited by other articles:

				J. Kameda, R. Miyawaki, V. A. Drits, and T. Kogure POLYTYPE AND MORPHOLOGICAL ANALYSES OF GUMBELITE, A FIBROUS Mg-RICH ILLITE Clays and Clay Minerals, October 1, 2007; 55(5): 453 - 466. [Abstract] [Full Text] [PDF]

				T. Chen and H. Wang Determination of layer stacking microstructures and intralayer transition of illite polytypes by high-resolution transmission electron microscopy (HRTEM) American Mineralogist, May 1, 2007; 92(5-6): 926 - 932. [Abstract] [Full Text] [PDF]