Trans-Vacant and cis-Vacant 2:1 Layer Silicates: Structural Features, Identification, and Occurrence

Abstract

A comprehensive study of clay minerals should include determination of the vacancy pattern of the dioctahedral sheet. The purpose of this report is to consider the advantages and limitations in various diffraction and non-diffraction methods for the determination of the layer types in clay minerals. Identification of trans-vacant (tv) and cis-vacant (cv) clay minerals reported here is based on powder X-ray diffraction (XRD) patterns calculated for different polytypes consisting of either tv or cv layers, on the simulation of experimental XRD patterns corresponding to illite or illite fundamental particles in which tv and cv layers are interstratified, and on the semi-quantitative assessment of the relative content of the layer types in the interstratified structures by generalized Méring's rules. A simple and effective method for identification of tv and cv layers in dioctahedral 2:1 layer silicates employs thermal analysis and is based on different dehydroxylation temperatures for tv and cv illite and smectite layers.Crystal chemical analysis of various dioctahedral 2:1 layer silicates consisting of tv and cv layers indicates that compositional control is present in the distribution of octahedral cations over trans-and cissites. In dioctahedral smectites the formation of tv and cv layers is related to the layer composition and local order-disorder in the distribution of isomorphous cations. Dioctahedral 1M micas with abundant Fe 3+ and Mg occur only as tv varieties. In contrast, 1M-cv illite, as well as cv layers in illite fundamental particles of I-S, can form only as Fe-and Mg-poor varieties. In illites and illite fundamental particles of I-S consisting of tv and cv layers, cv layers prevail when the amounts of Al in octahedra and tetrahedra are >1.55 and >0.35 atoms per O 10 (OH) 2 , respectively.The main factors responsible for the stability of cv and tv illites have been established. Monomineral cv 1M illite, its association with tv 1M illite, and interstratified cv/tv illite occur around ore deposits, in bentonites, and in sandstones mostly as a result of different types of hydrothermal activity. The initial material for their formation should be Al-rich, and hydrothermal fluids should be Mg-and Fe-poor.Tv and cv smectites of volcanic origin differ in terms of octahedral cation composition and distribution of isomorphous octahedral cations. Mg-rich cv smectites have random distribution of isomorphous octahedral cations, whereas in Mg-bearing tv smectites octahedral Mg cations are dispersed so as to minimize the amount of Mg-OH-Mg arrangements.

Cation arrangement in the octahedral and tetrahedral sheets of cis-vacant polymorph of dioctahedral 2:1 phyllosilicates by quantum mechanical calculations

“…Drits et al (1984) were the first to distinguish the reflections of each configuration and to deduce the crystal structure of a cv illite model. However, Drits and Zviagina (2009) noted that in the powder XRD patterns of trans-vacant 3T (3T-tv) and 1M-cv mica polymorphs, the hkl reflections had similar intensities and very close positions which makes them difficult to distinguish especially for interstratificates of cv and tv layers. Tsipursky and Drits (1984) revealed semi-quantitatively the cv/tv proportions of several dioctahedral 2:1 phyllosilicates, finding a higher cv proportion in montmorillonites than in nontronites and beidellites.…”

Electron diffraction study of 1M illites with interstratified trans- and cis-vacant 2 : 1 layers

“…Illites with -ccosβ/a values from 0.390 to 0.396 are assigned to dioctahedral micas with a vacant trans octahedron: tv1M. 1M illites with smaller -ccosβ/a values enriched in aluminum (Fe and Mg depleted species) can be assigned to interstratified tv/cv 1M illites [14,20]. For these illites the aforementioned decrease in the -ccosβ/a values (in comparison with the values found for tv1M illites) is related not to the variations in the cationic composition of micas but to the different ratios of irregularly interstratified tv and cv layers in them.…”

“…It was noted in [14,20] that the configuration of interlayer gaps that arise as a result of the rotation of adjacent tetrahedral sheets by 120° (where the inter layer cation K has an environment similar to that of the most stable mica (2М 1 muscovite)) is favorable for the formation of cv1М micas. This interlayer configu ration also reduces the mutual repulsion of the nearest basal oxygen atoms of adjacent layers.…”

“…This interlayer configu ration also reduces the mutual repulsion of the nearest basal oxygen atoms of adjacent layers. The manifesta tion of the stabilizing effect of these factors is facili tated by the decrease in the number of octahedral Mg, Fe 3+ , and Fe 2+ cations and the increase in the number of tetrahedral Al atoms [14,20]. It can be seen from the symbolic description of the 33 0 33 (tv1М) and 51 0 51 polytypes (cv1М) [2] that the tetrahedral sheets of adjacent layers in the tv1М polytype have the same orientation, whereas the tetrahedral sheets in cv1М, which form the interlayer gap, are rotated by 120°.…”

Mixed-layered structure formation during trans-vacant Al-rich illite partial dehydroxylation

“…The structure of the dioctahedral micas is described conventionally in terms of 1M, 2M 1 , 2M 2 , and 3T polytypes that differ in mutual arrangements of the adjacent layers (Bailey 1984). The octahedral cations in dioctahedral 2M 1 , 2M 2 , and 3T micas typically occupy only cis-sites (Bailey 1984), whereas 1M structures may consist of either trans-vacant (tv) or cis-vacant (cv) 2:1 layers, or of the interstratified layer types (Zvyagin, et al 1985;Reynolds and Thomson 1993;Zviagina et al 2007;Drits et al 1993Drits et al , 2006Drits et al , 2010Drits and Zviagina 2009). Dehydroxylation and the formation of H 2 O molecules occur in high-temperature thermal transformations of dioctahedral micas.…”