Crystal-Chemical Factors Responsible for the Distribution of Octahedral Cations Over trans- and cis-Sites in Dioctahedral 2:1 Layer Silicates

Abstract

Crystal chemical analysis of various dioctahedral 2:1 phyllosilicates consisting of trans-vacant (tv) and cis-vacant (cv) layers and interstratified cv and tv layers shows that there is compositional control over the distribution of octahedral cations over trans and cis sites. Fe3+ and Mg-rich dioctahedral micas (celadonite, glauconite, leucophyllite and most phengite) occur only as tv varieties. Similarly, the occurrence of tv illites and tv illite fundamental particles in illite-smectite (I-S) does not depend significantly on the cation composition of the 2:1 layers. In contrast, compositional restrictions exist to control the occurrence of pure cv1M illite, which can form only as Fe- and Mg-poor varieties. Similarly, proportions of cv and tv layers in illite fundamental particles depend on the amount of Al in octahedral and tetrahedral sheets of the 2:1 layers.Simulations of atomic coordinates and interatomic distances for periodic tv1M and cv1M illite structures allow us to reveal the main structural factors that favor the formation of cv layers in illite and I-S. It is shown that in contrast to the tv1M structure, interlayer K in cv1M illite has an environment which is similar to that in 2M1 muscovite. This similarity along with a high octahedral and tetrahedral Al content probably provides stability for cv1M illite in low-temperature natural environments. Because of structural control, the occurrence of monomineral cv1M illite, its association with tv 1M illite, and interstratified cv-tv illite fundamental particles is confined by certain physical and chemical conditions. These varieties are most often formed by hydrothermal activity of different origin. The initial material for their formation should be Al-rich and the hydrothermal fluids should be Mg- and Fe-poor. They occur mostly around ore deposits, in bentonites and in sandstone sedimentary rocks.The factors governing the formation of tv and cv layers in dioctahedral smectite are probably related to the layer composition and local order-disorder in the distribution of isomorphous octahedral cations, because there is no influence from fixed interlayer cations. In particular, the occurrence of Mg-OH-Mg cation arrangements is more favorable for the formation of cv montmorillonite layers.

Thermodynamic properties of illite, smectite and beidellite by calorimetric methods: Enthalpies of formation, heat capacities, entropies and Gibbs free energies of formation

“…M min is the molar mass of clay mineral, n is the number of moles of interlayer water per mole of clay, g is the mass fraction of dehydrated clay in the dehydrated sample (clay + impurities). also concluded in a tendency for Mg cations to avoid the formation of Mg-OH-Mg configurations in dioctahedral phyllosilicates from spectroscopic analyses and mechanistic calculations, although it was not verified for some particular cases, such as Mg-rich montmorillonites (Drits et al, 2006). Therefore, the configurational entropic term for the octahedral layer was calculated by considering a distribution of cations Al 3+ , Fe 3+ and Fe 2+ among two octahedral sites, whereas Mg 2+ was distributed only in one octahedral site.…”

Evaluation of redox-active iron sites in smectites using middle and near infrared spectroscopy

“…j Starting suspension for re-oxidation with oxygen and 4-AcNB corresponds to sample 7 G. k R þ 1:5 ðSi 6:5 FeðIIIÞ 1:5 ÞðFeðIIIÞ 4 ÞO 20 ðOHÞ 4 taken from (Decarreau et al, 2008). end-member of a tetrahedrally charged, iron-rich trans-vacant smectite (total iron content of 33.5 wt%). The main factor governing the formation of cis-and trans-vacant layers is the structural iron content with iron-rich smectites (Fe content >0.6/unit cell) being predominantly trans-vacant while those smectites containing lower amounts of iron (Fe content >0.6/unit cell) are cis-vacant (Drits et al, 2006;Wolters et al, 2009). Thus, iron-rich ferruginous smectite (SWa-1) and Ö lberg montmorillonite (for unit cell composition see Table 1), which contain approximately the same amount of total iron ranging in between the endmembers (12-13 wt%), exhibit both a trans-vacant configuration.…”

Computer simulations of cations order‐disorder in 2:1 dioctahedral phyllosilicates using cation‐exchange potentials and monte carlo methods

“…By means of neutron powder diffraction, Redfern [1] studied the VI Mg 21 and VI Al 31 ordering in phengite, finding ordering. For dioctahedral smectites, Drits et al [18] proposed that octahedral cation order-disorder probably had important consequences in the formation of cis-vacant and trans-vacant sheets, and that the pairs MgAOHAMg could favor the formation of cis-vacant montmorillonites. Tunega et al [19] reported quantum-mechanical studies, using plane-wave periodic calculations for different cationic arrangement in supercells, and found the short-range ordering is the usual ordering in dioctahedral phyllosilicates, but the longrange ordering would be difficult in these minerals.…”