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

Abstract

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 1Md 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 (K1.66Ca0.04)Σ1.70(Al3.32Fe0.31Mg0.57Mn0.06)Σ4.26(Si6.43Al1.57)Σ8O20(OH)4 and (K1.57Na0.31Ca0.03)Σ1.91(Al3.58Fe0.05Mg0.29Mn0.01)Σ3.93(Si6.70Al1.30)Σ8O20(OH)4, respectively. In addition, 1Mdillite, which is the polytype occurring in the common 1Mdto 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 Δz, 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.

Exhumation history and landscape evolution of the Sierra de San Luis (Sierras Pampeanas, Argentina) - new insights from low - temperature thermochronological data

“…In TEM (transmission electron microscopy) and SAED (selected area electron diffraction) analyses on sample APM 59-09, it was not possible to identify 1M illite, which may be attributed to the small TEM sample volume. However, other authors also reported difficulties to find the 1M polytype proposed by XRD studies during TEM analysis (e.g., Peacor et al, 2002;Solumn et al, 2005).…”

Polytype and morphological analyses of gümbelite, a fibrous Mg-rich illite

“…Dz decreases with increasing substitution of (Mg, Fe) because the sizes of the M1 and M2 octahedra become closer (Lee and Guggenheim, 1981;Brigatti et al, 1998). In fact, Peacor et al (2002) showed by TEM-AEM analyses that unusually high Mg contents in illite sequences are associated with the 1M polytype, which can be attributed to lower Dz values.…”

Determination of layer stacking microstructures and intralayer transition of illite polytypes by high-resolution transmission electron microscopy (HRTEM)

“…It has been found that 1M d and 1M illite are the predominant form in sedimentary and low-grade metamorphic rocks, but 2M 1 is the only stable polymorph at low and moderate temperatures, consistent with the transformation to the 2M 1 polytype with increasing K + and IV Al +3 content in illite (Grubb et al 1991). Some researchers (Grubb et al 1991;Zöller and Brockamp 1997;Peacor et al 2002) suggested that illite varieties are not polytypes in a strict sense because of their changes in composition accompanied with changes in stacking sequence. Nevertheless, we will still use the term polytype to describe the different stacking sequences in this paper.…”

“…Bauluz et al (2000) observed that there is no 1M polytype in any sequence of interstratifi ed illite-smectite and illite. Peacor et al (2002) studied high-Mg 1M illite and explained the absence of 1M polytype in the prograde diagenetic sequence of pelitic rocks.…”